class X
{
    public virtual void Blah() {
        Console.WriteLine("X.Blah()");
    }
}

class Y : X
{
    public string message;

    public override void Blah() {
        Console.WriteLine(message);
    }
}

interface HasBlah
{
    void Blah(object child);
}

class Demo<T> : HasBlah
{
    public void Blah(T child) {
        child.Blah();
    }

    public override void Blah(object child) {
        Blah(child as T); // This throws an exception if child can't be cast to T
    }
}

void ExampleFailure() {
    X x = new X();
    Y y = new Y();
    y.message = "y.message";

    HasBlah demoX = new Demo<X>();
    HasBlah demoY = new Demo<Y>();

    demoX.Blah(x); // outputs "X.Blah()"
    demoY.Blah(y); // outputs "y.message"
    demoY.Blah(x); // throws an exception
}