HTTP_X_FORWARDED_FOR未将对象引用设置到对象的实例，请问怎么改？

public string IPAddress
        {
            get
            {
                string result =String.Empty;

                result = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
                if (result != null && result != String.Empty)
                {

                    if (result.IndexOf(".") == -1)
                        result = null;
                    else
                    {
                        if (result.IndexOf(",") != -1)
                        {
                               result = result.Replace(" ", "").Replace("'", "");
                            string[] temparyip = result.Split(",;".ToCharArray());
                            for (int i = 0; i < temparyip.Length; i++)
                            {
                                if (this.IsIPAddress(temparyip[i]) && temparyip[i].Substring(0, 3) != "10." && temparyip[i].Substring(0, 7) != "192.168" && temparyip[i].Substring(0, 7) != "172.16.")
                                {
                                    return temparyip[i];
                                }
                            }
                        }
                        else if (this.IsIPAddress(result))
                            return result;
                        else
                            result = null;
                    }

                }

                string IpAddress = (HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] != null && HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] != String.Empty) ? HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] : HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];

                if (null == result || result == String.Empty)
                    result = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];

                if (result == null || result == String.Empty)
                    result = HttpContext.Current.Request.UserHostAddress;

                return result;
            }
        }

        public bool IsIPAddress(string str1)
        {
            if (str1 == null || str1 == string.Empty || str1.Length < 7 || str1.Length > 15) return false;

            string regformat = @"^\d{1,3}[\.]\d{1,3}[\.]\d{1,3}[\.]\d{1,3}$";

            Regex regex = new Regex(regformat, RegexOptions.IgnoreCase);
            return regex.IsMatch(str1);
        }

vs2005提示：result = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];未将对象引用设置到对象的实例，请问怎么改？ --------------------编程问答-------------------- 确定这一行错了？ --------------------编程问答-------------------- up --------------------编程问答-------------------- 是的，我也遇到同样的问题：
HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];上面这一行运行时提示：未将对象引用到实例求解决，谢谢、

补充：.NET技术 ,  C#