I have following class:

```
public class PairOfDice
{
    private Dice d1,d2;
    public int Value 
    {
       get { return d1.Value + d2.Value; }
    }
}

```

Now I would like to use a `PairOfDice` in my test which returns the value 1, although I use random values in my real dice:

```
[Test]
public void DoOneStep ()
{
    var mock = new Mock<PairOfDice>();
    mock.Setup(x => x.Value).Return(2);
    PairOfDice d = mock.Object;
    Assert.AreEqual(1, d.Value);
}

```

Unfortunately I get a `Invalid setup on non-overridable member` error. What can I do in this situation? 

Please note, that this is my first try to implement Unit-Tests.