ASP.NET 2.0: A Developeramp;#039;s Notebook [Electronic resources] نسخه متنی

1.6. Post to Another Page

Note: ASP.NET 2.0 now allows you to post values from one page to
another easily.

Most ASP.NET developers are familiar
with the postback feature of ASP.NET
server controls. However, in some cases, posting back to the same
page is not the desired actionyou might need to post to
another. For example, you might need to build an application to
perform some surveys. Instead of displaying 50 questions on one page,
you would like to break it down to 10 questions per page so that the
user need not wade through 50 questions in one go. Moreover, answers
to certain questions might trigger a related set of questions in
another page. In this case, using a conventional postback mechanism
for your web pages is clearly not the solution. You might want to
post the values (i.e., the answers to the questions) of one page to
another. In this situation, you need to be able to cross-post to
another page and, at the same time, be able to retrieve the values
from the previous page.

In ASP.NET 1.0 and 1.1, there is no easy way to transfer to another
page, and most developers resort to using
Server.Transfer. Even so, passing values from one
page to another this way is a challenge.

In ASP.NET 2.0, cross-page posting is much easier. Controls now
inherently support posting to another page via the PostBackUrl
attribute.

1.6.1. How do I do that?

To see cross-page posting in action, you will create an application
that contains two pages. One page lets you choose items from a
CheckBoxList control and then lets you cross-post to another page.
You will learn how the values that were cross-posted can be retrieved
in the destination page. In addition, you will see the difference
between cross-page posting and the old
Server.Transfer( ) method.

In Visual Studio 2005, create a new web site project and name it
C:\ASPNET20\chap01-CrossPagePosting.

Populate the default Web Form with the controls shown in Figure 1-18. The CheckBoxList1 control contains CheckBox
controls that let users choose from a list of values. When the
"Post to Default2.aspx" button is
clicked, you will post the values to
Default2.aspx using the new cross-page posting
feature in ASP.NET 2.0. The "Transfer to
Default2.aspx" button uses
Server.Transfer to load
Default2.aspx.

Figure 1-18. Populating the default Web Form

Add a new Web Form to your project and populate it with a Button
control (see Figure 1-19).

Figure 1-19. Populating the Default2 Web Form

In Default.aspx, switch to Source View and add
the
PostBackUrl attribute to btnPost so that it can
perform a cross-page post to Default2.aspx:

<asp:Button ID="btnPost" 
PostBackUrl="~/Default2.aspx" 
runat="server" 
Text="Post to Default2.aspx" />

For btnTransfer, add the following code-behind:

Protected Sub btnTransfer_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) _
Handles btnTransfer.Click
Server.Transfer("Default2.aspx")
End Sub

In order for Default2.aspx to access the value
of the CheckBoxList1 control in Default.aspx,
expose a public property in Default.aspx. To do
so, switch to the code-behind of Default.aspx
and type in the following code shown in bold:

Partial Class Default_aspx
Inherits System.Web.UI.Page
Public ReadOnly Property ServerSide( ) As CheckBoxList
Get
Return CheckBoxList1
End Get
End Property
Protected Sub btnTransfer_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) _
Handles btnTransfer.Click
Server.Transfer("Default2.aspx")
End Sub
End Class

Note: You need to expose public properties in the first page in
order for the other page to access it. This has the advantage of
early binding and facilitates strong typing.

In Default2.aspx, you need to specify that
Default.aspx is going to post to it by using
the
PreviousPageType directive. In Default2.aspx,
switch to Source View and add the PreviousPageType directive:

<%@ Page Language="VB" AutoEventWireup="false" 
CodeFile="Default2.aspx.vb" Inherits="Default2_aspx" %>
<%@ PreviousPageType VirtualPath="~/Default.aspx" %>

When Default.aspx posts to
Default2.aspx, all the information about
Default.aspx is encapsulated in a special
property known as PreviousPage. When
Default2.aspx loads,
you
must first determine whether it was posted by
Default.aspx or it gets loaded by itself. So
check if PreviousPage contains a reference:

Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If PreviousPage IsNot Nothing Then
...

