public interface IClass1
{
  void Info();
}

public class Class2 : IClass1
{
   public void Info(){//...
     System.Console.WriteLine("it's Class2!");
   }
}

public class Class3 : IClass1
{
   public void Info(){//...
     System.Console.WriteLine("it's a Class3!");
   }
}

public static void Main()
{
    IClass1 cls = new Class2();
    cls.Info();
    cls = new Class3();
    cls.Info();
}