C#截取字符串长度(整齐划一)的代码

/// <summary>
/// 截取字符串函数
/// </summary>
/// <param name="Str">所要截取的字符串</param>
/// <param name="Num">截取字符串的长度</param>
/// <returns></returns>
public static string GetSubString(string Str, int Num)
{
  if (Str == null || Str == "")
       return "";
       string outstr = "";
       int n = 0;
       foreach (char ch in Str)
       {
        n += System.Text.Encoding.Default.GetByteCount(ch.ToString());
        if (n > Num)
             break;
        else
             outstr += ch;
        }
     return outstr;
}