Response.Redirect() Turns Response.RedirectLocation NULL - asp.net

Question

I have a group of Asp.Net applications that all share a common HttpModule that contains some code like this:
public class MyModule : IHttpModule
{
public void Dispose()
{
}
public void Init( HttpApplication context )
{
context.Error += new EventHandler( context_Error );
}
private void context_Error( object sender, EventArgs e )
{
var lastError = HttpContext.Current.Server.GetLastError();
doSomeStuffWithError(lastError);
var response = HttpContext.Current.Response;
var redirect = GetErrorRedirect(lastError);
response.Redirect(redirect, true);
}
}
This works totally fine for all of my applications except for one. In the case of the one that doesn't work correctly, response.Redirect(...) doesn't seem to work. Instead of the redirect I expect, Asp.Net is redirecting to its standard error page. I've checked the configuration of this application and don't see anything wrong or significantly different from the other applications.
While investigating this issue, I modified the error handler with one more line of code as follows:
private void context_Error( object sender, EventArgs e )
{
var lastError = HttpContext.Current.Server.GetLastError();
doSomeStuffWithError(lastError);
var response = HttpContext.Current.Response;
var redirect = GetErrorRedirect(lastError);
//setting a break point here, i've verified that 'redirect' has a value in all cases
response.Redirect(redirect, true);
var wtf = response.RedirectLocation;
//inspecting the value of 'wtf' here shows that it is null for one application, but equal to 'redirect' in all others.
}
When I set a break point on 'wtf' I'm seeing some strange behavior. For applications that work, wtf contains the same value as redirect. However, for my app that isn't working, wtf is null.
Anyone have any ideas on what would cause wtf to be null in this way?

Answer 1

The overload of Response.Redirect you are using will call Response.End and throw a ThreadAbortException. It says so in the documentation. So, the fact that "it works" in other applications is interesting as it should never execute var wtf = response.RedirectLocation; During a debugging session, it is not surprising that it's null, either, as there is likely some reason that it allows to execute that line during debug.
In addition, it will of course execute the default error page if you have the mode set to On or RemoteOnly for the <customErrors> setting in Web.config unless you clear the error before redirecting. This is by design.
If you need to execute additional code after you have already called Response.Redirect, pass false as the second parameter to avoid the Response.End call and clear the error using HttpContext.Current.ClearError().
Based on your example, I would rewrite your HttpModule like so:
public class MyModule : IHttpModule
{
public void Dispose()
{
}
public void Init( HttpApplication context )
{
context.Error += new EventHandler( context_Error );
}
private void context_Error( object sender, EventArgs e )
{
var context = HttpContext.Current;
var lastError = context.Server.GetLastError();
doSomeStuffWithError(lastError);
var response = context.Response;
var redirect = GetErrorRedirect(lastError);
context.ClearError();
// pass true if execution must stop here
response.Redirect(redirect, false);
// do other stuff here if you pass false in redirect
}
}