.NET Remoting

This chapter is from the book 

Applying .NET Remoting

So far, I have discussed the architecture and various concepts related to the .NET remoting. In this section, you will learn how to apply these concepts to see remoting in action. In particular, you will learn how to

• Create a remotable class.

• Create a Server-activated object.

• Create a Client-activated object.

• Use configuration files to configure the remoting framework.

• Use interface assemblies to compile remoting clients.

Creating a Remotable Class

Creating a remotable class is simple. All you need to do is to inherit a class from the MarshalByRefObject class. Step By Step 3.1 creates a remotable class named DbConnect. This class connects to a specified SQL Server database and allows you to execute a SELECT SQL statement using its ExecuteQuery() method.

STEP BY STEP 3.1: Creating a Remotable Class

1. Launch Visual Studio .NET. Select File, New, Blank Solution, and name the new solution 310C03. Click OK.

2. Add a new Visual Basic .NET Class library named StepByStep3-1 to the solution.

3. In the Solution Explorer, rename the default Class1.vb to DbConnect.vb.

4. Open the DbConnect.vb class and replace the code with the following code:

Imports System
Imports System.Data
Imports System.Data.SqlClient


' Marshal-by-Reference Remotable Object
Public Class DbConnect
  Inherits MarshalByRefObject
  Private sqlconn As SqlConnection

  ' Default constructor connects to the Northwind
  ' database
  Public Sub New()
    sqlconn = New SqlConnection( _
     "data source=(local);" & _
     "initial catalog=Northwind;" & _
     "integrated security=SSPI")
    Console.WriteLine( _
     "Created a new connection " & _
     "to the Northwind database")
  End Sub

  ' Parameterized constructor connects to the
  ' specified database
  Public Sub New(ByVal DbName As String)
    sqlconn = New SqlConnection( _
     "data source=(local);" & _
     "initial catalog=" & DbName & ";" & _
     "integrated security=SSPI")
    Console.WriteLine( _
     "Created a new connection " & _
     "to the " & DbName & " database")
  End Sub

  Public Function ExecuteQuery( _
   ByVal strQuery As String) As DataSet
    Console.Write("Starting to execute " & _
     "the query...")
    ' Create a SqlCommand to represent the query
    Dim sqlcmd As SqlCommand = _
     sqlconn.CreateCommand()
    sqlcmd.CommandType = CommandType.Text
    sqlcmd.CommandText = strQuery

    ' Create a SqlDataAdapter object
    ' to talk to the database
    Dim sqlda As SqlDataAdapter = _
     New SqlDataAdapter()
    sqlda.SelectCommand = sqlcmd
    ' Create a DataSet to hold the results
    Dim ds As DataSet = New DataSet()
    Try
      ' Fill the DataSet
      sqlda.Fill(ds, "Results")
    Catch ex As Exception
      Console.WriteLine(ex.Message, _
       "Error executing query")
    End Try
    Console.WriteLine("Done.")
    ExecuteQuery = ds
  End Function
End Class

5. Select Build, Build StepByStep3-1. This step packages the remotable class into the file StepByStep3-1.dll, which is located in the bin or directory of your project. You can navigate to it through the Solution Explorer: Just select the project and click the Show All Files button in the Solution Explorer toolbar.

You've now created a remotable class, but it cannot yet be directly called from client application domains. For a remotable class to be activated, you need to connect the class to the remoting framework. You'll learn how to do that in the next section.

Creating a Server-Activated Object

A remotable class is usually connected with the remoting framework through a separate server program. The server program listens to the client request on a specified channel and instantiates the remote object or invokes calls on it as required.

It is a good idea to keep the remotable class and server program separate; this enables the design to be modular and the code to be reusable.

In this section, I'll show you how to create a remoting server. Here's an overview of the steps that the remoting server must take.

1. Create a server channel that listens on a particular port to the incoming object activation requests from other application domains. The following code segment shows how to create a TCP server channel and an HTTP server channel:

' Register a TCP server channel on port
Dim channel As TcpServerChannel = _
 New TcpServerChannel(1234)

' Register a HTTP server channel on port
Dim channel As HttpServerChannel = _
 New HttpServerChannel(1234)

2. Register the channel with the remoting framework. This tells the framework which requests should be directed to this particular server. This registration is performed through the RegisterChannel() method of the ChannelServices class:

' Register the channel with remoting framework
ChannelServices.RegisterChannel(channel)

3. Register the remotable class with the remoting framework. This tells the framework which classes this particular server can create for remote clients. For a server-activated object, this registration is performed using the RegisterWellKnownServiceType() method of the RemotingConfiguration class, as shown here:

4. ' Register a remote object with the remoting framework
   RemotingConfiguration.RegisterWellKnownServiceType( _
    GetType(DbConnect), "DbConnect", _
    WellKnownObjectMode.SingleCall)

   Here, the first parameter is the type of the remotable class. The second parameter specifies the uniform resource identifier (URI) through which the server publishes the location of the remote object. The last parameter specifies the activation mode. The activation mode can be one of the two possible values of the WellKnownObjectMode enumeration—SingleCall or Singleton.

4. With the channel and class both registered, the remoting server is ready to go. The remoting framework will direct all requests for that class via that channel to the registered server.

As I discussed, earlier, an SAO can be activated in two different modes—SingleCall and Singleton. In the next few sections, I'll cover how to create a remoting server for activating objects in each of these modes. I'll also tell the story on the other side of the channel; that is, how to connect the client program to the remoting framework so that it can instantiate the SAO and call methods on it.

Registering a Remotable Class As a Server-Activated Object Using the SingleCall Activation Mode

In this section, I'll demonstrate how to create a server that exposes the remotable class through the remoting framework. The server process here will be a long running user interface-less process that will continue to listen for incoming client requests on a channel.

Ideally, you should write this type of server program as a Windows service or use an existing Windows service such as Internet Information Services (IIS) to work as a remoting server. But I have chosen to write the server program as a console application mainly because I'll use the console Window to display various messages that will help you understand the workings of the remoting server.

Later in this chapter, in the section "Using IIS As an Activation Agent," I'll cover how to use IIS as a remoting server. I'll talk about Windows services in Chapter 6, "Windows Services."

STEP BY STEP 3.2: Registering a Server-Activated Object Using the SingleCall Activation Mode

1. Add a new Visual Basic .NET Console application named StepByStep3-2 to the solution.

2. In the Solution Explorer, right-click the project StepByStep3-2 and select Add Reference from the context menu. In the Add Reference dialog box (see Figure 3.4), select the .NET tab, select the System.Runtime.Remoting component from the list view, and click the Select button. Now select the Projects tab, select the Project named StepByStep3-1 (which contains the remotable object) from the list view, and click the Select button. Both the selected projects then appear in the Selected Components list, as shown in Figure 3.4. Click OK.

Figure 3.4 The Add Reference dialog box allows you to add references to components.

3. In the Solution Explorer, rename the default Module1.vb module to DbConnectSingleCallServer.vb. Open the file and change the name of the module to DbConnectSingleCallServer in the module declaration.

4. Add the following Imports directives above the module declaration:

Imports StepByStep3_1
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp

5. Add the following code in the Main() procedure:

Public Sub Main()
  ' Create and Register a TCP server channel
  ' that listens on port
  Dim channel As TcpServerChannel = _
    New TcpServerChannel(1234)
  ChannelServices.RegisterChannel(channel)

  ' Register the service that publishes
  ' DbConnect for remote access in SingleCall mode
  RemotingConfiguration. _
   RegisterWellKnownServiceType( _
  GetType(StepByStep3_1.DbConnect), "DbConnect", _
  WellKnownObjectMode.SingleCall)

  Console.WriteLine("Started server in the " & _
   "SingleCall mode")
  Console.WriteLine("Press <ENTER> to terminate " & _
   "server...")
  Console.ReadLine()
End Sub

6. Right-click on the StepByStep3-2 project in the Solution Explorer and select Properties. Change the Startup object to DbConnectSingleCallServer.

7. Build the project. This step creates a remoting server that is capable of registering the StepByStep3_1.DbConnect class for remote invocation using the SingleCall activation mode.

Step by Step 3.2 uses a receiver TCP channel (TcpServerChannel) to register a remotable class with the remoting framework. However, converting this program to use HTTP channel is not difficult—you just need to change all instances of Tcp to Http.

Step By Step 3.2 creates a remoting host that listens on port 1234. This is an arbitrary port number that might or might not work on your computer. A good idea is to check whether a port is already in use by some other application before running this program. You can do this from the command line on a particular computer by using the Windows netstat command.

This suggestion works only in a test scenario. It is not reasonable to instruct a customer to check whether the port is available before he starts the application. If the application will run entirely on your company's network, you can safely use a port in the private port range of 49152 through 65535—provided, of course, that the port number you choose is not used by any other internal application. In case you are distributing the application, you should get a port number registered with the IANA (Internet Assigned Numbers Authority). You can see a list of already assigned port numbers at http://www.iana.org/assignments/port-numbers.

Instantiating and Invoking a Server-Activated Object

At this stage, you have a remotable object as well as a remoting server ready. In this section, I'll show you how to create a remoting client and use it to send messages to the remoting server to activate the remote object. In order to achieve this, the remoting client needs to take the following steps:

1. Create and register a client channel that is used by the remoting framework to send messages to the remoting server. The type of the channel used by the client should be compatible with the channel used by server. The following examples show how to create a TCP client channel and an HTTP client channel:

   ' Create and register a TCP client channel
   Dim channel As TcpClientChannel = _
    New TcpClientChannel()
   ChannelServices.RegisterChannel(channel)
   
   ' Create and register a HTTP client channel
   Dim channel As HttpClientChannel = _
    New HttpClientChannel()
   ChannelServices.RegisterChannel(channel)

2. Register the remotable class as a valid type in the client's application domain. This registration is performed using the RegisterWellKnownClientType() method of the RemotingConfiguration class as shown here:

   ' Register the remote class as a valid
   ' type in the client's application domain
   RemotingConfiguration.RegisterWellKnownClientType( _
    GetType(DbConnect), "tcp://localhost:1234/DbConnect")

   Here, the first parameter is the type of the remotable class. The second parameter specifies the uniform resource identifier (URI) through which the server publishes the location of the remote object. Localhost maps to your local development machine. If the remote object is on some other computer, you'll replace localhost with the name of the computer.

3. Instantiate the SAO on the server. You can only use the default constructor.

   ' Instantiate the remote object
   DbConnect dbc = new DbConnect()

I'll demonstrate the preceding steps in Step by Step 3.3. In this example, I'll create the client program as a Windows application that accepts a SQL statement from the user and passes it to the remotable object. The rows returned by the remotable object are displayed in a DataGrid control.

