Cross-Page Posting in ASP.NET Web Pages

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Cross-Page Posting in ASP.NET Web Pages

By default, buttons and other controls that cause a postback on an ASP.NET Web page submit the page back to itself. This is part of the round-trip cycle that ASP.NET Web pages go through as part of their normal processing. For details, see Introduction to ASP.NET Web Pages.

Under some circumstances, you might want to post one page to another page. For example, you might be creating a multi-page form that collects different information on each page. In that case, you can configure certain controls (those that implement the IButtonControl interface, such as the Button control) on the page to post to a different target page. This is referred to as cross-page posting. Cross-page posting provides some advantages over using the Transfer method to redirect to another page. For details, see Redirecting Users to Another Page.

Note:
You can also use the Wizard control to create multi-view forms. For details, see Wizard Web Server Control Overview.

Because cross-page posting is configured for individual controls, you can create a page that posts to different pages depending on which button the user clicks.

Getting Information from the Source Page

When you configure a page for cross-page posting, you frequently want to get information from the source page. This might include the information from controls on the page—that is, the information being posted by the browser—as well as public properties of the source page.

Getting Control Values

The Page class exposes a property named PreviousPage. If the source page and target page are in the same ASP.NET application, the PreviousPage property in the target page contains a reference to the source page. (If the page is not the target of a cross-page posting, or if the pages are in different applications, the PreviousPage property is not initialized.) By default, the PreviousPage property is typed as Page.

Note:
If the source and target page are in different applications, you cannot directly get the values of controls on the page, but you can read the posted data from the Form dictionary. You cannot read view state from the source page, because it is hashed. If you want to store values in the source page and make them available in a target page in another application, you can store the values as strings inside hidden fields on the source page and access them through Request.Form on the target page.

Note:

If the source and target page are in different applications, you cannot directly get the values of controls on the page, but you can read the posted data from the Form dictionary. You cannot read view state from the source page, because it is hashed. If you want to store values in the source page and make them available in a target page in another application, you can store the values as strings inside hidden fields on the source page and access them through Request.Form on the target page.

Using the reference in the PreviousPage property, you can search for controls on the source page and extract their value. You typically do this with the FindControl method.

Note:
If you are coding the source page specifically to be able to share information with target pages, an easier way to make control values available to the target page is to expose them as public properties. For details, see Getting Public Property Values from the Source Page later in this topic.

The following code example shows how you can get the value of the TextBox1 control on the source page.

Visual Basic

If Not Page.PreviousPage Is Nothing Then
    Dim SourceTextBox As TextBox
    SourceTextBox = CType(PreviousPage.FindControl("TextBox1"), _
        TextBox)
    If Not SourceTextBox Is Nothing Then
        Label1.Text = SourceTextBox.Text
    End If
End If

if (Page.PreviousPage != null)
{
    TextBox SourceTextBox = 
        (TextBox)Page.PreviousPage.FindControl("TextBox1");
    if (SourceTextBox != null)
    {
        Label1.Text = SourceTextBox.Text;
    }
}

The FindControl method finds controls in the current naming container. If the control you are looking for is inside another control (typically, inside a template), you must first get a reference to the container and then search the container to find the control you want to get. In the following code example, the source page contains a Login control with a LayoutTemplate container that in turn contains a TextBox control named UserName. The code gets the value of the UserName control.

Visual Basic

Dim LoginControl As Login
LoginControl = CType(PreviousPage.FindControl("Login1"), Login)
If Not LoginControl Is Nothing Then
    Dim UserName As TextBox
    UserName = CType(LoginControl.FindControl("UserName"), TextBox)
    If Not UserName Is Nothing Then
        Label1.Text = UserName.Text
    End If
Else
    Label1.Text = "Cannot find user name in Login control."
End If

Login LoginControl = (Login)PreviousPage.FindControl("Login1");
if (LoginControl != null)
{
    TextBox UserName = (TextBox)LoginControl.FindControl("UserName");
    if (UserName != null)
    {
        Label1.Text = UserName.Text;
    }
}
else
{
    Label1.Text = "Cannot find user name in Login control.";
}

Getting Public Property Values from the Source Page

In the target page of a cross-page posting, you can also get the values of public members of the source page. The most common scenario is that the source page defines public properties and you want to get their values on the target page.

Security Note:
It is recommended that you expose only the information you need as public properties to reduce the amount of information available to potentially malicious users.

To get public members of the source page, you must first get a strongly typed reference to the source page.

You can do so in a number of ways. The first is to include an @ PreviousPageType directive in the target page, which allows you to specify the source page, as in this example:

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

When this directive is included, the PreviousPage property is strongly typed to the class of the referenced source page. As a consequence, you can directly reference public members of the source page. You can specify the type of the source page either directly, using a type attribute, or indirectly by explicitly referencing the source page in a VirtualPath attribute, as shown in the example.

The following code example shows a portion of a source page containing a public property named CurrentCity that exposes the value of a TextBox control named textCity.

Visual Basic

Public ReadOnly Property CurrentCity() As String
    Get
        Return textCity.Text
    End Get
End Property

public String CurrentCity
{
    get
    {
        return textCity.Text;
    }
}

Note:
Properties on the source page that are created primarily to expose values for cross-page posting are usually read-only properties. Although the source page can contain public read/write properties, setting a source page property from the target page property generally has no purpose, because the value will not be persisted.

If the target page contains a PreviousPageType directive that points to the source page, you can access the source page's CurrentCity property using code such as the following.

