using System;

​

public interface IAb

{

    int Prop1

    {

        get;

        set;

    }

}

​

public class A : IAb

{

    public int Prop1

    {

        get;

        set;

    }

}

​

public class B : A, IAb

{

    public string Prop2

    {

        get;

        set;

    }

}

​

public class Program

{

    static class MyMethods

    {

        public static IAb CreateObject(Type t)

        {

            if (t == typeof(A))

            {

                return new A()

                {Prop1 = 123};

            }

            else

            {

                return new B()

                {Prop1 = 456, Prop2 = "Foo"};

            }

        }

    }

​

    public void Main()

    {

        IAb AorB = MyMethods.CreateObject(typeof(B));

        Console.WriteLine(AorB.Prop1);

        if (AorB is B)

        {

            // fails

            // Console.WriteLine((B)AorB.Prop2); // 'IAb' does not contain a definition for 'Prop2'

            // works

            B newVar = (B)AorB;

            Console.WriteLine(newVar.Prop2);

        }

​

        Console.ReadLine();

    }

}