c#:Why when I try to use coalesce operator in a mock it doesn't work

Question

Why when I am trying to use a mock like this:

public void Test(string param=null){
    MyMock.Setup(x=>x.foo(param ?? It.IsAny<string>));
 }

This obviously works fine

public void Test(string param=null){
   if(param==null)
      MyMock.Setup(x=>x.foo(It.IsAny<string>));
   else
      MyMock.Setup(x=>x.foo(param));
 }

But why is that? the "param?? It.IsAny" returns param or It.IsAny what am I missing here?

I saw this and this but I still don't get it.

Answer 1

When setting up Mock in Moq you actually pass not a value of function parameter (say, of tpye T ), but an Expression<T> , which is then analyzed by Moq .
So, when you pass value Moq will analyze it and setup expectation for method call with that value.
When you pass It.IsAny<T> Moq will again analyze it and setup expectation without any arguments.
But when you pass your param?? It.IsAny<string> param?? It.IsAny<string> Moq simply doesn't know how to analyze it.

Answer 2

MyMock.Setup(x=>x.foo(param==null ? It.IsAny<string>:param));