I found this question while looking to do exactly the same thing. Couldn’t find an answer anywhere, but after a bit more digging found that the .Net Framework has built in support for this.
You can register a factory object with
WebRequest.Create will call when using that prefix (or url). The factory object must implement
IWebRequestCreate which has a single method
Create which returns a
WebRequest. Here you can return your mock
I’ve put some sample code up at