Stack data structure implementation
The code and the test look very good. I only found these nit-picks:


There's a typo in the method name Poppint.
The test sometimes uses Assert.IsTrue(a == b), which should be Assert.IsEqual(a, b).
The name underTop does not sound perfect to me, since it refers to a top, which is meaningless for a single node. A single node doesn't have a top, therefore I would call it next or below.
The property Size should be renamed to Count to align with the other collection types.
I prefer to have the same namespace for the main code and the testing code. But I don't know the official convention for organizing tests.