STEP BY STEP 3.3: Instantiating and Invoking a Server-Activated Object

1. Add a new Visual Basic .NET Windows Application named StepByStep3-3 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

3. In the Solution Explorer, delete the default Form1.vb. Add a new form named DbConnectClient.vb. Set the new form as the Startup object for the project.

4. Add the following directives to the form's module:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp
Imports StepByStep3_1

5. Place two GroupBox controls, a TextBox control (txtQuery), a Button control (btnExecute) and a DataGrid control (dgResults) on the form. Set the Multiline property of txtQuery to True. Arrange the controls as shown in Figure 3.5.

Figure 3.5 This form invokes a method of a remote object to execute the given query.

6. Add the following code to the form directly after the designer-generated code:

' Declare a Remote object
Dim dbc As DbConnect

7. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  ' Register a TCP client channel
  Dim channel As TcpClientChannel = _
   New TcpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' Register the remote class as a valid
  ' type in the client's application domain
  RemotingConfiguration. _
   RegisterWellKnownClientType( _
   GetType(DbConnect), _
   "tcp://localhost:1234/DbConnect")

  ' Instantiate the remote class
  dbc = New DbConnect()
End Sub

8. Double-click the Button control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
     dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
     "Query Execution Error")
  End Try
End Sub

9. Right-click on the name of the solution in the Solution Explorer window and select Properties. This opens the Solution Property Pages dialog box. In the dialog box, select the Multiple Startup Projects check box, select the action Start for StepByStep3-2 and StepByStep3-3, and set the action to None for other projects as shown in Figure 3.6. Make sure that StepByStep3-2 is placed above StepByStep3-3. If it isn't already there, click the Move Up and Move Down buttons to get the right order.

Figure 3.6 Use the Solution Property Pages dialog box to control which projects are started and in what order.

10. Build the project. Select Debug, Start to run the project. You should see a command window displaying a message that the server is started in the SingleCall mode.

11. Shortly afterward, you'll see a Windows form for the client program. Enter a query in the text box and click the Execute Query button. The client invokes a method on the remote object and binds the results from the remote method to the DataGrid control as shown in Figure 3.7.

Figure 3.7 The remoting client populates the data grid with the result returned from the method invocation on a remote object.

In the preceding steps, I have chosen to start the client and server programs within the Visual Studio .NET IDE. You can also start these programs by clicking on StepByStep3-2.exe and StepByStep3-3.exe, respectively, from the Windows explorer. Note that the server program should be running before you click on the Execute Query button on the client program.

At the end of Step By Step 3.3 when you look at the console window of the server program, you'll note the output as shown in Figure 3.8. You'll note that the remote object is created every time you click on the Execute Query button. That's normal behavior based on what you've learned so far in the chapter; however, what's peculiar is that for the first call, the constructor is called twice.

Figure 3.8 The SingleCall remoting server creates a new instance of the remote object with each request.

In fact, calling the constructor twice for the first request on a server is also a regular behavior of SingleCall object activation. The following two points explain why this happens:

• The first call to constructor is made by the remoting framework at the server side to check whether it is okay to call this object remotely and to check the activation mode of the object.

• The second constructor is called because of the client's call on the remote object. In the case of SingleCall activation, the server does not preserve the state of the constructor that was called earlier; therefore, the object has to be re-created with each client request.

You might wonder why I included a reference to StepByStep3-1.dll in this project. You might support your argument by saying that the DbConnect class contained in the StepByStep3-1.dll is a remotable class and its right place is on the server and not on the client.

Well said, but I have included StepByStep3-1.dll because of the following reasons:

• The client project StepByStep3-3 won't compile without it—The reason is that I am referring to the DbConnect class in the project StepByStep3-3, and the project StepByStep3-3 by itself has no definition of DbConnect. When I include a reference to StepByStep3-1.dll, the project StepByStep3-3 can resolve the definition for DbConnect from there and allow me to compile the project.

• The client program StepByStep3-3.exe won't execute without it—I can't remove StepByStep3-1.dll from the project directory after the compilation is successfully completed. The StepByStep3-1.dll is required again at the time of running the client. This is because to create the proxy object for the DbConnect class, the CLR must have the metadata that describes DbConnect. This metadata is read from the assembly stored in StepByStep3-1.dll.

In some cases, this isn't a good real-life solution because the StepByStep3-1.dll might contain important business logic that you do not want your customers to decompile.

You're right! And, I have a solution for that in the form of interface assemblies. With interface assemblies, you just share the interface of an assembly with your customers, not the actual business logic. I'll show you how to create interface assemblies later in this chapter.

Registering a Remotable Class As a Server-Activated Object Using the Singleton Activation Mode

The same remotable object can be activated in different modes without making any changes to the remotable object itself. In case of SAO, the choice of activation mode is totally with the server. In this section, I'll show you how to create a remoting server that publishes the DbConnect class as an SAO using the Singleton activation mode. I'll use the same client program that was created in Step By Step 3.3 to test this Singleton server.

STEP BY STEP 3.4: Registering a Server-Activated Object Using the Singleton Activation Mode

1. Add a new Visual Basic .NET Console application named StepByStep3-4 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

3. In the Solution Explorer, rename the default Module1.vb to DbConnectSingletonServer.vb. Open the file and change the name of the Module to DbConnectSingletonServer in the module declaration.

4. Add the following directives:

Imports StepByStep3_1
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp

5. Add the following code in the Main() method:

Public Sub Main()
  ' Create and Register a TCP server channel
  ' that listens on port
  Dim channel As TcpServerChannel = _
    New TcpServerChannel(1234)
  ChannelServices.RegisterChannel(channel)

  ' Register the service that publishes
  ' DbConnect for remote access in SingleCall mode
  RemotingConfiguration. _
   RegisterWellKnownServiceType( _
  GetType(StepByStep3_1.DbConnect), "DbConnect", _
  WellKnownObjectMode.Singleton)

  Console.WriteLine("Started server in the " & _
   "Singleton mode")
  Console.WriteLine("Press <ENTER> to terminate " & _
   "server...")
  Console.ReadLine()
End Sub

6. Right-click on the StepByStep3-4 project in the Solution Explorer and select Properties. Change the Startup object to DbConnectSingletonServer.

7. Build the project. This step creates a remoting server capable of registering the StepByStep3_1.DbConnect class for remote invocation using the Singleton activation mode.

8. Set the StepByStep3-4, the remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Singleton mode.

9. Now, set StepByStep3-3, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Enter a query in the text box and click the button. The code invokes a method on the remote object, which is created when the form loads. The code binds the results from the remote method to the DataGrid control.

Although there has been no change in the output for the client, if you note the messages generated by server (see Figure 3.9), you'll see that just one instance of the connection is created and is shared by all the clients that connect to this server.

Figure 3.9 The Singleton remoting server uses the same instance of remote object with subsequent requests.

Guided Practice Exercise 3.1

The objective of this exercise is to create a remoting server that exposes the DbConnect class of StepByStep3-1 as a Singleton SAO. However, the server and client should communicate via the HTTP channels and SOAP formatter. Other than this, the server and the client programs are similar to those created in Step by Step 3.4 and Step by Step 3.3, respectively.

How would you establish communication between a remoting server and client using HTTP and SOAP?

This exercise helps you practice creating a remoting server and clients using the HTTP channels and SOAP formatter.

You should try working through this problem on your own first. If you get stuck, or if you'd like to see one possible solution, follow these steps:

1. Add a new Visual Basic .NET Console application named GuidedPracticeExercise3-1_Server to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

3. In the Solution Explorer, rename the default Module1.vb to DbConnectSingletonServer.vb. Open the file and change the name of the class to DbConnectSingletonServer in the class declaration.

4. Add the following directives:

Imports StepByStep3_1
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Http

5. Add the following code in the Main() method:

Sub Main()
  ' Create and Register a HTTP server channel
  ' that listens on port
  Dim channel As HttpServerChannel = _
   New HttpServerChannel(1234)
  ChannelServices.RegisterChannel(channel)

  ' Register the service that publishes
  ' DbConnect for remote access in Singleton mode
  RemotingConfiguration. _
   RegisterWellKnownServiceType( _
   GetType(StepByStep3_1.DbConnect), "DbConnect", _
   WellKnownObjectMode.Singleton)
  Console.WriteLine("Started server in the " & _
   "Singleton mode")
  Console.WriteLine("Press <ENTER> to terminate " & _
    "server...")
  Console.ReadLine()
End Sub

6. Right-click on the GuidedPracticeExercise3-1 Server project in the Solution Explorer and select Properties. Change the startup object to DbConnectSingletonServer.

7. Build the project. This step creates a remoting server that is capable of registering the StepByStep3_1.DbConnect class (the remotable object) for remote invocation using the Singleton activation mode via the HTTP channel.

8. Add a new Visual Basic .NET Windows Application named GuidedPracticeExercise3-1_Client to the solution.

9. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

10. In the Solution Explorer, delete the default Form1.vb. Add a new form named DbConnectClient.vb. Set the new form as the startup object for the project.

11. Add the following directives to the form's module:

Imports StepByStep3_1
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Http

12. Place two GroupBox controls, a TextBox control (txtQuery), a Button control (btnExecute) and a DataGrid control (dgResults) on the form. Set the Multiline property of txtQuery to True. Refer to Figure 3.5 for the design of the form.

13. Add the following code to the form directly after the designer-generated code:

' Declare a Remote object
Dim dbc As DbConnect

14. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  ' Register a TCP client channel
  Dim channel As HttpClientChannel = _
   New HttpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' Register the remote class as a valid
  ' type in the client's application domain
  RemotingConfiguration. _
   RegisterWellKnownClientType( _
   GetType(DbConnect), _
   "http://localhost:1234/DbConnect")

  ' Instantiate the remote class
  dbc = New DbConnect()
End Sub

15. Double-click the Button control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
     dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
     "Query Execution Error")
  End Try
End Sub

16. Build the solution. Set the GuidedPracticeExercise3-1_Server, the remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Singleton mode. The remoting server is now ready to receive SOAP messages via HTTP.

17. Now, set GuidedPracticeExercise3-1_Client, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Enter a query in the text box and click the button. The code invokes a method on the remote object and sends the messages in SOAP format via HTTP.

If you have difficulty following this exercise, review the sections "Channels," "Formatters," "Creating a Remotable class," and "Creating a Server-Activated Object" earlier in this chapter. Make sure that you also perform Step By Step 3.1 through Step By Step 3.4. After doing that review, try this exercise again.

