// 4
      double d = 5.214;
      double res = Math.Round(d, 2, MidpointRounding.ToEven);
      Console.WriteLine(res);//5.21

      //6
      d = 5.216;
      res = Math.Round(d, 2, MidpointRounding.ToEven);
      Console.WriteLine(res);//5.22

      //5
      d = 5.215;
      res = Math.Round(d, 2, MidpointRounding.ToEven);
      Console.WriteLine(res);//5.22
      d = 5.225;
      res = Math.Round(d, 2, MidpointRounding.ToEven);
      Console.WriteLine(res);//5.22




      //不止小数点后3位时
      d = 0.7865666;
      res = Math.Round(d, 2, MidpointRounding.ToEven);
      Console.WriteLine(res);//0.79

      d = 0.786;
      res = Math.Round(d, 2, MidpointRounding.ToEven);
      Console.WriteLine(res);//0.79

      d = 0.785;
      res = Math.Round(d, 2, MidpointRounding.ToEven);
      Console.WriteLine(res);//0.78