显示的金额进行格式化千分位逗号隔开NET

public string FormatAmount(decimal? amount = null)
      {
          amount = amount ?? Amount;

          var s = amount.ToString();
          if (s.Length < 2)
          {
              return s;
          }

          var len = s.IndexOf('.');
          if (len < 0)
          {
              len = s.Length;
          }
          if (len < 4)
          {
              return s;
          }

          for (var i = len-3; i > 0; i -= 4)
          {
              s = s.Insert(i, ",");
              i++;
          }
          return s;
      }