This chapter is from the book
This chapter is from the book
This chapter is from the book
Navigation Between Pages
Implement navigation for the user interface.
A typical Web application is a collection of Web pages linked to each other. In Chapter 2, I discussed the HyperLink control that allows a user to navigate to a different Web page when the hyperlink is clicked. However, there is also a need to navigate to a Web page programmatically. ASP.NET provides the following methods for programmatically navigating between pages:
Response.Redirect()
Server.Transfer()
Server.Execute()
I'll discuss each of these methods in the following sections.
The Response.Redirect() Method
The Response.Redirect() method causes the browser to connect to the specified URL. When the Response.Redirect() method is called, it creates a response whose header contains a 302 (Object Moved) status code and the target URL. When the browser receives this response from the server, it uses the header information to generate another request to the specified URL. When using the Response.Redirect() method, the redirection happens at the client side and involves two roundtrips to the server.
Using the Response.Redirect() method is recommended in the following cases:
You want to connect to a resource on ANY Web server.
You want to connect to a non-ASPX resource (such as an HTML file).
You want to pass query string as part of the URL.
The Server.Transfer() Method
The Server.Transfer() method transfers the execution from the current ASPX page to the specified ASPX page. The path specified to the ASPX page must be on the same Web server and must not contain a query string.
When the Server.Transfer() method is called from an executing ASPX page, the current ASPX page terminates execution and control is transferred to another ASPX page. The new ASPX page still uses the response stream created by the prior ASPX page. When this transfer occurs, the URL in the browser still shows the original page because the redirection occurs on the server side and the browser remains unaware of the transfer.
When you want to transfer control to an ASPX page residing on the same Web server, you should use Server.Transfer() instead of Response.Redirect() because Server.Transfer() will avoid an unnecessary roundtrip and provide better performance and user experience.
The default use of the Server.Transfer() method does not pass the form data and the query string of the original page request to the page receiving the transfer. However, you can preserve the form data and query string of the original page by passing a true value to the optional second argument, of the Server.Transfer() method. The second argument takes a Boolean value that indicates whether to preserve the form and query string collections.
When you set the second argument to true, you need to be aware of one thing: that the destination page contains the form and query string collections that were created by the original page. As a result, the hidden _VIEWSTATE field of the original page is also preserved in the form collection. The view state is page scoped and is valid for a particular page only. This causes the ASP.NET machine authentication check (MAC) to announce that the view state of the new page is tampered with. Therefore, when you choose to preserve the form and query string collections of the original page, you must set the EnableViewStateMac attribute of the Page directive to false for the destination page.
The Server.Execute() Method
The Server.Execute() method allows the current ASPX page to execute a specified ASPX page. The path to the specified ASPX page must be on the same Web server and must not contain a query string.
Bad HTML Code
The output returned to the browser by Server.Execute() and Server.Transfer() might contain multiple <html> and <body> tags because the response stream remains the same while executing another ASPX page. Therefore, the output that results from calling these methods might contain bad HTML code.
After the specified ASPX page is executed, control transfers back to the original page from which the Server.Execute() method was called. This technique of page navigation is analogous to making a method call to an ASPX page.
The called ASPX page has access to the form and query string collections of the calling page: Thus, for the reasons explained in the previous section, you need to set the EnableViewStateMac attribute of the Page directive to false on the called ASPX page.
By default, the output of the executed page is added to the current response stream. This method also has an overloaded version in which the output of the redirected page can be fetched in a TextWriter object instead of adding the output to the response stream. This helps you control where the output is placed on the original page.
STEP BY STEP
3.13 Using the Response.Redirect(), Server.Transfer(), and Server.Execute() Methods
-
Add a new Web form to the project. Name the Web form StepByStep3_13.aspx. Change the pageLayout property of the DOCUMENT element to FlowLayout.
-
Add three Label controls, a Literal control (litDynamic), two TextBox controls (txtRows, txtCells), and three Button controls (btnTransfer, btnExecute, and btnRedirect) to the Web form, as shown in Figure 3.12.
-
Double-click the three Button controls to add an event handler to the Click event of each button. Add the following code in their event handlers:
-
Add a new Web form to the project. Name the Web form StepByStep3_13a.aspx. Change the pageLayout property of the DOCUMENT element to FlowLayout.
-
Add a Table control (tblDynamic) from the Web Forms tab of the Toolbox to the Web form.
-
Switch to the HTML view of the StepByStep3_13a.aspx file and modify the Page directive to add the EnableViewStateMac="false" attribute:
-
Switch to code view and add the following method in the class definition:
-
Add the following code in the Page_Load() event handler:
-
Set StepByStep3_13.aspx as the start page in the project.
