Detecting Asp.net Ajax Request In Custom HttpModule

The UpdatePanel is one of the coolest feature of Microsoft's ASP.NET AJAX. For some reason if you want to detect an ajax request on the page code-behind it's so easy. Take a look at the code snipped below :

if(ScriptManager.GetCurrent(this).IsInAsyncPostBack)
{
	//this is an ajax postback
}

On the other hand if you use a custom httpmodule with asp.net ajax web application, you will probably get this error  :

Sys.WebForms.PageRequestManagerParserErrorExeption: The message received from the server could not be parsed.  Common causes for this error are when the response is modified by calls to Response.Write(), response filters, HttpModules, or server trace is enabled.  Details: ...

In this situation, we can not use ScriptManager's IsInAsyncPostBack property cause we're working at the httpmodule level. We need something else to catch ajax request.So I began to look for a special property which represents the ajax request. Look at the page's request headers :

Whoala ! Asp.Net Ajax adds an extra header tag for it's request. Just look for the 'x-microsoftajax' tag on HttpModule's BeginRequest event. If it is existing then it's an ajax request. 

if (Request.Headers["x-microsoftajax"] != null)
{
	//ajax request
}