语法问题,大家看看哪里出问题了

C# code

using System;class Hex{private static string str_key;static string decode(string str){    string str2 = str_key;    string str3 = "";    int num5 = 0;    if (str == "")    {        return "";    }    int length = str2.Length;    if (length == 0)    {        str2 = "Think Space";    }    int num2 = 0;    int num3 = Convert.ToInt32(str.Substring(0, 2), 0x10);    int startIndex = 2;    while (true)    {        try        {            num5 = Convert.ToInt32(str.Substring(startIndex, 2), 0x10);        }        catch (Exception)        {        }        int num6 = num5 ^ Convert.ToInt32(str2[num2]);        if (num6 <= num3)        {            num6 = (0xff + num6) - num3;        }        else        {            num6 -= num3;        }        str3 = str3 + Convert.ToChar(num6);        num3 = num5;        startIndex += 2;        if (num2 < length)        {            num2++;        }        else        {            num2 = 1;        }        if (startIndex >= str.Length)        {            return str3;        }    }}static void Main()    {    Console.WriteLine(decode("870098489"));    }}

[解决办法]
string str2 = str_key;
=>
string str2 = str;