urlencode，urldecode详解

urlencode ：将目标字符串按照特定的编码（比如utf-8、gb2312) 解码成byte数组，

然后对除这些字符集（a-z,A-Z,\,(,),*,-,.,_,!） 之外的所有字节（8位）转换成%加16进制字符，其中空格特殊处理将转换成+号。

比如？号，十进制为63，二进制为11 1111 ，经过urlencode后变为%3f

具体算法如下：（reflector编译的）

/// <summary>
        /// 10进制转16进制( 10->a ,11->b 
        /// </summary>
        /// <param name="n"></param>
        /// <returns></returns>
        internal static char IntToHex(int n)
        {
            if (n <= 9)
            {
                return (char)(n + 0x30);
            }
            return (char)((n - 10) + 0x61);
        }
        private static int HexToInt(char h)
        {
            if ((h >= '0') && (h <= '9'))
            {
                return (h - '0');
            }
            if ((h >= 'a') && (h <= 'f'))
            {
                return ((h - 'a') + 10);
            }
            if ((h >= 'A') && (h <= 'F'))
            {
                return ((h - 'A') + 10);
            }
            return -1;
        }

        private static byte[] UrlDecodeBytesFromBytesInternal(byte[] buf, int offset, int count)
        {
            int length = 0;
            byte[] sourceArray = new byte[count];
            for (int i = 0; i < count; i++)
            {
                int index = offset + i;
                byte num4 = buf[index];
                if (num4 == 0x2b)//+
                {
                    num4 = 0x20;//空格
                }
                else if ((num4 == 0x25) && (i < (count - 2)))//0x25=% 
                {
                    int num5 = HexToInt((char)buf[index + 1]);//高4位
                    int num6 = HexToInt((char)buf[index + 2]);//低4位
                    if ((num5 >= 0) && (num6 >= 0))
                    {
                        num4 = (byte)((num5 << 4) | num6);
                        i += 2;
                    }
                }
                sourceArray[length++] = num4;
            }
            if (length < sourceArray.Length)
            {
                byte[] destinationArray = new byte[length];
                Array.Copy(sourceArray, destinationArray, length);
                sourceArray = destinationArray;
            }
            return sourceArray;
        }



        private static byte[] UrlEncodeBytesToBytesInternal(byte[] bytes, int offset, int count, bool alwaysCreateReturnValue)
        {
            int num = 0;
            int num2 = 0;
            for (int i = 0; i < count; i++)
            {
                char ch = (char)bytes[offset + i];
                if (ch == ' ')
                {
                    num++;
                }
                else if (!IsSafe(ch))
                {
                    num2++;
                }
            }
            if ((!alwaysCreateReturnValue && (num == 0)) && (num2 == 0))
            {
                return bytes;
            }
            byte[] buffer = new byte[count + (num2 * 2)];
            int num4 = 0;
            for (int j = 0; j < count; j++)
            {
                byte num6 = bytes[offset + j];
                char ch2 = (char)num6;
                if (IsSafe(ch2))
                {
                    buffer[num4++] = num6;
                }
                else if (ch2 == ' ')
                {
                    buffer[num4++] = 0x2b;//+
                }
                else
                {
                    buffer[num4++] = 0x25;//%
                    buffer[num4++] = (byte)IntToHex((num6 >> 4) & 15);//高4位
                    buffer[num4++] = (byte)IntToHex(num6 & 15);//1111  与15与 取低4位
                }
            }
            return buffer;
        }