-
Run the project. Enter the number of rows and cells and click all three buttons one by one. When you click the Redirect button, the browser is redirected to StepByStep3_13a.aspx by passing the Rows and Cells values as the query string data as shown in Figure 3.13. When you click the Transfer button, the browser doesn't change the page name in the location bar, but the control gets transferred to the StepByStep3_13a.aspx page as shown in Figure 3.14. Finally, when you click the Execute button, the StepByStep3_13a.aspx page is executed and control comes back to the calling page, where the output of the Server.Execute() method is displayed as shown in Figure 3.15.
){kind=small}
Figure
3.12 The design of a form that allows you to specify rows and columns
to create a table dynamically.
private void btnRedirect_Click(object sender,
System.EventArgs e)
{
// Calling Response.Redirect by passing
// Rows and Cells values as query strings
Response.Redirect("StepByStep3_13a.aspx?Rows="
+ txtRows.Text + "&Cells=" + txtCells.Text);
}
private void btnTransfer_Click(object sender,
System.EventArgs e)
{
// Writing into Response stream
Response.Write(
"The following table is generated " +
"by StepByStep3_13a.aspx page:");
// Calling the Server.Transfer method
// with the second argument set to true
// to preserve the form and query string data
Server.Transfer("StepByStep3_13a.aspx", true);
// Control does not come back here
}
private void btnExecute_Click(object sender,
System.EventArgs e)
{
// Creating a StringWriter object
StringWriter sw = new StringWriter();
// Calling the Server.Execute method by
// passing a StringWriter object
Server.Execute("StepByStep3_13a.aspx", sw);
// Control comes back
// Displaying the output in the StringWriter
// object in a Literal control
litDynamic.Text = sw.ToString();
}
<%@ Page language="c#" Codebehind="StepByStep3_13a.aspx.cs" AutoEventWireup="false" Inherits="_315C03.StepByStep3_13a" EnableViewStateMac="false" %>
private void CreateTable(Int32 intRows, Int32 intCells)
{
// Create a new table
TableRow trRow;
TableCell tcCell;
// Iterate for the specified number of rows
for (int intRow=1; intRow <= intRows; intRow ++)
{
// Create a row
trRow = new TableRow();
if(intRow % 2 == 0)
{
trRow.BackColor = Color.LightBlue;
}
// Iterate for the specified number of columns
for (int intCell=1; intCell <= intCells;
intCell++)
{
// Create a cell in the current row
tcCell = new TableCell();
tcCell.Text = "Cell (" + intRow + ","
+ intCell + ")";
trRow.Cells.Add(tcCell);
}
// Add the row to the table
tblDynamic.Rows.Add(trRow);
}
}
private void Page_Load(
object sender, System.EventArgs e)
{
if(Request.Form["txtRows"] != null)
{
// If the request contains form data then the
// page is called from the Server.Transfer or
// Server.Execute methods from the
// StepByStep3_13.aspx page. Get the Rows and
// Cells values from the form collection and
// Create a table.
CreateTable(
Int32.Parse(Request.Form["txtRows"]),
Int32.Parse(Request.Form["txtCells"]));
}
else if(Request.QueryString["Rows"] != null)
{
// If the request contains query string data
// that means the response is redirected from
// the StepByStep3_14.aspx page. Get the Rows
// and Cells value from the query string and
// Create a table.
Response.Write("StepByStep3_13a.aspx:");
CreateTable(
Int32.Parse(Request.QueryString["Rows"]),
Int32.Parse(Request.QueryString["Cells"]));
}
}
)
Figure
3.13 The Response.Redirect() method can be used to navigate to
a URL that contains query strings.
)
Figure
3.14 The Server.Transfer() method is used to navigate to an ASPX
page on the same server without causing an additional roundtrip.
)
Figure
3.15 The Server.Execute() method executes the specified ASPX page
and returns the control back to the calling page.
In fact, it's a good idea to keep the second argument set to false when using the Server.Transfer() method. When you want to pass some values from one page to another in the current HTTP request, instead of using the form and query string collections, you should use the HttpContext object. The HttpContext object gives access to all the information about the current HTTP request. It exposes a key-value collection via the Items property in which you can add values that will be available for the life of the current request. The Page class contains a property called Context that provides access to the HttpContext object for the current request.
You can use two techniques to access the values of one page from another page in the current HTTP request using the HttpContext objects—HttpContext.Handler and HttpContext.Items. I have demonstrated these techniques in the Guided Practice Exercise 3.2 and in the Exercise 3.2, respectively.
Guided Practice Exercise 3.2
When using the Server.Transfer() method, for security reasons, you might not want to disable the machine authentication check for the view state of a page or for an application. How would you pass the state of one page to another in that case?
The Page class contains a property called Context that provides access to the HttpContext object for the current request. The Handler property of the HttpContext object provides the instance of the page that first received the HTTP request.
To use the HttpContext.Handler property in your code, you should take the following steps:
Expose the control values as public properties in the page that transfers the control.
In the target page, create an instance of the previous page's class.
