moq- why can't operate function with default parameter - unit-testing

Question

I am trying to make a unit test on a very simple interface.
my interface is:
public interface Interface1
{
string retStr(string dd);
string retStr2(string dd,string fff);
}
this is the mock:
var myMoq = new Mock<Interface1>();
myMoq.Setup(d => d.retStr("David")).Returns("retStr");
Console.WriteLine(myMoq.Object.retStr("fdf").ToString());
I GOT runtime error: Object reference not set to an instance of an object.
and another error on implementation:
myMoq.Setup(d => d.retStr2(It.Is<string>(e=>e=="qqq"), It.IsAny<string>())).Returns("2 parameters");
Console.WriteLine(myMoq.Object.retStr2("fdf","wewew").ToString());
Why is it?

Answer 1

In your setup, you are setting the expectation that a specific string will be passed in (for example "David").
You are telling Moq, "Pass back "retStr" if the method invoked with the string "David", otherwise return a default value (for string, null). Because of this, when you do a .ToString() on the result of the method, the object is null.
The same thing applies to the second example.
In order to make a more general return value, use It.IsAny<string>() when setting up a method. Or, do as you expect in the test and send in "David" when you call the method.