How to mock a private static inner class?

Question

I'd like to ask a question which bother me a lot...

How can I mock the return value of MyPublicStaticMethod_C?? MyPrivateStaticClass_B is really private and static

public static partial class MyPublicStaticClass_A
{
    private static partial class MyPrivateStaticClass_B
    {
        public static int MyPublicStaticMethod_C(string para)
        {
            //...
        }
        //...
    }
}

Answer 1

You must reference the private type through the reflection API using GetNestedType(). Then you can use the non-public mocking API to arrange the method. Here's a working example:

    public static partial class MyPublicStaticClass_A
    {
        public static int Test(string str)
        {
            return MyPrivateStaticClass_B.MyPublicStaticMethod_C(str);
        }

        private static partial class MyPrivateStaticClass_B
        {
            public static int MyPublicStaticMethod_C(string para)
            {
                return 1;
            }
        }
    }

    [TestMethod]
    public void ShouldArrangeInnerPrivateClassMethod()
    {
        var privateType = typeof(MyPublicStaticClass_A).GetNestedType("MyPrivateStaticClass_B", BindingFlags.NonPublic);
        Mock.NonPublic.Arrange<int>(privateType, "MyPublicStaticMethod_C").Returns(5);
        var result = MyPublicStaticClass_A.Test(null);
        Assert.Equal(5, result);
    }

But, yeah, having such code should be a last resort - only if refactoring is impossible or unfeasible.