using System;

​

public class A {}

public class B {}

public class C {}

public class Program

{

    public static void Main()

    {

        Process(new A());

        Process(new B());

        Process(new C());       

    }

    public static void Process(A parameter) => Console.WriteLine("A");  

    public static void Process(B parameter) => Console.WriteLine("B");  

    public static void Process(object parameter) => throw new NotSupportedException();

}