I am sorry for the confusion. The Assert.xxxx functions do not return any value at all. As a result they cannot be used in IF statements as you're trying to do. The purpose of the Assert class is it will throw an exception (which will immediately abort the test) if the condition being tested is not true. In other words Assert.IsNotNull(s) will throw an exception and make the test immediately abort if s is null. Else the test will continue to run normally.
If you can explain what you're trying to accomplish with your code perhaps we can offer you a better solution to what you've attempted to code.
All the best,
the Telerik team