Monday, April 27, 2009

ASP.NET 2.0 : Cross Page Posting

Cross Page Posting in ASP.NET 2.0

By default, button controls in ASP.NET pages post back to the same page that contains
the button, where you can write an event handler for the post. In most cases this is the
desired behavior, but occasionaly you will also want to be able to post to another page in
your application. The Server.Transfer method can be used to move between pages, however
the URL doesn't change. Instead, the cross page posting feature in ASP.NET 2.0 allows
you to fire a normal post back to a different page in the application. In the target page,
you can then access the values of server controls in the source page that initiated the post
To use cross page posting, you can set the PostBackUrl property of a Button, LinkButton
or ImageButton control, which specifies the target page. In the target page, you can then
access the PreviousPage property to retrieve values from the source page. By default, the
PreviousPage property is of type Page, so you must access controls using the FindControl
method. You can also enable strongly-typed access to the source page by setting the
@PreviousPageType directive in the target page to the virtual path or Type name of the
source page.
Here is a step-by-step guide for implementing the cross-page post back using controls
that implement the IButtonControl interface.
√ Create a Web Form and insert a Button control on it using the VS .NET designer.
√ Set the button's PostBackUrl property to the Web Form you want to post back. For
instance in this case it is "nextpage.aspx"
PostBackUrl="~/nextpage.aspx" Text="Post to nextpage" />
When the PostBackUrl property of the IButtonControl is set, the ASP.NET framework
binds the corresponding HTML element to new JavaScript function named
WebForm_DoPostBackWithOptions. The corresponding HTML rendered by the ASP.NET
2.0 will look like this:
< input type="submit" name="Button1" value="Post to Page 2"
WebForm_PostBackOptions("Button1", ",false”,"Page2.aspx", false, false))"
id="Button1" / >

How do we access viewstate value of this page in the next page ?
View state is page specific; it contains information about controls embedded on the
particular page. ASP.NET 2.0 resolves this by embedding a hidden input field name,
__POSTBACK . This field is embedded only when there is an IButtonControl on the
page and its PostBackUrl property is set to a non-null value. This field contains the view
state information of the poster page. To access the view state of the poster page, you can
use the new PreviousPage property of the page:
Page poster = this.PreviousPage;
Then you can find any control from the previous page and read its state:
Label posterLabel = poster.findControl("myLabel");
string lbl = posterLabel.Text;
This cross-page post back feature also solves the problem of posting a Form to multiple
pages, because each control, in theory, can point to different post back URL.

Can we post and access view state in another application?

You can post back to any page and pages in another application, too. But if you are
posting pages to another application, the PreviousPage property will return null. This is a
significant restriction, as it means that if you want to use the view state, you are confined,
for example, posting to pages in the same virtual directory. Even so, this is a highly
acceptable addition to the functionality of ASP.NET.


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