Creating a Client-Activated Object

When exposing a remotable class as a CAO, no changes are required to be made on the remotable class. Instead, only the server and client differ on how the remotable class is registered with the remoting system.

In this section, I'll show you how to register a remotable class as a CAO and how to instantiate and invoke a CAO from a remoting client.

Registering a Remotable Class As a Client-Activated Object

You'll have to take the following steps to register a remotable class as a CAO on the server:

1. Create a server channel that listens on a particular port to the incoming object activation requests from other application domains. The following examples show how to create a TCP server channel and an HTTP server channel:

' Register a TCP server channel on port
Dim channel As TcpServerChannel = _
 new TcpServerChannel(1234)

' Register a HTTP server channel on port
Dim channel As HttpServerChannel = _
 new HttpServerChannel(1234)

2. Register the channel with the remoting framework. This registration is performed through the RegisterChannel() method of the ChannelServices class:

' Register the channel with remoting framework
ChannelServices.RegisterChannel(channel);

3. Register the remotable class with the remoting framework. For a client-activated object, this registration is performed using the RegisterActivatedServiceType() method of the RemotingConfiguration class as shown here:

' Register a remote object as CAO
' with the remoting framework
RemotingConfiguration.RegisterActivatedServiceType(  _
 GetType(DbConnect), "DbConnect"))

Here, the first parameter is the type of the remotable class. The second parameter specifies the URI through which the server publishes the location of the remote object.

Step By Step 3.5 shows how to expose the now-familiar DbConnect class from Step By Step 3.1 as a CAO.

STEP BY STEP 3.5: Registering a Remotable Class As a Client-Activated Object

1. Add a new Visual Basic .NET Console application named StepByStep3-5 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

3. In the Solution Explorer, rename the default Module1.vb to DbConnectCAOServer.vb. Open the file and change the name of the module to DbConnectCAOServer in the module declaration.

4. Add the following directives:

Imports StepByStep3_1
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp

5. Add the following code in the Main() method:

Sub Main()
  ' Create and Register a TCP Channel on port
  Dim channel As TcpServerChannel = _
   New TcpServerChannel(1234)
  ChannelServices.RegisterChannel(channel)

  ' Register the client activated object
  RemotingConfiguration. _
   RegisterActivatedServiceType( _
   GetType(DbConnect))
  Console.WriteLine( _
   "Started server in the Client Activation mode")
  Console.WriteLine( _
   "Press <ENTER> to terminate server...")
  Console.ReadLine()
End Sub

6. Right-click on the StepByStep3-5 project in the Solution Explorer and select Properties. Change the Startup object to DbConnectCAOServer.

7. Build the project. This step creates a remoting server that is capable of registering the StepByStep3_1.DbConnect (the remotable object) class for remote invocation using the client activation mode.

Instantiating and Invoking a Client-Activated Object

To instantiate and invoke a client-activated object, the remoting client needs to take the following steps:

1. Create and register a client channel used by the remoting framework to send messages to the remoting server. The type of channel used by the client should be compatible with the channel used by the server. The following examples show how to create a TCP client channel and an HTTP client channel:

' Create and register a TCP client channel
Dim channel As TcpClientChannel = _
 New TcpClientChannel()
ChannelServices.RegisterChannel(channel)

' Create and register a HTTP client channel
Dim channel As HttpClientChannel = _
 New HttpClientChannel()
ChannelServices.RegisterChannel(channel)

2. Register the remotable class as a valid type in the client's application domain. This registration is performed using the RegisterActivatedClientType() method of the RemotingConfiguration class as shown here:

' Register DbConnect as a type on client,
' which can be activated on the server
RemotingConfiguration.RegisterActivatedClientType( _
 GetType(DbConnect), "tcp://localhost:1234")

Here, the first parameter is the type of the remotable class. The second parameter specifies the uniform resource identifier (URI) through which the server publishes the location of the remote object.

3. Instantiate the CAO on the server using the desired constructor.

   ' Instantiate the remote object
   Dim dbc As DbConnect = new DbConnect("Pubs")

I'll demonstrate the preceding steps in Step By Step 3.6. This example is similar to the client program created in Step By Step 3.3, but this time the client allows users to choose between the databases on the server.

STEP BY STEP 3.6: Instantiating and Invoking a Client-Activated Object

1. Add a new Visual Basic .NET Windows Application named StepByStep3-6 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

3. In the Solution Explorer, delete the default Form1.vb. Add a new form named DbConnectClient.vb. Set the new form as the startup object for the project.

4. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp
Imports StepByStep3_1

5. Place three GroupBox controls (grpDatabases, grpQuery and grpResults), a ComboBox control (cboDatabases), a TextBox control (txtQuery), two Button controls (btnSelect and btnExecute), and a DataGrid control (dgResults) on the form. Refer to Figure 3.10 for the design of this form.

6. Select the Items property of the cboDatabases control in the Properties window and click on the (...) button. This opens the String Collection Editor dialog box. Enter the following names of databases in the editor:

   Northwind
   Pubs

   Click OK to add the databases to the Items collection of the cboDatabases control.

7. Add the following code just after the Windows form designer generated code:

' Declare a Remote object
Dim dbc As DbConnect

8. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  cboDatabases.SelectedIndex =
  grpQuery.Enabled = False
End Sub

9. Double-click the btnSelect control and add the following code in the Click event handler:

Private Sub btnSelect_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnSelect.Click
  ' Disable the Databases group box and
  ' Enable the Query group box
  grpDatabases.Enabled = False
  grpQuery.Enabled = True

  ' Register a TCP client channel
  Dim channel As TcpClientChannel _
   = New TcpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' Register the remote class as a valid
  ' type in the client's application domain
  ' by passing the Remote class and its URL
  RemotingConfiguration. _
   RegisterActivatedClientType( _
  GetType(DbConnect), "tcp://localhost:1234")

  ' Instantiate the remote class
  dbc = New DbConnect( _
   cboDatabases.SelectedItem.ToString())
End Sub

10. Double-click the btnExecute control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
      dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
     "Query Execution Error")
  End Try
End Sub

11. Build the project. You now have a remoting client ready to use.

12. Set StepByStep3-5, the CAO remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Client Activation mode.

13. Now, set StepByStep3-6, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Select a database from the combo box and click the Select button. An instance of the remotable object, DbConnect, is created with the selected database. Now, enter a query in the text box and click the button. The code invokes a method on the remote object. The code binds the results from the remote method to the DataGrid control, as shown in Figure 3.10.

Figure 3.10 A CAO client allows database selection by taking advantage of the capability to call various constructors.

14. Now, again select Debug, Start Without Debugging to run one more instance of the remoting client. Select a different database from the combo box and click the Select button. An instance of the remotable object, DbConnect, is created with the selected database. Now, enter a query in the text box and click the button. You should see that the second instance of the client fetches the data from the selected database. Now switch to the Server command window. You see that the remote object is instantiated twice with different databases, as shown in Figure 3.11. This shows that the client activation creates an instance of remotable object for each client.

Figure 3.11 The client-activated remoting server creates a new instance of the remote object for each client.

REVIEW BREAK

• To create a remotable class, inherit the remotable class from the MarshalByRefObject class.

• A remotable class is usually connected with the remoting framework through a separate server program. The server program listens to the client request on a specified channel and instantiates the remote object or invokes calls on it as required.

• You should create a server channel that listens on a given port number and register the channel with the remoting framework before you register the remotable class.

• The type of channel registered by the client must be compatible with the type of channel used by the server for receiving messages.

• You do not specify a port number when you register the client channel. The port number is instead specified at the time of registering the remote class in the client's domain.

• To register SAO objects on the server side, you call the RemotingConfiguration.RegisterWellKnownServiceType() method; and to register SAO objects on the client side, you call the RemotingConfiguration.RegisterWellKnownClientType() method.

• To register CAO objects on the server side, you call the RemotingConfiguration.RegisterActivatedServiceType() method; and to register CAO objects on the client side, you call the RemotingConfiguration.RegisterActivatedClientType() method.

• You can only instantiate SAO objects at the client side using their default constructors, whereas you can instantiate CAO using any of the object's constructors.

Using Configuration Files to Configure the Remoting Framework

In all the examples so far, I have written code to register the channel and remote object with the remoting framework. This approach to specifying settings is also known as programmatic configuration. Although this approach works fine, there is a drawback—every time you decide to make any change in how the channel or the remote objects are registered, you'll have to recompile the code to see the effect of the changes.

Alternatively, you can store the remoting settings in an XML-based configuration file instead of in the code file. Any changes made to the configuration file can be automatically picked up by the program when it executes the next time. You need not recompile the sources. This approach of specifying configuration settings is also known as declarative configuration.

Declarative configuration can be specified at two levels:

• Machine-Level—The configuration settings at the machine-level can be specified through the machine.config file. The machine.config file is present in the CONFIG subdirectory of the .NET Framework installation (typically, Microsoft.NET\Framework\v1.0.3705\CONFIG in the Windows directory of your computer, if you're running version 1.0 of the .NET Framework). Any settings specified in this file apply to all the .NET applications running on the machine.

• Application-Level—The configuration settings for a specific application can be specified through the application configuration file. In a Windows application, the name of the application-level configuration file includes the full application name and the extension, with .config appended to that extension. For example, the configuration file name for StepByStep3-1.exe is StepByStep3-1.exe.config. In an ASP.NET application, the name of the configuration file is web.config.

The general format of a configuration file is a follows:

 <configuration>
 <system.runtime.remoting>
 <application>
  <lifetime>
   <!-- Use this section to specify the -->
   <!-- lifetime information for all  -->
   <!-- the objects in the application. -->
  </lifetime>
  <service>
   <!-- Use this section to specify how a remote-->
   <!-- object is exposed by the remoting server-->
   <!-- Use the <wellknown> tag to configure a -->
   <!-- a SAO while use the <activated> tag   -->
   <!-- to configure a CAO           -->
   <wellknown />
   <activated />
  </service>
  <client>
   <!-- Use this section to specify how a remote-->
   <!-- object is consumed by the client    -->
   <!-- Use the <wellknown> tag to configure a -->
   <!-- a call to SAO while use the <activated> -->
   <!-- tag to configure a call to CAO     -->
   <wellknown />
   <activated />
  </client>
  <channels>
   <!-- Use this section to configure the    -->
   <!-- channels that the application uses   -->
   <!-- to communicate with the remote objects -->
  </channels>
 </application>
</system.runtime.remoting>
</configuration>

Server-Side Configuration

In Step By Step 3.7, I'll create a remoting server with Singleton activation mode similar to the one created in Step By Step 3.4. However, you'll note that the code itself is reduced because most of the configuration-related code is now moved to a separate configuration file.

