判断是否为数字

private bool isNumeric(string str)
{
         if (str == null || str.Length == 0)
             return false;
         System.Text.ASCIIEncoding ascii = new System.Text.ASCIIEncoding();
         byte[] bytestr = ascii.GetBytes(str);
         foreach (byte c in bytestr)
         {
             if (c < 48 || c > 57)
             {
                 return false;
             }
         }
         return true;
}