Then check if this is a cross-page post or a
Server.Transfer by using the new
IsCrossPagePostBack property. This
property
will be true if Default.aspx cross-posts to
Default2.aspx, false if
Default.aspx performs a
Server.Transfer to
Default2.aspx:

If PreviousPage IsNot Nothing Then
'--checks the type of posting
If PreviousPage.IsCrossPagePostBack Then
Response.Write("This is a cross-post")
Else
Response.Write("This is a Server.Transfer")
End If

Note: Check the IsCrossPagePostBack property to see if there is a
cross posting.

Finally, display the selections made in the CheckBoxList control in
Default.aspx:

Response.Write("<br/>You have selected :")
Dim i As Integer
For i = 0 To PreviousPage.ServerSide.Items.Count - 1
If PreviousPage.ServerSide.Items(i).Selected Then
Response.Write( _
PreviousPage.ServerSide.Items(i).ToString & " ")
End If
Next

Tip: The ServerSide property refers
to the property exposed in Default.aspx. The
advantage to exposing properties in the page is that the data is
strongly typed.

The entire block of code is as shown:

Protected Sub Page_Load(ByVal sender As Object, _
ByVal e As System.EventArgs) _
Handles Me.Load
If PreviousPage IsNot Nothing Then
'--checks the type of posting
If PreviousPage.IsCrossPagePostBack Then
Response.Write("This is a cross-post")
Else
Response.Write("This is a Server.Transfer")
End If
Response.Write("<br/>You have selected :")
Dim i As Integer
For i = 0 To PreviousPage.ServerSide.Items.Count - 1
If PreviousPage.ServerSide.Items(i).Selected Then
Response.Write( _
PreviousPage.ServerSide.Items(i).ToString & " ")
End If
Next
End If
End Sub

To see the example in action, Figure 1-20 shows a
cross-page post.

Figure 1-20. Cross-posting from Default.aspx to Default2.aspx

Figure 1-21 shows posting to
Default2.aspx via the Server.Transfer method.
Note the URL of both pages (they are the same for
Server.Transfer).

Figure 1-21. Using Server.Transfer to post from Default.aspx to Default2.aspx

If you click the button "Postback to
myself" in Default2.aspx, you
will notice that the information from
Default.aspx is no longer displayed; a postback
will clear the object reference in the PreviousPage property.

Tip: The main difference between a cross-page post and the use of
Server.Transfer is that, in the case of the
Server.Transfer, the URL does not change to the
new page.

1.6.2. What about...

...having multiple pages post to the same page?

In this case, you would not be able to use the early binding
mechanism accorded by the PreviousPage property, because the
PreviousPageType directive predefines the type of a previous page:

<%@ PreviousPageType VirtualPath="~/default.aspx" %>

So if there are different pages posting to a common page, having this
directive is not useful, because those pages may have different
controls and types. Hence, a better way would be to use late-binding
via the FindControl( ) method.

Example 1-1 shows how you can use the
AppRelativeVirtualPath property of the PreviousPage property to get
the address of the last page posted to it. The application then uses
the FindControl( ) method to locate the controls within the source
page.

Note: Use the FindControl( ) method to locate values in the
previous page if multiple pages post to one page.

Example 1-1. Locating the controls of the last page to post

If PreviousPage.AppRelativeVirtualPath = "~/Default.aspx" Then
Dim serverSide As CheckBoxList
serverSide = CType(PreviousPage.FindControl("checkboxlist1"), _
System.Web.UI.WebControls.CheckBoxList)
If serverSide IsNot Nothing Then
Dim i As Integer
For i = 0 To serverSide.Items.Count - 1
If serverSide.Items(i).Selected Then
Response.Write(serverSide.Items(i).ToString & " ")
End If
Next
End If
ElseIf PreviousPage.AppRelativeVirtualPath = "~/Default3.aspx" Then
Dim userName As TextBox
userName = CType(PreviousPage.FindControl("txtName"), _
System.Web.UI.WebControls.TextBox)
If userName IsNot Nothing Then
Response.Write(userName.Text)
End If
End If

Tip: If you attempt to cross-post between two different applications, the
PreviousPage property will be set to Nothing (or null in C#).

1.6.3. Where can I learn more?

To learn more about the various ways to redirect users to another
page, check out the Microsoft Visual Studio 2005 Documentation Help
topic "Redirecting Users to Another
Page."