public static class DecimalExtension  {    /// <summary>    /// decimal保留指定位數小數    /// </summary>    /// <param name="num">原始數量</param>    /// <param name="scale">保留小數位數</param>    /// <returns>截取指定小數位數后的數量字符串</returns>    public static string ToString(this decimal num, int scale)    {      string numToString = num.ToString();      int index = numToString.IndexOf(".");      int length = numToString.Length;      if (index != -1)      {        return string.Format("{0}.{1}",          numToString.Substring(0, index),          numToString.Substring(index + 1, Math.Min(length - index - 1, scale)));      }      else      {        return num.ToString();      }    }  }