Question

I am wondering if someone can please explain this to me:

class Program
{
    static void Main()
    {
        AnotherDerivedClass d = new AnotherDerivedClass();
        Console.WriteLine(d.PrintMessage());

        IMsg m = d as IMsg;
        //Why this prints BaseClass.
        //How does it know that IMsg is implemented in the BaseClass.
        Console.WriteLine(m.PrintMessage());

        IMsg n = d as DerivedClass;
        //Why this prints BaseClass and not DerivedClass
        Console.WriteLine(n.PrintMessage());

        Console.Read();
    }
}

public interface IMsg
{
    string PrintMessage();
}

public class BaseClass : IMsg
{
    public string PrintMessage()
    {
        return "BaseClass";
    }
}

public class DerivedClass : BaseClass
{
    public new string PrintMessage()
    {
        return "DerivedClass";
    }
}

public class AnotherDerivedClass : DerivedClass
{
    public new string PrintMessage()
    {
        return "AnotherDerivedClass";
    }
}

Answer 1

You have replaced the implementation in your derived classes, not overridden them. If you use the BaseClass, the original implementation will be used.

You need to make the method in the base virtual:

public class BaseClass : IMsg
{

    public BaseClass()
    {

    }

    public virtual string PrintMessage()
    {
        return "BaseClass";
    }
}

and override in the derived class:

public class DerivedClass : BaseClass
{
    public DerivedClass()
    {

    }

    public override string PrintMessage()
    {
        return "DerivedClass";
    }
}

to get the behaviour you specified.