If I may contribute a minor enhancement, you can implement it this way :

listener
.Expect(a => a.BeginAcceptTcpClient(null, null))
.IgnoreArguments()
.Return(asyncResult)
.WhenCalled
(
invocation =>
{
var callbackIndex = invocation.Arguments.Length – 2;
var callback = invocation.Arguments[callbackIndex] as AsyncCallback;
callback(asyncResult);
}
);

You may even take it a step further and create an extension method for it that wraps all this up neatly, so that you can call it as follows :

listener
.Expect(a => a.BeginAcceptTcpClient(null, null))
.IgnoreArguments()
.ReturnAsync(asyncResult)

ReturnAsync is the extension method and can implement both the Return and WhenCalled logic; I didn’t put the code here because it’s so trivial anyone can figure it out.

Thanks again for a great article, and I hope my contribution helps!