今天在用HttpWebRequest類向一個遠端頁面post資料時，遇到了一個怪問題，總是出現500的內部伺服器錯誤，通過檢視遠端伺服器的log，發現報的是“無效的檢視狀態”錯誤：

通過對比自己post的__VIEWSTATE和伺服器接收到的__VIEWSTATE的值（通過伺服器的HttpApplication的BeginRequest事件可以取到Request裡的值），發現__VIEWSTATE中的一個+號被替換成了空格。（由於ViewState太長，這個差異也是仔細觀察了很久才看出來的）

造成這個錯誤的原因在於+號在url中是特殊字元，遠端伺服器在接受request的時候，把+轉成了空格。同樣的，如果想

修改後的post資料的示例程式碼如下，注意下面加粗的那句話：

public HttpWebResponse GetResponse(string url)

        {

            var req = (HttpWebRequest)WebRequest.Create(url);

            req.CookieContainer = CookieContainer;
 

if (Parameters.Count > 0)

            {

                req.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";

                req.ContentType = "application/x-www-form-urlencoded";

                req.Method = "POST";

//data要UrlEncode

                var postData = 
 
string.Join("&", Parameters.Select(

                                               p =>

string.Format("{0}={1}", p.Key,

                                                             System.Web.HttpUtility.UrlEncode(p.Value, Encoding))).ToArray());

                var data = Encoding.GetBytes(postData);

                req.ContentLength = data.Length;

using (var sw = req.GetRequestStream())

                {

                    sw.Write(data, 0, data.Length);

                }

            }

            req.Timeout = 40 * 1000;

            var response = (HttpWebResponse)req.GetResponse();

return response;

        }