public static void Main()
{
IMyInterface im;
Plus p = new Plus();
Minus m = new Minus();
im = p;
Console.WriteLine("im.Calc = {0}", im.Calc(3, 5));
Console.WriteLine("im.Calc = {0}", p.Calc(3, 5));
im = m;
Console.WriteLine("im.Calc = {0}", im.Calc(3, 5));
Console.WriteLine("im.Calc = {0}", m.Calc(3, 5));
}