STEP BY STEP 3.7: Registering a Server-Activated Object in the Singleton Activation Mode Using Configuration Files

1. Add a new Visual Basic .NET Console application named StepByStep3-7 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

3. In the Solution Explorer, right-click project StepByStep3-7 and select Add, Add New Item from the context menu. Add an Item named StepByStep3-7.exe.config based on the XML File template.

4. Open the StepByStep3-7.exe.config file and modify it to contain the following code:

<configuration>
  <system.runtime.remoting>
    <application>
      <service>
        <!-- Set the activation mode,
        remotable object and its URL -->
        <wellknown mode="Singleton"
          type=
       "StepByStep3_1.DbConnect, StepByStep3-1"
          objectUri="DbConnect" />
      </service>
      <channels>
        <!-- Set the channel and port -->
        <channel ref="tcp server"
          port="1234" />
      </channels>
    </application>
  </system.runtime.remoting>
</configuration>

5. In the Solution Explorer, select the project and click the Show All Files button in the toolbar. Move the StepByStep3-7.exe.config file from the project folder to the bin folder under the project, where the StepByStep3-7.exe file will be created when the project is compiled.

6. In the Solution Explorer, rename the default Module1.vb to DbConnectSingletonServer.vb. Open the file and change the name of the module to DbConnectSingletonServer in the module declaration. Set the renamed module as the startup object for the project.

7. Add the following directive:

Imports System.Runtime.Remoting

8. Add the following code in the Main() method:

Sub Main()
  ' Load remoting configuration
  RemotingConfiguration.Configure( _
   "StepByStep3-7.exe.config")
  Console.WriteLine("Started server in the " & _
  "Singleton mode")
  Console.WriteLine("Press <ENTER> to terminate " & _
   "server...")
  Console.ReadLine()
End Sub

9. Build the project. This step creates a remoting server capable of registering the StepByStep3-1.DbConnect class for remote invocation using the Singleton activation mode via its settings in the configuration file.

10. Set StepByStep3-7, the remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Singleton mode by configuring from its settings in the configuration file.

11. Now, set StepByStep3-3, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Enter a query in the text box and click the button. The code invokes a method on the remote object, which is created when the form loads. The code binds the results from the remote method to the DataGrid control.

The most important thing to note in Step By Step 3.7 is the way I have written the <service> and the <channels> elements in the configuration file.

The <service> element is written as

<service>
  <!-- Set the activation mode,
     remotable object and its URL -->
  <wellknown mode="Singleton"
        type=
       "StepByStep3_1.DbConnect, StepByStep3-1"
       objectUri="DbConnect" />
</service>

For an SAO, you need to use the <wellknown> element in which you specify the activation mode, type, and the object URI. The type attribute is specified as a pair of qualified class names (StepByStep3_1.DbConnect) and the name of the assembly (StepByStep3-1). The objectUri attribute specifies the endpoint of the URI where the client program will attempt to connect.

The <channels> element can be used to specify the channels used by the server to expose the remotable class. The ref attribute specifies the ID of the channel you want to use. The value ref="tcp server" specifies that the channel is a TCP server channel. If I instead write ref="tcp", the channel becomes a receiver-sender channel.

<channels>
  <!-- Set the channel and port -->
  <channel ref="tcp server" port="1234" />
</channels>

With the use of configuration files, the remoting code inside the Main() method of the server is now just one statement:

' Load remoting configuration
RemotingConfiguration.Configure( _
 "StepByStep3-7.exe.config")

The Configure() method of the RemotingConfiguration class loads the configuration file into memory, parses its contents to locate the <system.runtime.remoting> section, and—based on the settings—calls the relevant methods to register the channels and the remoting objects.

Client-Side Configuration

The configuration of a remoting client is quite similar to that of a remoting server. However, you'll configure the <client> element of the configuration file instead of the <service> element.

Step By Step 3.8 demonstrates how to use client-side configuration files.

STEP BY STEP 3.8: Instantiating and Invoking a Server-Activated Object

1. Add a new Visual Basic .NET Windows Application named StepByStep3-8 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

3. In the Solution Explorer, right-click the project StepByStep3-8 and select Add, Add New Item from the context menu. Add an Item named StepByStep3-8.exe.config based on the XML File template.

4. Open the StepByStep3-8.exe.config file and modify it to contain the following code:

<configuration>
  <system.runtime.remoting>
    <application>
      <client>
        <!-- Set the remotable object
        and its URL -->
        <wellknown
         type=
       "StepByStep3_1.DbConnect, StepByStep3-1"
         url="tcp://localhost:1234/DbConnect"
        />
      </client>
    </application>
  </system.runtime.remoting>
</configuration>

5. In the Solution Explorer, select the project and click the Show All Files button in the toolbar. Move the StepByStep3-8.exe.config file from the project folder to the bin folder under the project, where the StepByStep3-8.exe file will be created when the project is compiled.

6. In the Solution Explorer, delete the default Form1.vb. Add a new form named DbConnectClient.vb. Set the new form as the startup object for the project.

7. Place two GroupBox controls (grpQuery and grpResults), a TextBox control (txtQuery), a Button control (btnExecute) and a DataGrid control (dgResults) on the form. Refer to Figure 3.5 for the design of this form.

8. Add the following directives:

Imports System.Runtime.Remoting
Imports StepByStep3_1

9. Add the following code just after the Windows form designer generated code:

' Declare a Remote object
Dim dbc As DbConnect

10. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  ' Load remoting configuration
  RemotingConfiguration.Configure( _
   "StepByStep3-8.exe.config")

  ' Instantiate the remote class
  dbc = New DbConnect()
End Sub

11. Double-click the Button control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
     dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
     "Query Execution Error")
  End Try
End Sub

12. Build the project. Set StepByStep3-7, the remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Singleton mode by configuring from its settings in the configuration file.

13. Now, set StepByStep3-8, the remoting client as the startup project. Select Debug, Start Without Debugging to run the project. Enter a query in the text box and click the button. The code invokes a method on the remote object, which is created from the settings stored in the configuration file. The code binds the results from the remote method to the DataGrid control.

The most important thing to note from this example is the use of the <client> element in the configuration file. The mode attribute is not part of the <wellknown> element of the client, and the objectUri attribute is replaced with the url attribute that specifies the address to connect to.

<client>
  <!-- Set the remotable object
     and its URL -->
  <wellknown
      type=
       "StepByStep3_1.DbConnect, StepByStep3-1"
      url="tcp://localhost:1234/DbConnect"
  />
</client>

You can note that unlike the server configuration file, the <channel> element is not required for the client as it determines the protocol and the port number using the specified URL.

Guided Practice Exercise 3.2

In this exercise, you are required to expose the DbConnect class from Step By Step 3.1 as a CAO through a remoting server using an HTTP channel. You also want to invoke methods on this client-activated object by creating a form similar to the one created in Step By Step 3.6.

However, you should be able to change various parameters, such as the channel protocol, port number, URL, name, and type of the remote object, without the need to recompile the server.

How would you design such a remoting client and server?

This exercise helps you practice creating configuration files for both server and client that remote a CAO. You should try working through this problem on your own first. If you get stuck, or if you'd like to see one possible solution, follow these steps:

1. Add a new Visual Basic .NET Console application named GuidedPracticeExercise3-2_Server to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3_1 (the remotable class assembly).

3. In the Solution Explorer, right-click the project GuidedPracticeExercise3-2_Server and select Add, Add New Item from the context menu. Add an Item named GuidedPracticeExercise3-2_Server.exe.config based on the XML File template.

4. Open the GuidedPracticeExercise3-2_Server.exe.config file and modify it to contain the following code:

<configuration>
  <system.runtime.remoting>
    <application>
      <service>
        <!-- Set the remotable object -->
        <activated
          type=
       "StepByStep3_1.DbConnect, StepByStep3-1"
         />
      </service>
      <channels>
        <channel ref="http" port="1234" />
      </channels>
    </application>
  </system.runtime.remoting>
</configuration>

5. In the Solution Explorer, select the project and click the Show All Files button in the toolbar. Move the GuidedPracticeExercise3-2_Server.exe.config file from the project folder to the bin folder under the project, where the GuidedPracticeExercise3-2_Server.exe file will be created when the project is compiled.

6. In the Solution Explorer, rename the default Module1.vb to DbConnectCAOServer.vb. Open the file and change the name of the module to DbConnectCAOServer in the module declaration. Set the module as the Startup object for the project.

7. Add the following directive:

Imports System.Runtime.Remoting

8. Add the following code in the Main() method:

Sub Main()
  ' Load remoting configuration
  RemotingConfiguration.Configure( _
   "GuidedPracticeExercise3-2_Server.exe.config")

  Console.WriteLine( _
   "Started server in the Client Activation mode")
  Console.WriteLine( _
   "Press <ENTER> to terminate server...")
  Console.ReadLine()
End Sub

9. Build the project. This step creates a remoting server capable of registering the StepByStep3_1.DbConnect class for remote invocation using the client activation mode via its settings in the configuration file.

10. Add a new Visual Basic .NET Windows Application named GuidedPracticeExercise3-2_Client to the solution.

11. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-1 (the remotable class assembly).

12. In the Solution Explorer, right-click the project GuidedPracticeExercise3-2_Client and select Add, Add New Item from the context menu. Add an Item named GuidedPracticeExercise3-2_Client.exe.config based on the XML File template.

13. Open the GuidedPracticeExercise3-2_Client.exe.config file and modify it to contain the following code:

<configuration>
  <system.runtime.remoting>
    <application>
      <!-- Set the url for the
       client activation -->
      <client url="http://localhost:1234">
        <!-- Set the remote object
        type and its assembly -->
        <activated
         type=
       "StepByStep3_1.DbConnect, StepByStep3-1"
        />
      </client>
    </application>
  </system.runtime.remoting>
</configuration>

14. In the Solution Explorer, select the project and click the Show All Files button in the toolbar. Move the GuidedPracticeExercise3-2_Client.exe.config file from the project folder to the bin\Debug folder under the project, where the GuidedPracticeExercise3-2_Client.exe file will be created when the project is compiled.

15. In the Solution Explorer, remove the default Form1.vb. Add a new form named DbConnectClient.vb. Set the new form as the startup object for the project.

16. Place three GroupBox controls (grpDatabases, grpQuery and grpResults), a ComboBox control (cboDatabases), a TextBox control (txtQuery), two Button controls (btnSelect and btnExecute), and a DataGrid control (dgResults) on the form. Set the Multiline property of the txtQuery control to True. Refer to Figure 3.10 for the design of this form.

