FakeConcreteClass

Fake.cs

public class TestingAnAbstractClassWithAFake
{

    private class FakeConcreteInstance : MyAbstractClass
    {}


    [Fact]
    public void Should_return_correct_message()
    {
        var fake = new FakeConcreteInstance();
        var result = fake.MyMethodOnAbstractClass("fish");
        Assert.Equal("I like fish in an abstract sense.", result);
    }
}

public class MyAbstractClass
{
    public string MyMethodOnAbstractClass(string input)
    {
        return "I like " + input + " in an abstract sense.";
    }
}