using System;

​

public class A

{

    public string PropertyA

    {

        get; set;   

    }   

}

public class B : A

{

    public string PropertyB

    {

        get; set;   

    }   

}

public class C : B

{

    public string PropertyC

    {

        get; set;   

    }   

}

public class Program

{

    public static void Main()

    {

        A a = new A(){ PropertyA = "BASE"};

        B b = new B(){ PropertyB = "Class B"};

        C c = new C(){ PropertyC = "Class C"};

        //Error

        //C a1 = new A(){ PropertyA = "BASE"};

        //Error

        //C B1 = new B(){ PropertyB = "Class B"};

        C c1 = new C(){ PropertyC = "Class C"};

        //Error

        //B a2 = new A(){ PropertyA = "BASE"};

        B B2 = new B(){ PropertyB = "Class B"};

        B c2 = new C(){ PropertyC = "Class C"};     

        A a3 = new A(){ PropertyA = "BASE"};

        A B3 = new B(){ PropertyB = "Class B"};

        A c3 = new C(){ PropertyC = "Class C"}; 

        var aAsB = a as B;

        Console.WriteLine("A as B Posible?:" + (aAsB is not null));

        try

        {

            var aAsBErrorExecution = (B)a;

            Console.WriteLine("A as B Posible?:" + (aAsBErrorExecution is not null));

        }

        catch(InvalidCastException)

        {

            Console.WriteLine("EXCEPTION: A as B Posible?:NO");

        }

        var aAsC = a as C;

        Console.WriteLine("A as C Posible?:" + (aAsC is not  null));

        var bAsA = b as A;

        Console.WriteLine("B as A Posible?:" + (bAsA is  not null));

        var bAsC = b as C;

        Console.WriteLine("B as C Posible?:" + (bAsC is not null));

        var cAsA = c as A;

        Console.WriteLine("C as A Posible?:" + (cAsA is  not null));

        var cAsB = c as B;

        Console.WriteLine("B as B Posible?:" + (cAsB is not null));

        Console.WriteLine("***************");

        A bInA = MethodAB();

        Console.WriteLine("B in C Posible?:" + (bInA is B));

        A CInA = MethodAC();

        Console.WriteLine("C in A Posible?:" + (CInA is C));

    }

    public static A MethodAB()

    {

        return new B();

    }

    public static A MethodAC()

    {

        return new C();

    }

    //Error

    //public static B MethodBA()

    //{

        //return new A();

    //}

    public static B MethodBC()

    {

        return new C();

    }

    //public static C MethodCA()

    //{

        //return new A();

    //}

    //public static C MethodCB()

    //{

        //return new B();

    //}

}