.net decimal保留指定的小數位數(不四舍五入)

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();
 }
 }
 }