Is it possible to determine whether the current request is an asynchronous postback (partial page update) from within the Application_Error event?
What's the best way to handle application errors when asynchronous postbacks are used?
In Application_Error, we are redirecting to different error pages but that doesn't work properly when the error is thrown during an asynchronous postback. We noticed that this holds true even when AllowCustomErrorsRedirect = false and we have an OnAsyncPostBackError handler to set the AsyncPostBackErrorMessage. During asynchronous postbacks, our AsyncPostBackErrorMessage is overwritten and the client receives a generic web page error instead.
In the Application_Error method you no longer have direct access to the <asp:ScriptManager> control on the page. So it's too late to handle its AsyncPostBackError event.
If you want to prevent a redirect, you should check the request to see if it is in fact an asynchronous request. The <asp:UpdatePanel> causes a post back with the following HTTP header:
X-MicrosoftAjax:Delta=true
(also see: ScriptManager Enables AJAX In Your Web Apps)
A check for this header would look something like this:
HttpRequest request = HttpContext.Current.Request;
string header = request.Headers["X-MicrosoftAjax"];
if(header != null && header == "Delta=true")
{
// This is an async postback
}
else
{
// Regular request
}
As to what would be an appropriate way to handle the exception is a different question imho.