public static class DecimalExtension
  {
    /// <summary>
    /// decimal保留指定位数小数
    /// </summary>
    /// <param name="num">原始数量</param>
    /// <param name="scale">保留小数位数</param>
    /// <returns>截取指定小数位数后的数量字符串</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();
      }
    }
  }