17. Select the Items property of the cboDatabases control in the Properties window and click on the (...) button. This opens the String Collection Editor dialog box. Enter the following names of databases in the editor:

    Northwind
    Pubs

    Click OK to add the databases to the Items collection of the cboDatabases control.

18. Add the following directives to the form's module:

Imports System.Runtime.Remoting
Imports StepByStep3_1

19. Add the following code directly after the Windows Forms designer generated code:

' Declare a Remote object
Dim dbc As DbConnect

20. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  cboDatabases.SelectedIndex =
  grpQuery.Enabled = False
End Sub

21. Double-click the btnSelect control and add the following code in the Click event handler:

Private Sub btnSelect_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnSelect.Click
  grpDatabases.Enabled = False
  grpQuery.Enabled = True

  ' Load remoting configuration
  RemotingConfiguration.Configure( _
   "GuidedPracticeExercise3-2_Client.exe.config")

  ' Instantiate the remote class
  dbc = New DbConnect( _
   cboDatabases.SelectedItem.ToString())
End Sub

22. Double-click the btnExecute control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
     dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
    "Query Execution Error")
  End Try
End Sub

23. Build the project. Set the GuidedPracticeExercise3-2_Server, the remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Client Activation mode by configuring from its settings in the configuration file.

24. Now, set GuidedPracticeExercise3-2_Client, the remoting client as the startup project. Select Debug, Start Without Debugging to run the project. Select a database from the combo box. Enter a query in the text box and click the button. The code invokes a method on the remote object, which is created from the settings stored in the configuration file. The code binds the results from the remote method to the DataGrid control.

If you have difficulty following this exercise, review the sections "Creating a Client-Activated Object" and "Using Configuration Files to Configure the Remoting Framework" earlier in this chapter. After doing that review, try this exercise again.

Using Interface Assemblies to Compile Remoting Clients

So far, in all the examples, you had to copy the assembly containing the remotable class to the client project (which happens automatically when you set a reference to the assembly) in order for the client to work. However, this is not a desirable solution in many cases because you might not want to share the implementation of the remotable object with your customers or business partners who are writing the client application.

An advisable solution in that case is to only share the interface of the remotable class with the client application. An interface does not contain any implementation; instead, it just contains the members that are supplied by the class.

In the following sections, I'll demonstrate how you can create interfaces that will allow you to create a remote SAO and CAO without sharing the implementation. I'll also introduce the Soapsuds tool, which can help you in automatically creating the interfaces for a remotable class.

Creating an Interface Assembly

In this section, I'll create an assembly that implements an interface named IDbConnect class. The interface defines a contract. All the classes or structs that implement the interface must adhere to this contract.

STEP BY STEP 3.9: Creating an Interface Assembly

1. Add a new Visual Basic .NET Class library named StepByStep3-9 to the solution.

2. In the Solution Explorer, rename the default Class1.vbs to IDbConnect.vb.

3. Open the IDbConnect.vb and replace the code with the following code:

Imports System
Imports System.Data

Public Interface IDbConnect
  Function ExecuteQuery(ByVal strQuery As String) _
   As DataSet
End Interface

4. Build the project. This step creates an assembly that contains the definition of the IDbConnect interface.

Creating a Remotable Object That Implements an Interface

Now that you created the interface IDbConnect, you'll implement this interface in a class named DbConnect. You must ensure that the class DbConnect adheres to the contract defined by the IDbConnect interface. This means that you must implement the method ExecuteQuery() and that the data type of parameters and the return value must match exactly with that defined in the interface.

Step By Step 3.10 creates the class DbConnect that adheres to the IDbConnect interface.

STEP BY STEP 3.10: Creating a Remotable Object That Implements an Interface Assembly

1. Add a new Visual Basic .NET Class library named StepByStep3-10 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-9 (the interface assembly).

3. In the Solution Explorer, rename the default Class1.vbs to DbConnect.vb.

4. Open the DbConnect.vb and replace the code with the following code:

Imports System
Imports System.Data
Imports System.Data.SqlClient
Imports StepByStep3_9

' Marshal-by-Reference Remotable Object
Public Class DbConnect
  Inherits MarshalByRefObject
  ' Implement the IDbConnect interface
  Implements IDbConnect

  Private sqlconn As SqlConnection

  ' Default constructor connects to the Northwind
  ' database
  Public Sub New()
    sqlconn = New SqlConnection( _
     "data source=(local);" & _
     "initial catalog=Northwind;" & _
     "integrated security=SSPI")
    Console.WriteLine( _
     "Created a new connection " & _
     "to the Northwind database")
  End Sub

  ' Parameterized constructor connects to the
  ' specified database
  Public Sub New(ByVal DbName As String)
    sqlconn = New SqlConnection( _
     "data source=(local);" & _
     "initial catalog=" & DbName & ";" & _
     "integrated security=SSPI")
    Console.WriteLine( _
     "Created a new connection " & _
     "to the " & DbName & " database")
  End Sub

  Public Function ExecuteQuery( _
   ByVal strQuery As String) As DataSet _
   Implements IDbConnect.ExecuteQuery
    Console.Write("Starting to execute " & _
     "the query...")
    ' Create a SqlCommand to represent the query
    Dim sqlcmd As SqlCommand = _
     sqlconn.CreateCommand()
    sqlcmd.CommandType = CommandType.Text
    sqlcmd.CommandText = strQuery

    ' Create a SqlDataAdapter object
    ' to talk to the database
    Dim sqlda As SqlDataAdapter = _
     New SqlDataAdapter()
    sqlda.SelectCommand = sqlcmd
    ' Create a DataSet to hold the results
    Dim ds As DataSet = New DataSet()
    Try
      ' Fill the DataSet
      sqlda.Fill(ds, "Results")
    Catch ex As Exception
      Console.WriteLine(ex.Message, _
       "Error executing query")
    End Try
    Console.WriteLine("Done.")
    ExecuteQuery = ds
  End Function
End CLass

5. Build the project. This step creates StepByStep3_10.DbConnect, a remotable class that implements the IDbConnect interface.

This program is similar to the one created in Step By Step 3.4, with the difference being that the DbConnect class is also implementing the IDbConnect interface in addition to deriving from the MarshalByRefObject class:

' Marshal-by-Reference Remotable Object
Public Class DbConnect
  Inherits MarshalByRefObject
  ' Implement the IDbConnect interface
  Implements IDbConnect

You can expose this remotable object to the clients via the remoting framework using the techniques you already know. Step By Step 3.11 creates a remoting server that registers the DbConnect class created in Step By Step 3.10 as an SAO in Singleton mode.

STEP BY STEP 3.11: Creating a Remoting Server to Register the Remotable Object that Implements an Interface Assembly

1. Add a new Visual Basic .NET Console application named StepByStep3-11 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting, the project StepByStep3-9 (the interface assembly), and StepByStep3-10 (the remotable class assembly).

3. In the Solution Explorer, rename the default Module1.vb to DbConnectSingletonServer.vb. Open the file and change the name of the module to DbConnectSingletonServer in the module declaration. Set the module to be the Startup object of the project.

4. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp

5. Add the following code in the Main() method:

Sub Main()
  ' Register a TCP server channel that
  ' listens on port
  Dim channel As TcpServerChannel = _
   New TcpServerChannel(1234)
  ChannelServices.RegisterChannel(channel)

  ' Register the service that publishes
  ' DbConnect for remote access in Singleton mode
  RemotingConfiguration. _
   RegisterWellKnownServiceType( _
   GetType(StepByStep3_10.DbConnect), "DbConnect", _
   WellKnownObjectMode.Singleton)
  Console.WriteLine("Started server in the " & _
   "Singleton mode")
  Console.WriteLine("Press <ENTER> to terminate " & _
   "server...")
  Console.ReadLine()
End Sub

6. Build the project. This step creates a remoting server capable of registering StepByStep3_10.DbConnect, the remotable object that implements the StepByStep3_9.IDbConnect interface for remote invocation using the Singleton activation mode.

Creating a Remoting Client That Uses an Interface Instead of the Implementation

When the remotable class is implementing an interface, you can just include the reference to the interface assembly instead of the implementation assembly at the client side. The client will then extract the necessary type information and metadata for compiling and running the program from the interface. Step By Step 3.12 shows you how to create such a client.

STEP BY STEP 3.12: Creating a Remoting Client to Invoke the Remotable Object That Implements an Interface Assembly

1. Add a new Visual Basic .NET Windows Application named StepByStep3-12 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-9 (the interface assembly).

3. In the Solution Explorer, delete the default Form1.vb. Add a new form named DbConnectClient.vb, and set it as the startup object for the project.

4. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp
Imports StepByStep3_9

5. Place two GroupBox controls (grpQuery and grpResults), a TextBox control (txtQuery), a Button control (btnExecute), and a DataGrid control (dgResults) on the form. Set the Multiline property of txtQuery to True. Refer to Figure 3.5 for the design of this form.

6. Add the following code directly after the Windows Form designer generated code:

' Declare a Remote object
Dim dbc As IDbConnect

7. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  ' Register a TCP client channel
  Dim channel As TcpClientChannel = _
   New TcpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' Instantiate the remote class
  dbc = CType( _
   Activator.GetObject(GetType(IDbConnect), _
   "tcp://localhost:1234/DbConnect"), IDbConnect)
End Sub

8. Double-click the Button control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
     dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
     "Query Execution Error")
  End Try
End Sub

9. Build the project. Set the StepByStep3-11, the remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Singleton mode.

10. Now, set StepByStep3-12, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Enter a query in the text box and click the button. The code invokes a method on the remote object, which is created when the form loads. The code binds the results from the remote method to the DataGrid control.

In the preceding program, you add the reference to StepByStep3-9.dll. This assembly contains the interface IDbConnect. However, an interface variable cannot be instantiated, and, therefore, you cannot create the instance of the remote object by using the new operator.

But there is an alternative way of creating instances of remote objects, which is especially helpful in the case of interfaces. This is by using the methods of Activator class as described here:

• The Activator.GetObject() Method—Calls the proxy to send messages to the remote object. No messages are sent over the network until a method is called on the proxy. This method is useful for activating server-activated objects.

• The Activator.CreateInstance() Method—Creates an instance of the specified type using the constructor that best matches the specified parameters. This method is useful for activating client-activated objects.

In Step By Step 3.12, I use the Activator.GetObject() method to create a proxy for the server-activated object indicated by the type IDbConnect and the specified server URL.

