验证字符串是否为纯数字

/// <summary>
        /// 验证是否为纯数字
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static bool IsNumeric(string str)
        {
            if (str == null || str.Length == 0)
                return false;
            ASCIIEncoding ascii = new ASCIIEncoding();
            byte[] bytestr = ascii.GetBytes(str);

            foreach (byte c in bytestr)
            {
                if (c < 48 || c > 57)
                {
                    return false;
                }
            }
            return true;
        }