Passing data between two pages with Cross-page Postback and Server.Transfer

The ASP.NET control, that cause postback, post back to the page for processing. That page can be current page or another page. Every control that cause postback can be configured to post to a different page - this postback is called cross postback and is often used for getting information from the pages in the multi page form, especially convenient for creating wizards.

<asp:Button
ID="Button1"
PostBackUrl="~/TargetPage.aspx"
runat="server"
Text="Submit" />

Cross-page post back is similar to Server.Transfer method, but there are some imporrtant differences. Server.Transfer conserves the post data between pages and also it keeps the url. And also it has a documented bug with getting url parameters from the source page. The Server.Transfer method is a server-based operation whereas cross-page postback is a client-based transfer. The main aim of the Server.Transfre method is to reduce client request to the server which makes it faster than Response.Redirect().
Anyway the cross-page postaback can throw the user on another page but with possibility to get the post data.

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

there is also possibility to get public properties from the source page with decalring the source:

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

getting the whole source page:

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

With PreviousPage.IsCrossPagePostBack we can determine whether the page was invoked from a cross-page posting.

Resources:
http://msdn2.microsoft.com/en-us/library/ms178139.aspx
http://www.developer.com/net/asp/article.php/3299641
http://www.codeproject.com/useritems/CrossPagePosting.asp