Multiple NSubstitute call configurations on method accessing different properties on a reference type parameter (to avoid NullReferenceException)

Question

I have the following test code using NSubstitute:

[TestMethod]
public void Test()
{
    var foo = Substitute.For<IFoo>();
    foo.Foo(Arg.Is<Bar>(b => !b.X)).Returns(0); // Line 1
    foo.Foo(Arg.Is<Bar>(b => b.X)).Returns(1); // Line 2
}

public interface IFoo
{
    int Foo(Bar b);
}

public class Bar
{
    public bool X;
}

When line 2 is executed, an exception is thrown:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

However, the exception is not thrown if I change !bX to b != null && !bX . It seems that the lambda expression in line 1 is being evaluated with a null lambda variable when line 2 is called.

My intention is to have more than one call configuration for the method I'm mocking. So, am I doing this wrong? Is there another way to do this?

Answer 1

The problem is that the last setup on a mocked member overrides any previous arrangements.

The desired behavior can be achieved with

//Arrange
var foo = Substitute.For<IFoo>();
foo.Foo(Arg.Any<Bar>()).Returns(args => args.Arg<Bar>().X ? 1 : 0); 

//...