调用方法时如何设置事件的期望值?

这就是我到目前为止所拥有的:

var _mock = MockRepository.GenerateMock<ITimer>();
//this is what I'd like to do but it doesn't work
Expect
    .Call(_mock.Start())
    .WhenCalled( () =>
        {
            if (_mock.Elapsed != null)
                _mock.Elapsed();
        });