Use the HttpContext.Handler property to retrieve the instance of the handler for the previous page. Initialize the object created in the preceding step with this value.
Use the instance of the previous page's class to access its properties.
In this exercise, you are required to modify Step by Step 3.13 to use the HttpContext.Handler property to retrieve the properties of the first page in the second page. How would you make this modification?
You should try working through this problem on your own first. If you are stuck, or if you'd like to see one possible solution, follow these steps:
-
Open the project 315C03. Add a new Web form GuidedPracticeExercise3_2 to the project. Change the pageLayout property of the DOCUMENT element to FlowLayout.
-
Add three Label controls, two TextBox controls (txtRows, txtCells), and one button control (btnTransfer) on the Web form.
-
Switch to the code view and add the following property definition in the class definition:
-
Double-click the Transfer button control and add the following code in the Click event handler:
-
Add a new Web form to the project. Name the Web form GuidedPracticeExercise3_2a.aspx. Change the pageLayout property of the DOCUMENT element to FlowLayout.
-
Add a Table control (tblDynamic) from the Web Forms tab of the Toolbox to the Web form.
-
Switch to code view and add the following code in the class definition to create an instance of the GuidedPracticeExercise3_2 page:
-
Add the following code in the Page_Load() event handler:
-
Switch to code view and add the following method in the class definition:
-
Set GuidedPracticeExercise3_2.aspx as the start page in the project.
-
Run the project. Enter the number of rows and cells and click the Transfer button. You should see that the browser doesn't changes the page name in the location bar, but the control gets transferred to the GuidedPracticeExercise3_2a.aspx page. The GuidedPracticeExercise3_2a.aspx page is able to access the Rows and Cells properties of the GuidedPracticeExercise3_2.aspx page via the HttpContext.Handler property.
// Declaring properties to expose
// the number of Rows and Cells entered
public Int32 Rows
{
get
{
return Int32.Parse(txtRows.Text);
}
}
public Int32 Cells
{
get
{
return Int32.Parse(txtCells.Text);
}
}
private void btnTransfer_Click(object sender,
System.EventArgs e)
{
// Writing into Response
Response.Write(
"The following table is generated " +
"by GuidedPracticeExercise3_2a.aspx page:");
// Calling the Server.Transfer method
Server.Transfer("GuidedPracticeExercise3_2a.aspx");
// Control does not come back here
}
// Declare a variable to store an instance of // the GuidedPracticeExercise3_2 Web form public GuidedPracticeExercise3_2 pageExercise3_2;
private void Page_Load(
object sender, System.EventArgs e)
{
if(!IsPostBack)
{
// Because its the same request context,
// get the reference to the
// GuidedPracticeExercise3_2 page
// through the current request Context.
// Get the Rows and Cells value from the
// GuidedPracticeExercise3_2 Web form's
// properties and Create a table.
pageExercise3_2 = (GuidedPracticeExercise3_2)
Context.Handler;
CreateTable(pageExercise3_2.Rows,
pageExercise3_2.Cells);
}
}
private void CreateTable(Int32 intRows, Int32 intCells)
{
// Create a new table
TableRow trRow;
TableCell tcCell;
// Iterate for the specified number of rows
for (int intRow=1; intRow <= intRows; intRow ++)
{
// Create a row
trRow = new TableRow();
if(intRow % 2 == 0)
{
trRow.BackColor = Color.LightBlue;
}
// Iterate for the specified number of columns
for (int intCell=1; intCell <= intCells;
intCell++)
{
// Create a cell in the current row
tcCell = new TableCell();
tcCell.Text = "Cell (" + intRow + ","
+ intCell + ")";
trRow.Cells.Add(tcCell);
}
// Add the row to the table
tblDynamic.Rows.Add(trRow);
}
}
If you have difficulty following this exercise, review the section "Navigation between Pages" earlier in this chapter and perform Step by Step 3.13. After doing this review, try this exercise again.
REVIEW BREAK
The Response.Redirect() method can be used to redirect the browser to any specified URL. The specified URL can point to any resource and might also contain query strings. Use of Response.Redirect() method causes an additional roundtrip.
The Server.Transfer() method performs a server-side redirection of a page, avoiding extra roundtrip to the server. The Server.Transfer() method can only be used to redirect to an ASPX page residing on the same Web server. The URL to the redirected ASPX page cannot contain query strings, although you can preserve the form and query string collection of the main page to the redirected page by passing the true value to an optional second parameter.
The Server.Execute() method is like a method call to an ASPX page. This method executes the specified ASPX page and returns the execution to the calling ASPX page. The page specified as an argument to the Server.Execute() must be an ASPX page residing on the same Web server, and the argument should not contain query string data. The ASPX page to be executed contains access to the form and query string collections of the main page.
You should set the EnableViewStateMac attribute of the Page directive to false for the destination page; in case of calling the page via the Server.Execute() method or the Server.Transfer() method with the second argument, set it to true.