This is a general recommendation if you're planning to use a tool like Pex in the future: make sure that preconditions (i.e. parameter validation) fails in a different fashion that other assertions.

Here's a snippet that shows the problem:

// don't do this
void Clone(ICloneable o) {
     Debug.Assert(o != null); // pre-condition
     ...
     object clone = o.Clone();
     Debug.Assert(clone); // assertion
}

Why is this bad?

A tool like Pex will explore your code and try to trigger every Debug.Assert it finds on its way. When the assertion is a precondition, it is likely expected and one would like to emit a negative test case (i.e. 'expected exception').

The problem in the snippet above is that both failure will yield to the same assertion exception and it will very difficult to *automatically* triage the failure as expected or not.

How do I fix this?

Make sure different classes of assertions can be differentiated automatically, through different exception types, tags in the message, etc...