I've created an http handler and have a condition that requires I
throw a 404 message.  I want to use ASP.NET's 404 message, just incase
there is a custom 404 in the web.config I would like ASP.NET to handle
the 404.  So if there is no custom 404, it will be the basic IIS 404
page.  And if there is a custom 404 in the web.config, it will
redirect to that page.

I tried asking this question on asp.net's forums, but didn't like the
answer I received... here is the link
http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=762031

I need it to satisfy both conditions (custom 404 and non custom 404). 
Ideally, the functionality I want would be if I could set a flag to
ignore the request, but there doesn't seem to be one in IHttpHandler. 
So now I'm looking for another way.

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Request for Question Clarification by mathtalk-ga on 22 Dec 2004 05:36 PST

Hi, joelnet-ga:

Are you trying to "trap" handling of all 404 errors in an IIS
instance, or only those associated with requests for ASPX pages?

regards, mathtalk-ga

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Clarification of Question by joelnet-ga on 22 Dec 2004 13:42 PST

no, please read original post.

Reponse object is having a StatusCode property which is the HTTP error code itself.

So instead of redirecting the page using Response.Redirect()

Set the response error code as 404 and set the Error message also.
the code would be somewhat similar to : 

If ( your condition to check whether you need redirection) Then
Response.StatusCode = 404
Response.StatusDescription ="File Not found"

End If

note that Response.Redirect will stop processing the remainging code
in the page while this code snippet will not do that. So you need to
make sure that the remaining code is not processed by putting those
lines in the Else part of the condition.

There may be another way for doing this.. But I did it this way when
there was a need for this in one of my projects.

Ok, I believe the true answer is "it is not possible".