Using the Soapsuds Tool to Automatically Generate an Interface Assembly

In the previous section, you learned how you can distribute interface assemblies to the client instead of the implementation assembly. Clients will still be able to instantiate the remote objects in this case.

However, in case of large numbers of clients, distribution of interface files to each of them can become another big issue. However, the .NET Framework SDK provides you the soapsuds tool (soapsuds.exe) to overcome this issue.

Given the URL for a remotable object, the soapsuds tool can automatically generate an interface assembly for the remote object. So all the client needs to know is the URL of the remotable object, and they can generate the interface assembly on their own by using the soapsuds tool. A typical usage of the soapsuds tool is

soapsuds –NoWrappedProxy -url:http://MyServer.com/DbConnect?wsdl 
-outputAssemblyFile:DbConnectInterface.dll

In this command line, the url switch specifies the URL of the remote object. You normally have to append ?wsdl to the URL to allow soapsuds to generate the metadata from the URL. The outputAssemblyFile switch specifies the name of the file in which you want the output assembly to be created. The NoWrappedProxy (or nowp) switch instructs the soapsuds tool to generate an unwrapped proxy.

By default, the soapsuds tool generates wrapped proxies—proxies that store various connection details, such as the channel formatting and the URL within the proxy itself. Although this means that you need not write the code for these things yourself, it's not recommended because it reduces the flexibility to change the configuration. If you want to avoid specifying these details in the code, a better way is to use the configuration files.

Step By Step 3.13 creates a client that uses the soapsuds generated unwrapped proxy to connect with the HTTP server created in the Guided Practice Exercise 3.1.

STEP BY STEP 3.13: Using the Soapsuds Tool to Automatically Generate an Interface Assembly

1. Add a new Visual Basic .NET Windows Application to your solution. Name it StepByStep3-13.

2. Set the GuidedPracticeExercise3-1_Server as the startup project. Select Debug, Start Without Debugging to run the project.

3. Select Start, Programs, Microsoft Visual Studio .NET, Visual Studio .NET Tools, Visual Studio .NET Command Prompt to launch a .NET command prompt.

4. Make sure that the GuidedPracticeExercise3-1_Server is running. Navigate to the bin folder of the StepByStep3-13 project and run the following command to automatically generate an interface assembly for the remotable object DbConnect, registered by the remoting server, GuidedPracticeExercise3-1_Server:

soapsuds -url:http://localhost:1234/DbConnect?wsdl 
-oa:DbConnectInterface.dll -nowp

5. In Visual Studio .NET, add references to the .NET assembly System.Runtime.Remoting and DbConnectInterface.dll (the interface assembly auto generated by the soapsuds.exe in step 4).

6. In the Solution Explorer, delete the default Form1.vb. Add a new form named DbConnectClient.vb and set it as the Startup object for the project.

7. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Http
Imports StepByStep3_1

8. Place two GroupBox controls (grpQuery and grpResults), a TextBox control (txtQuery), a Button control (btnExecute), and a DataGrid control (dgResults) on the form. Set the Multiline property of txtQuery to True. Refer to Figure 3.5 for the design of this form.

9. Add the following code directly after the Windows Form designer generated code:

' Declare a Remote object
Dim dbc As DbConnect

10. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  ' This step is not required if you use
  ' a wrapped proxy
  ' Register a HTTP client channel
  Dim channel As HttpClientChannel = _
   New HttpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' This step is not required if you use
  ' a wrapped proxy.
  ' Register the remote class as a valid
  ' type in the client's application domain
  RemotingConfiguration. _
   RegisterWellKnownClientType( _
   GetType(DbConnect), _
  "http://localhost:1234/DbConnect")

  ' Instantiate the remote class
  dbc = New DbConnect()
End Sub

11. Double-click the Button control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
     dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
     "Query Execution Error")
  End Try
End Sub

12. Build the project. If the GuidedPracticeExercise3-1_Server project is not running, set the GuidedPracticeExercise3-1_Server as the startup project and select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Singleton mode.

13. Now, set StepByStep3_13, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Enter a query in the text box and click the button. The code invokes a method on the remote object, which is created when the form loads. The code binds the results from the remote method to the DataGrid control.

In Step By Step 3.13, you learned how to use the soapsuds tool to automatically generate the interface class for the remotable object.

Also, the interface class generated by the soapsuds tool allows you to directly use the name of remotable class. This means that you can access the remotable class directly in the client as if you have a direct reference to the remotable class at the client side. For this reason, I can use the RegisterWellKnownClientType() method of the RemotingConfiguration class to register the remote class on the client side instead of using the Activator.GetObject() method.

Creating an Interface Assembly That Works with the Client-Activated Objects

Neither of the techniques for generating interface assemblies discussed so far (in the sections, "Creating an Interface Assembly" and "Using the Soapsuds Tool to Automatically Generate an Interface Assembly") is useful for CAO.

The problem lies with the constructors. CAO has capabilities of invoking even the non-default constructors of the remotable class. However, an interface cannot contain the declaration for a constructor as it does with a method or a property.

This common problem is generally solved using the following steps:

1. Create an interface and a remotable class as shown in the previous examples. For easy reference, I'll call them IDbConnect and DbConnect, respectively. IDbConnect contains definitions of all the methods that the DbConnect class wants to be exposed to the clients.

2. Create an interface that declares as many different methods as there are constructors in the remotable class. Each of these methods is responsible for creating an object in a way defined by its corresponding constructor. This technique is also called an abstract factory pattern. The name contains the word "factory" because it allows you to create objects in different ways. Let's call this interface IDbConnectFactory.

3. Create a class that derives from the MarshalByRefObject class and implements the interface created in step 2 (IDbConnectFactory). Implement all methods to actually create the remotable object (DbConnect) based on the given parameters and return the created instance. Let's call this class DbConnectFactory.

4. Create a remoting server that registers the class created in step 3 (DbConnectFactory) as the remotable class.

5. Create a client that connects to the server and create an instance of the remotable class created in step 3 (DbConnectFactory).

6. Invoke the methods corresponding to a constructor on the remotable object created in step 5. The return value of the constructor will be a remotable object of type defined in step 1 (DbConnect).

7. You now have an object of a type that you originally wanted to remote (DbConnect). You can use this object to invoke methods.

In this section, I'll show you how to implement the preceding technique to create a client-activated object that enables the creation of objects using any of its available constructors.

I'll use the same remotable class that I defined in Step By Step 3.10 (DbConnect) and its interface (IDbConnect) that I defined in Step By Step 3.9. So I'll directly start with step 2 of the preceding list by creating an IDbConnectFactory interface in Step By Step 3.14.

STEP BY STEP 3.14: Creating an Assembly That Works As an Abstract Factory for the IDbConnect Object

1. Add a new Visual Basic .NET Class library named StepByStep3-14 to the solution.

2. Add a reference to the project StepByStep3-9 (the interface assembly).

3. In the Solution Explorer, rename the default Class1.vb to IDbConnectFactory.vb.

4. Open the IDbConnectFactory.vb and replace the code with the following code:

Imports System
Imports System.Data
Imports StepByStep3_9

Public Interface IDbConnectFactory
  Function CreateDbConnectInstance() As IDbConnect
  Function CreateDbConnectInstance( _
   ByVal dbName As String) As IDbConnect
End Interface

5. Build the project. The class library now contains an interface to a class that can act as a factory of objects implementing the IDbConnect interface.

The next step is to create a class that implements the IDbConnectFactory interface and then exposes that class as a remotable class through the remoting framework. Step By Step 3.15 shows how to do so.

STEP BY STEP 3.15: Creating a Remoting Server That Exposes DbConnectFactory As a Remotable Class

1. Add a new Visual Basic .NET Console application named StepByStep3-15 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting, the projects StepByStep3-9 (the interface assembly containing IDbConnect), StepByStep3-14 (the new interface assembly containing the IDbConnectFactory), and StepByStep3-10 (the remotable class assembly).

3. Add a new class to the project that will create another remotable object that inherits from MarshalByRefObject class and implements the IDbConnectFactory interface, created in Step By Step 3.14. Name it DbConnectFactory.vb. Open the DbConnectFactory.vb class and replace the code with the following code:

Imports System
Imports StepByStep3_9
Imports StepByStep3_10
Imports StepByStep3_14

Public Class DbConnectFactory
  Inherits MarshalByRefObject
  Implements IDbConnectFactory

  Public Function CreateDbConnectInstance() _
   As IDbConnect Implements _
   IDbConnectFactory.CreateDbConnectInstance
    Return New DbConnect()
  End Function

  Public Function CreateDbConnectInstance( _
   ByVal dbName As String) As IDbConnect _
   Implements IDbConnectFactory. _
   CreateDbConnectInstance
    Return New DbConnect(dbName)
  End Function
End Class

4. In the Solution Explorer, rename the default Module1.vb as DbConnectFactoryServer.vb. Open the file and change the name of the module to DbConnectFactoryServer in the module declaration. Set this module to be the Startup object of the project.

5. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp

6. Add the following code in the Main() method:

Sub Main()
  ' Register a TCP server channel that
  ' listens on port
  Dim channel As TcpServerChannel = _
   New TcpServerChannel(1234)
  ChannelServices.RegisterChannel(channel)

  RemotingConfiguration. _
   RegisterWellKnownServiceType( _
   GetType(DbConnectFactory), _
   "DbConnectFactory", _
   WellKnownObjectMode.Singleton)
  Console.WriteLine("Started server in the " & _
   "client activated mode")
  Console.WriteLine("Press <ENTER> to terminate " & _
   "server...")
  Console.ReadLine()
End Sub

7. Build the project. This step creates a remoting server capable of registering StepByStep3_10.DbConnect, the remotable object that implements the StepByStep3_9.IDbConnect interface, for remote invocation using the Singleton activation mode.

Step By Step 3.15 exposes the DbConnectFactory as an SAO in Singleton mode. Although the DbConnectFactory object is registered as an SAO, the objects returned by various methods of DbConnectFactory are CAO because they are created only on the explicit request from the client.

Step By Step 3.16 is the final step for creating CAO using the abstract factory pattern.

STEP BY STEP 3.16: Instantiating and Invoking a Client-Activated Object Using the Abstract Factory Pattern

1. Add a new Visual Basic .NET Windows Application named StepByStep3-16 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the projects StepByStep3-9 (the IDbConnect interface assembly) and StepByStep3-14 (the IDbConnectFactory interface assembly).

3. In the Solution Explorer, delete the default Form1.vb. Add a new form and name it DbConnectClient.vb. Set the new form as the startup object for the project.

4. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Tcp
Imports StepByStep3_9 ' contains IDbConnect
Imports StepByStep3_14 ' contains IDbConnectFactory

5. Place three GroupBox controls (grpDatabases, grpQuery and grpResults), a ComboBox control (cboDatabases), a TextBox control (txtQuery), two Button controls (btnSelect and btnExecute), and a DataGrid control (dgResults) on the form. Set the Multiline property of txtQuery to True. Refer to Figure 3.10 for the design of this form.

6. Select the Items property of the cboDatabases control in the Properties window and click on the (...) button. This opens String Collection Editor dialog box. Enter the following names of databases in the editor:

   Northwind
   Pubs

   Click OK to add the databases to the Items collection of the cboDatabases control.

7. Add the following code directly after the Windows Form designer generated code:

' Declare a Remote object
Dim dbc As IDbConnect

8. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  cboDatabases.SelectedIndex =
  grpQuery.Enabled = False
End Sub

9. Double-click the btnSelect control and add the following code in the Click event handler:

Private Sub btnSelect_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnSelect.Click
  ' Disable the Databases group box and
  ' Enable the Query group box
  grpDatabases.Enabled = False
  grpQuery.Enabled = True

  ' Register a TCP client channel
  Dim channel As TcpClientChannel = _
   New TcpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' Register the remote class as a valid
  ' type in the client's application domain
  ' by passing the Remote class and its URL
  Dim dbcf As IDbConnectFactory = _
   CType(Activator.GetObject(GetType(IDbConnect), _
   "tcp://localhost:1234/DbConnectFactory"), _
   IDbConnectFactory)

  dbc = dbcf.CreateDbConnectInstance _
   (cboDatabases.SelectedItem.ToString())
End Sub

10. Double-click the btnExecute control and add the following code in the Click event handler:

Private Sub btnExecute_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnExecute.Click
  Try
    ' Invoke a method on the remote object
    Me.dgResults.DataSource = _
     dbc.ExecuteQuery(Me.txtQuery.Text)
    dgResults.DataMember = "Results"
  Catch ex As Exception
    MessageBox.Show(ex.Message, _
     "Query Execution Error")
  End Try
End Sub

11. Build the project. This step creates a remoting client that is capable of activating DbConnect as a CAO.

12. Set the StepByStep3-15, the remoting server, as the startup project. Select Debug, Start Without Debugging to run the project. You should see a command window displaying a message that the server is started in the Client Activation mode.

13. Now, set StepByStep3-16, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Select a database from the combo box and click the Select button. An instance of the remotable object, DbConnect, is created with the selected database. Now, enter a query in the text box and click the button. The code invokes a method on the remote object. The code binds the results from the remote method to the DataGrid control.

14. Now, again select Debug, Start Without Debugging to run the remoting client. Select a different database from the combo box and click the Select button. An instance of the remotable object, DbConnect, is created with the selected database. Now, enter a query in the text box and click the button. You should see that the second instance of the client fetches the data from the selected database. Now switch to the Server command window. You see that the remote object is instantiated twice with different databases. This shows that the client activation creates an instance of remotable object per client.

In Step By Step 3.16, it is important to note that although the object for DbConnectFactory is created as a server-activated object, the DbConnect object is always created as the client-activated object. For the DbConnectFactory object, there will be only one instance for all the clients; however, there will be one DbConnect object for each client.

• The .NET Framework allows you to store remoting configuration details in an XML-based configuration file instead of the code file. This causes any changes in the configuration file to be automatically picked up, rather than recompiling the code files.

• To configure remoting configuration details from configuration files, you should call the RemotingConfiguration.Configure() method and pass the name of the configuration file.

• You can distribute the interface assembly to the client instead of the implementation assembly by creating an interface that defines the contract and exposes the member definitions to the client. The remotable class should implement this interface.

• You can use the soapsuds tool to automatically generate the interface class for the remotable object instead of manually defining the interface. However, the soapsuds tool only works when the HTTP channel is used for communication.

• To create an interface that allows creating client-activated remote objects, you should create an additional interface that declares as many different methods as there are constructors in the remotable class. Each of these methods should be able to create the remote object in a way defined by its corresponding constructor.

Using IIS As an Activation Agent

So far, in this chapter, you are hosting the remotable class by creating your own server that is a console application. A major disadvantage with this approach is that you have to manually start the server if it is not already running.

As discussed before, remoting provides two alternatives to overcome this disadvantage. You can either run the server process as a Windows service instead of a console application, or use IIS (which is a built-in Windows service) as an activation agent for the server process. I'll talk about the former alternative in Chapter 6 and the latter in this section.

Using IIS as an activation agent offers the following advantages:

• You need not write a separate server program to register the remotable classes.

• You need not worry about finding an available port for your server application. You can just host the remotable object, and IIS automatically uses the port 80.

• IIS can provide other functionality such as authentication and secure sockets layer (SSL).

The following list specifies what you need to do in order to host a remotable class in IIS:

• Place the assembly containing the remotable objects into the \bin directory of an IIS Web application or place the assembly in the GAC (Global Assembly Cache) on the IIS computer.

• Configure the remoting settings by placing the <system.runtime.remoting> configuration section into the web.Config file for the Web application. Alternatively, you can write the configuration code in the Application_Start() method of the global.asax file in the same way you would register a remote object in an .exe host.

• You should not specify a channel. IIS already listens on port 80. Specifying a port for a channel causes exceptions to be thrown when new IIS worker processes are started.

• The well-known object URIs must end with ".rem" or ".soap" because these are the two extensions, which are registered with both IIS (via the aspnet_isapi.dll) as well as the remoting system (in machine.config).

Step By Step 3.17 demonstrates how to use IIS for activating the DbConnect remotable class from Step By Step 3.10.

STEP BY STEP 3.17: Using IIS As an Activation Agent

1. Add a new Empty Web Project named StepByStep3-17 to the solution.

2. Add references to the project StepByStep3-9 (the interface assembly) and StepByStep3-10 (the remotable object) by selecting Add Reference from the context menu.

3. Add a Web Configuration File to the project. Open the web.config file and add the following <system.runtime.remoting> element inside the <configuration> element:

<configuration>
  <system.runtime.remoting>
    <application>
      <service>
      <!-- Set the activation mode,
        remotable object and its URL -->
        <wellknown mode="Singleton"
          type=
    "StepByStep3_10.DbConnect, StepByStep3-10"
          objectUri="DbConnect.rem"
        />
      </service>
    </application>
  </system.runtime.remoting> 
...
</configuration>

4. IIS is now hosting StepByStep3_10.DbConnect, the remotable class, as a server activated object using the Singleton activation mode.

If you note the objectUri of the SAO in the preceding example, it ends with the extension .rem.

Step By Step 3.18 demonstrates how to invoke a remote object hosted by IIS.

STEP BY STEP 3.18: Instantiating and Invoking an IIS-Hosted Remote Object

1. Add a new Visual Basic .NET Windows Application named StepByStep3-18 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting and the project StepByStep3-9 (the interface assembly).

3. In the Solution Explorer, delete the default Form1.vb. Add a new form named DbConnectClient.vb. Set the new form as the startup object for the project.

4. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Http
Imports StepByStep3_9 ' contains IDbConnect

5. Place two GroupBox controls, a TextBox control (txtQuery), a Button control (btnExecute), and a DataGrid control (dgResults) on the form. Set the Multiline property of txtQuery to True. Refer to Figure 3.5 for the design of this form.

6. Add the following code directly after the Windows Form designer generated code:

' Declare a Remote object
Dim dbc As IDbConnect

7. Double-click the form and add the following code in the Load event handler:

Private Sub DbConnectClient_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  ' Register a Http client channel
  Dim channel As HttpClientChannel = _
   New HttpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' Instantiate the remote class
  dbc = CType( _
  Activator.GetObject(GetType(IDbConnect), _
  "http://localhost/StepByStep3-17/DbConnect.rem"), _
   IDbConnect)
End Sub

8. Double-click the Button control and add the following code in the Click event handler:

  Private Sub btnExecute_Click( _
   ByVal sender As System.Object, _
   ByVal e As System.EventArgs) _
   Handles btnExecute.Click
    Try
      ' Invoke a method on the remote object
      Me.dgResults.DataSource = _
       dbc.ExecuteQuery(Me.txtQuery.Text)
      dgResults.DataMember = "Results"
    Catch ex As Exception
      MessageBox.Show(ex.Message, _
       "Query Execution Error")
    End Try
End Sub

9. Build the project. This step creates the remoting client that can invoke an IIS-hosted remote object.

10. Set StepByStep3-18, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Enter a query in the text box and click the button. The code invokes a method on the remote object, which is created when the form loads. The code binds the results from the remote method to the DataGrid control.

This client was similar to the client created in Step By Step 3.12; however, there are a few things to note. The client is using the HTTP channel to communicate with the server, no port number is specified in the URL, and the URL ends with .rem.

Asynchronous Remoting

So far, all the method invocations that you studied in this chapter have been synchronous. In a synchronous method call, the thread that executes the method call waits until the called method finishes execution.

Synchronous calls can make the user interface very non-responsive if the client program is waiting for a long process to finish execution. This is especially true for remote method calls in which additional time is involved because the calls are made across the network.

The .NET framework has a solution for this in the form of asynchronous methods. An asynchronous method calls the method and returns immediately, leaving the invoked method to complete its execution.

Understanding the Model of Asynchronous Programming in the .NET Framework

In the .NET Framework, asynchronous programming is implemented with the help of delegate types. Delegates are types that are capable of storing references to methods of a specific signature. A delegate declared as follows is capable of invoking methods that take a string parameter and return a string type:

Delegate Function _
 LongProcessDelegate(param As String) As String

So, if there is a method definition such as

Function LongProcess(Param As String) As String
...
End Function

Then the LongProcessDelegate can hold references to the method like this:

Dim delLongProcess As LongProcessDelegate
delLongProcess = New LongProcessDelegate( _
 AddressOf LongProcess)

Note the use of the AddressOf operator in returning the location of the LongProcess function.

Once you have the delegate object available, you can use its BeginInvoke() method to call the LongProcess() method asynchronously, such as follows:

Dim ar As IAsyncResult = _
 delLongProcess.BeginInvoke("Test", Nothing, Nothing)

The IAsync interface is used to monitor an asynchronous call and relate the beginning and the end of an asynchronous method call. When you use the BeginInvoke() method, the control will immediately come back to the next statement while the LongProcess() method might still be executing.

