C# 实例化接口对象的方法

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
  interface Itemp
  {
    double plus();
  }
  public class num : Itemp
  {
    double aa, bb;
    public num(double a, double b)
    {
      this.bb = b;
      this.aa = a;
    }
    public double plus()
    {
      return (aa * bb);
    }
  }
  class Program
  {
    static void Main(string[] args)
    {
      num n = null;//声明类对象引用
      Itemp tm = null;//声明接口对象引用
      tm = new num(1.1, 2.2);//接口回调(向上转型)
      Console.WriteLine(tm.plus());
      Console.ReadKey();
    }
  }
}