If I want to mock a class that returns a string that is used to determine whether while loop should continue (imagine read while string != null), how can I set the expectation.
I'm not familiar with the syntax you're using. I would write this as:
r.ReadLine();
LastCall.Return("1,10,20").Repeat.Once();
r.ReadLine();
LastCall.Return(null).Repeat.Once();
To ensure that you're specifying the number of times that things are to be repeated. (Don't have Visual Studio to hand, syntax may not be exact.)