字符串转化成为utf8 Encoding (c#)

private string Convert(String str)  {    char[] hexDigits = {         '0', '1', '2', '3', '4', '5', '6', '7',            '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};

   Encoding utf8 = Encoding.UTF8;

   StringBuilder result = new StringBuilder();

   for (int i=0;i<str.Length;i++)   {    string sub = str.Substring(i,1);    byte[] bytes = utf8.GetBytes(sub);

    if (bytes.Length == 1) //普通英文字母或数字    {     result.Append(sub);    }    else  //其它字符，转换成为编码    {     for (int j=0;j<bytes.Length;j++)     {      result.Append("%"+hexDigits[bytes[j]>>4]+hexDigits[bytes[j] & 0XF]);     }    }   }      return result.ToString();  }