Is this you are trying to do?

    class A
    {
        public int MyName { get; set; }
    }

    class B : A
    {
        public int MyName1 { get; set; }
    }

    public class TestAB
    {
        public static void Test()
        {
            A objA = new A();
            B objB = new B();



            objB = (B)objA; // Unable to cast object of type
'AdvanceLib.A' to type 'AdvanceLib.B'.
            objA = objB;

        }

    }

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