asp.net web api - Exception in ExceptionFilterAttribute is always of type HttpResponseException -

following this post on exception handling in asp.net web api, created exceptionfilterattribute looks this:

public class notimplexceptionfilterattribute : exceptionfilterattribute {     public override void onexception(httpactionexecutedcontext context)     {         if (context.exception notimplementedexception)         {             context.response = new httpresponsemessage(httpstatuscode.notimplemented);         }     } } 

... , asp.net web api controller looks this:

public class productscontroller : apicontroller {     [notimplexceptionfilter]     public string get()     {         throw new notimplementedexception("this method not implemented");     } } 

the problem context.exception always of type system.web.http.httpresponseexception no matter exception throw in actionmethod. such, code inside if statement never gets executed. message property on exception is:

processing of http request resulted in exception. please see http response returned 'response' property of exception details.

the response property referenced in message 500 internal server error.

my routes configured correctly because actionmethod hit. don't have other exceptionfilterattribute filters applied.

any idea how can context.exception reference exception thrown?