To return the value of an asynchronous method call, you can call the EndInvoke() method on the same delegate, such as

Dim result As String = delLongProcess.EndInvoke(ar)

However, it is important to know where to place the preceding method call because when you call EndInvoke() and, if the LongProcess() method has not yet completed execution, EndInvoke() will cause the current thread to wait for the completion of LongProcess(). A poor use of EndInvoke(), such as placing it as the very next statement after BeginInvoke(), can potentially cause an asynchronous method call to turn into a synchronous method call.

One of the alternatives to this problem is to use the IsCompleted property of the ISyncResult object to check if the method has completed the execution and call the EndInvoke() method only in such cases.

Dim result As String
If (ar.IsCompleted) Then
  result = delLongProcess.EndInvoke(ar)
End If

But regular polling of the ar.IsCompleted property requires additional work at the client side. In fact, there's a better way to do this in the form of the callback methods. In this technique, you can register a method that is automatically invoked as soon as the remote method finishes execution. You can then place a call to EndInvoke() method inside the callback method to collect the result of method execution.

To implement a callback method with an asynchronous method invocation, you need to take the following steps in the client program:

1. Define a callback method that you want to execute when the remote method has finished execution.

2. Create an object of delegate type AsyncCallback to store the reference to the method created in step 1.

3. Create an instance of an object that can receive a remote call to a method.

4. Declare a delegate type capable of storing references of the remote method.

5. Using the object from step 3, create a new instance of the delegate declared in step 4 to refer to the remote method.

6. Call BeginInvoke on the delegate created in step 5, passing any arguments and the AsyncCallback object.

7. Wait for the server object to call your callback method when the method has completed.

Applying Asynchronous Programming

In the following sections, I'll create a set of three projects to demonstrate the use of callback methods for an asynchronous method call:

• The Remotable Class—The Remotable class exposes the remote methods that are called from the client program.

• The Remote Server—I'll use IIS to host the remotable object.

• The Client Program—The client program calls the remote method asynchronously.

I'll create a remotable class that is different from other remotable classes used in this chapter to help you properly understand synchronous and asynchronous method calls.

The remotable class used in Step By Step 3.19 is named RemotableClass, and it has a single method named LongProcess(). The LongProcess() method waits for about five seconds to simulate a long process call.

STEP BY STEP 3.19: Creating a Remotable Class

1. Add a new Visual Basic .NET Class Library named StepByStep3-19 to the solution.

2. In the Solution Explorer, rename the default Class1.vb to RemotableClass.vb.

3. Open the RemotableClass.vb and replace the code with the following code:

Imports System
Imports System.Threading

Public Class RemotableClass
 Inherits MarshalByRefObject
 Public Function LongProcess(ByVal param As String) _
  As String
   Thread.Sleep(5000)
   Return param
 End Function
End Class

4. Select Build, Build StepByStep3-19. This step generates the code for your class library and packages it into the file StepByStep3-19.dll, which is located in the bin\ directory of your project.

RemotableClass is now ready to be hosted in a remoting host. In Step By Step 3.20, I decided to host the remotable class using IIS because it requires only a minimal amount of code. I'll, of course, control the remoting configuration by modifying the web.config file.

STEP BY STEP 3.20: Using IIS to Host a Remotable Class

1. Add a new Visual Basic ASP.NET Web Application named StepByStep3-20 to the solution.

2. Add a reference to the project StepByStep3-19 (the remotable class assembly).

3. Open the web.config file and add the following <system.runtime.remoting> element inside the <configuration> element:

<configuration>
 <system.runtime.remoting>
   <application>
    <service>
      <!-- Set the activation mode,
        remotable object and its URL -->
      <wellknown mode="Singleton"
 type="StepByStep3_19.RemotableClass, StepByStep3-19"
        objectUri="RemotableClass.rem" />
    </service>
   </application>
 </system.runtime.remoting>
...
</configuration>

4. The remoting server is now hosting the RemotableClass contained in the assembly StepByStep3-19 for remote invocation using the Singleton activation mode.

Neither RemotableClass nor the remoting host contains any additional information that supports an asynchronous method call. The asynchronous call is completely managed in the client code. So in the final step, I'll create a client application that calls a remote method synchronously as well as asynchronously. Showing both ways of calling a method will help you to understand the difference.

STEP BY STEP 3.21: Instantiating and Invoking a Client That Calls Remote Object Methods Synchronously and Asynchronously

1. Add a new Visual Basic .NET Windows Application named StepByStep3-21 to the solution.

2. Add references to the .NET assembly System.Runtime.Remoting.

3. Add a new class file named StepByStep3-19.vb.

4. Replace the code in the StepByStep3-19.vb file with code to define the interface and location of the remote object:

   Imports System
   Imports System.Runtime.Remoting.Messaging
   Imports System.Runtime.Remoting.Metadata
   Imports System.Runtime.Remoting.Metadata.W3cXsd2001
   
   <Serializable()> _
   Public Class RemotableClass
     Inherits System.MarshalByRefObject
   
     <SoapMethod(SoapAction:=
      "http://schemas.microsoft.com/clr/nsassem/StepByStep3_19.RemotableClass
                  /StepByStep3-19#LongProcess")> _
     Public Function LongProcess(ByVal param As String) _
      As String
       Return (CType(CType(Nothing, Object), String))
     End Function
   
   End Class

5. In the Solution Explorer, delete the default Form1.vb. Add a new form named SyncAsync.vb. Set this form as the startup object for the project.

6. Add the following directives:

Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels
Imports System.Runtime.Remoting.Channels.Http

7. Place a TextBox control (txtResults) and two Button controls (btnSync and btnAsync) on the form. Refer to Figure 3.12 for the design of this form.

8. Add the following code directly after the Windows Form designer generated code:

' Remotable object
Dim remObject As RemotableClass

' Create a delegate for the LongProcess method
' of the Remotable object
Delegate Function LongProcessDelegate( _
 ByVal param As String) As String

' Declare a LongProcessDelegate
' object, an AsyncCallback
' delegate object and an IAsyncResult object
Dim delLongProcess As LongProcessDelegate
Dim ab As AsyncCallback
Dim ar As IAsyncResult

' Declare an integer variable to hold
' number of times the method is called
Dim counter As Integer

9. Double-click the form and add the following code in the Load event handler:

Private Sub SyncAsync_Load( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles MyBase.Load
  ' Register a Http client channel
  Dim channel As HttpClientChannel = _
   New HttpClientChannel()
  ChannelServices.RegisterChannel(channel)

  ' Instantiate the remote class
  remObject = CType(Activator.GetObject( _
   GetType(RemotableClass), _
   "http://localhost/StepByStep3-20/RemotableClass.rem"), _
   RemotableClass)

  ' Create a AsyncCallback delegate object to hold
  ' the reference of the
  ' LongProcessCompleted method, which is
  ' called when the asynchronous call is completed
  ab = New AsyncCallback( _
   AddressOf LongProcessCompleted)

  ' Create a LongProcessDelegate
  ' delegate object to hold
  ' the reference of the LongProcess method
  delLongProcess = New LongProcessDelegate( _
   AddressOf remObject.LongProcess)
End Sub

10. Double-click the btnSync control and add the following code in the Click event handler:

Private Sub btnSync_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnSync.Click
  ' Increment the method call counter
  counter +=
  Dim param As String = String.Format( _
   "Call: {0}, Type=Synchronous", counter)

  ' Append the start message to the text box
  txtResults.AppendText(String.Format( _
   "{0}, Started at: {1}" & vbCrLf, _
   param, DateTime.Now.ToLongTimeString()))

  ' Call the LongProcess method
  ' of the remotable object
  remObject.LongProcess(param)

  ' Append the completed message to the text box
  txtResults.AppendText( _
    String.Format("{0}, Completed at: {1}" & _
     vbCrLf, _
    param, DateTime.Now.ToLongTimeString()))
End Sub

11. Double-click the btnAsync control and add the following code in the Click event handler:

Private Sub btnAsync_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles btnAsync.Click
  ' Increment the method call counter
  counter +=
  Dim param As String = String.Format( _
   "Call: {0}, Type=Asynchronous", _
   counter)
  ' Append the start message to the text box
  txtResults.AppendText( _
   String.Format("{0}, Started at: {1}\n", _
   param, DateTime.Now.ToLongTimeString()))

  ' Call the BeginInvoke method to start
  ' the asynchronous method call
  ar = delLongProcess.BeginInvoke(param, ab, Nothing)
End Sub

12. Add the following callback, LongProcessCompleted() method definition, to the form's code-behind file at the class level:

Sub LongProcessCompleted(ByVal ar As IAsyncResult)
  ' Call the EndInvoke method to retrieve the return
  ' value of the asynchronous method call
  Dim result As String = delLongProcess.EndInvoke(ar)

  ' Append the completed message to the text box
  txtResults.AppendText( _
  String.Format("{0}, Completed at: {1}\n", _
  result, DateTime.Now.ToLongTimeString()))
End Sub

13. Build the project. This step creates a remoting client for the RemotableClass remotable object.

14. Set the StepByStep3_21, the remoting client, as the startup project. Select Debug, Start Without Debugging to run the project. Click the Call LongProcess Synchronously button to call the LongProcess() method synchronously. You should see the start message appended to the text box, and the application freezes for the next five seconds. After the method call is completed, you notice a completed message and you are now able to work with the application. Now, click the Call LongProcess Asynchronously button to call the LongProcess() method asynchronously. As soon as the method starts, the start message is appended to the text box. While the method is executing, you are able to still work with the application (such as moving the form, clicking buttons and so on, although clicking the synchronous call button freezes the application). When the method is completed, you are notified by appending the completed message in the text box as shown in Figure 3.12.

Figure 3.12 The asynchronous method call invokes the method and transfers the control back to the form. The asynchronous call uses a callback method to get a notification when the remote method finishes the execution.

In this program, I followed the same steps for an asynchronous method call that were discussed earlier in the previous section, "Understanding the Model of Asynchronous Programming in the .NET Framework."

REVIEW BREAK

• You can choose to run the remoting host as a console application, as a Windows service, or as an IIS application.

• You can use IIS as an activation agent only when the underlying communication is in HTTP channel. Using IIS as an activation agent eliminates the need to write a separate server program that listens on a unique port number (IIS uses the port 80).

• When creating IIS hosted remote objects, you cannot specify constructor parameters; therefore, activating CAO is not possible using IIS.

• You can invoke a method asynchronously by calling the BeginInvoke() method on the delegate of that method.

• You can automatically get a notification when an asynchronous method ends. However, you must first create another delegate object (of AsyncCallback type) that refers to the callback method that you need to execute when the remote method ends. And then you should pass the delegate to the callback method as an argument to the BeginInvoke() method.