Visual Basic

Label1.Text = PreviousPage.CurrentCity

Label1.Text = PreviousPage.CurrentCity;

Another way to get a strongly typed reference to the source page is to include an @ Reference directive in the target page that references the source page, as you would reference any type that you wanted to use in your page. In that case, in the target page you can get the target page's PreviousPage property and cast it to the source page type, as in the following code example.

Visual Basic

Dim sourcePage As SourcePage_aspx
sourcePage = CType(PreviousPage, SourcePage_aspx)
Label1.Text = p.CurrentCity

SourcePage_aspx sourcePage;
sourcePage = (SourcePage_aspx) PreviousPage;
Label1.Text = sourcePage.CurrentCity;

Checking for Postbacks in the Target Page

During a cross-page postback, the contents of the source page's controls are posted to the target page, and the browser executes an HTTP POST operation (not a GET operation). However, in the target page, the IsPostBack property is false immediately after a cross-page post. Although the behavior is that of a POST, the cross-posting is not a postback to the target page. Therefore, IsPostBack is set to false and the target page can go through its first-time code.

If it is useful in your application, you can determine whether the target page is running as the result of a cross-page post. To do so, you can test the IsCrossPagePostBack property of the page reference returned by the target page's PreviousPage property, as in the following code example.

Visual Basic

If PreviousPage IsNot Nothing Then
    If PreviousPage.IsCrossPagePostBack = True Then
         Label1.Text = "Cross-page post."
    End If
Else
    Label1.Text = "Not a cross-page post."
End If

if(PreviousPage != null)
{
    if(PreviousPage.IsCrossPagePostBack == true)
    {
         Label1.Text = "Cross-page post.";
    }
}
else
{
    Label1.Text = "Not a cross-page post.";
}

Note that if the current page is not the target of a cross-page post, the PreviousPage property returns null (Nothing in Visual Basic).

For more information, see How to: Determine How ASP.NET Web Pages Were Invoked.

Cross-Page Posting versus Server.Transfer

The PreviousPage property and the PreviousPageType directive are useful in two situations where you invoke the target page: in a cross-page postback, which is a client-based transfer, and with the Transfer method, which is a server-based operation. In both operations, code in the target page can get a reference to the source page using the PreviousPage property.

It might be important in the target page to determine whether the page was invoked from a cross-page posting or a Server.Transfer operation. To help you do this, the Page class exposes a property named IsCrossPagePostBack. For details, see How to: Determine How ASP.NET Web Pages Were Invoked.

Tasks

How to: Post ASP.NET Web Pages to a Different Page

How to: Determine How ASP.NET Web Pages Were Invoked

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Circular Reference

Keyur.Parekh ... Thomas Lee | Edit | Show History

In my scenairo -

I am having two pages A and B.

In Page A there is one public property ABC

In Page B there is one public Property DEF

In Page B I have added reference of Page A

Now I am using Server.Transfer("B") and on page B I am accessing PreviousPage.ABC property and doing some calculation,

after that I am setting some value in DEF, which I want to use on Page A.

So I have to again use Server.Transfer("A").

For that I have give reference of Page B in Page A.

But when I am giving reference .... System is giving error of Circular Reference??

Can Anybody know?

[tfl - 25 09 09] Hi - and thanks for your post. You should post questions like this to the MSDN Forums at http://forums.microsoft.com/msdn or the MSDN Newsgroups at http://www.microsoft.com/communities/newsgroups/en-us/. You are much more likely get a quicker response using the forums than through the Community Content. For specific help about:
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Use an interface to cast the pages to

margre | Edit | Show History

Hi,

here's one solution - define a custom interface and have both pages implement it. For example:

public interface IForm {

    string Param{get;set;}

Each page implements the interface. Then you can cast the PreviousPage reference to it in the Page.Load method and get the value that way:

protected void Page_OnLoad() {

    if (PreviousPage != null) {

        IForm form = PreviousPage as IForm;

        // do something with the parameter...

        string param = form.Param;

Hope that is clear.

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Posting to another page..which is not yours

ThisIsTrendy ... Thomas Lee | Edit | Show History

There is a big problem I've been working on for a whole week. I want to post to antoher page on another website which is not mine and which I can't control. The scenario is this:

- A user fills a form on my website

- When the user pressess the submit button, the form values collected have to pass on to another website by using the POST method and not GET

- However, before I post to the other website, I would like to save the form data in my database.

Basically:

- I cannot use Response.Redirect, because with Response.Redirect I can pass the values only by using the GET method.

- I cannot use Server.Transfer because the URL on the browser would remain referencing my own website which is undesirable in my case since my website does not have SSL (https) whilst the other website has SSL and the users would enjoy seeing it.

- I cannot create a server-side OnClick event handler on the submit button with a PostBackURL posting to somewhere else because since the page is not posting back on itself, the event handler would simply not be called.

- I know its stupid but I've also researched on URL rewriting just in case I manage to server.transfer to another page but I would rewrite the actual URL like response.redirect would have done. No sucess, you can't rewrite the domain part of the URL (if you know how please tell me).

After hard work having tried all of these, I am out of ideas. It would be really stupid to have the user go to another page on my website and make another submit click just to enable me to save the form data before he leaves.

Can you help me solve this issue?

You must probably use an http handler to do something so low level as that.

HRM

-- As you want to post the data to other site you can set the form action property to the desired URL and submit the page.

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