Server.Transfer Vs. Response.Redirect

Server.Transfer() doesn't end the current request, it only instructs ASP.NET to stop rendering the current page and start rendering the new page instead. The client is none the wiser, from its point of view the server is still responding to the initial request, so the URL displayed in the address bar does not change.

• The page transferred to should be another .aspx page. For instance, a transfer to an .asp or .asmx page is not valid.
• The Transfer method preserves the QueryString and Form collections.
• Transfer calls Response.End(), which throws a ThreadAbortException exception upon completion.

Response.Redirect() ends the current request and sends a 302 response code to the client. The client then issues another HTTP request to the redirected URL and processes the response. Since the client knows that the URL has changed, it displays the redirected URL in its address bar.

Response.Redirect simply sends a message down to the browser, telling it to move to another page.

Server.Transfer is similar in that it sends the user to another page with a statement such as Server.Transfer("newpage.aspx"). However, the statement has a number of distinct advantages and disadvantages.

Firstly, transferring to another page using Server.Transfer conserves server resources. Instead of telling the browser to redirect, it simply changes the "focus" on the Web server and transfers the request. This means you don't get quite as many HTTP requests coming through, which therefore eases the pressure on your Web server and makes your applications run faster.

But watch out: because the "transfer" process can work on only those sites running on the server, you can't use Server.Transfer to send the user to an external site. Only Response.Redirect can do that.

Secondly, Server.Transfer maintains the original URL in the browser. This can really help streamline data entry techniques, although it may make for confusion when debugging.

That's not all: The Server.Transfer method also has a second parameter—"preserveForm". If you set this to True, using a statement such as Server.Transfer("newpage.aspx", True), the existing query string and any form variables will still be available to the page you are transferring to.

For example, if your newpage.aspx has a TextBox control called TextBox1 and you transferred to WebForm2.aspx with the preserveForm parameter set to True, you'd be able to retrieve the value of the original page TextBox control by referencing Request.Form("TextBox1").

This technique is great for wizard-style input forms split over multiple pages. But there's another thing you'll want to watch out for when using the preserveForm parameter. ASP.NET has a bug whereby, in certain situations, an error will occur when attempting to transfer the form and query string values. You'll find this documented at http://support.microsoft.com/default.aspx?id=kb;en-us;Q316920.

The unofficial solution is to set the enableViewStateMac property to True on the page you'll be transferring to, then set it back to False. This records that you want a definitive False value for this property and resolves the bug.

So, in brief: Response.Redirect simply tells the browser to visit another page. Server.Transfer helps reduce server requests, keeps the URL the same and, with a little bug-bashing, allows you to transfer the query string and form variables.

Top Tip: Don't confuse Server.Transfer with Server.Execute, which executes the page and returns the results. It was useful in the past, but, with ASP.NET, it's been replaced with fresher methods of development. Ignore it.