【茶包射手專欄】Cassini's Response Header Encoding

先前有一篇文章討論ASP.NET如何正確傳回中文下載檔名，網友帆歷經九九八十一難後，終於修成正果，還揪出一隻鬼 --- VS 2005內建的ASP.NET Development Server似乎不支援HeaderEncoding!

為了解開這個謎團，茶包射手再次整裝出發!

這回我們使用的辦案工具是HttpWatch Pro(原因是在我的Vista上Fiddler啟動時的UAC升等視窗都會被埋掉，啟動後會導致IE7 Hang住，最要命的是它抓不到我存取Localhost的記錄，怕明明要射茶包變成修理小提琴，我很有魄力地立刻換了武器)，用HttpWatch的錄製封包功能，Dev Server與IIS7的Header差異馬上現形!

ASP.NET Development Server

HTTP/1.1 200 OK
Server: ASP.NET Development Server/9.0.0.0
Date: Sat, 08 Sep 2007 08:07:38 GMT
X-AspNet-Version: 2.0.50727
content-disposition: attachment;filename=銝剜?瑼?.xls
Cache-Control: private
Content-Type: application/octet-stream
Content-Length: 16384
Connection: Close

IIS 7

HTTP/1.1 200 OK
Cache-Control: private
Content-Length: 16384
Content-Type: application/octet-stream
Server: Microsoft-IIS/7.0
X-AspNet-Version: 2.0.50727
content-disposition: attachment;filename=中文檔名.xls
X-Powered-By: ASP.NET
Date: Sat, 08 Sep 2007 08:08:21 GMT

我發現ASP.NET Dev Server傳回的Header中，明明要求用Big5 Encoding，但傳回的卻是亂碼，而依多年來跟中文亂碼打交道的經驗，直覺上像是UTF-8被解成Big5(長度變長，而且有問號)。我寫了兩行Code驗證! Bingo! 就是UTF-8被硬轉成Big5。

由這個現象來推斷，ASP.NET Dev Server並未依我們的要求，在Header區使用Big5 Encoding，而是用UTF-8囉?

要確證這一點，得靠Source Code，但沒有ASP.NET Dev Server的Code怎麼辦? 用Reflector? 可以。但我想到另一招，ASP.NET Dev Server的前身是一個Sample專案Cassini，所以我們可以找它爸爸驗一下DNA吧!

在我的機器上把Cassini Sample Web Server架起來，第一件事先確定是不是有其父必有其子? 下載中文檔名也會失敗?

答案是!!! Cassini傳回的Response Header與ASP.NET Dev Server除了Server Name外，完全相同。

Cassini

HTTP/1.1 200 OK
Server: Microsoft-Cassini/1.0.0.0
Date: Sat, 08 Sep 2007 08:02:56 GMT
X-AspNet-Version: 2.0.50727
content-disposition: attachment;filename=銝剜?瑼?.xls
Cache-Control: private
Content-Type: application/octet-stream
Content-Length: 16384
Connection: Close

追蹤了程式碼，我在Cassini.Connection Class中找到了答案，在Cassini的邏輯中，並不會因為指定Response.HeaderEncoding而變更編碼方式，一律用UTF-8解成byte[]後送至Socket，而在IE端則是以Big5去解析它，符合先前的實驗與分析。Case Closed!

private static String MakeResponseHeaders(

int statusCode, String moreHeaders, int contentLength, 

bool keepAlive) {

    StringBuilder sb = new StringBuilder();

    sb.Append("HTTP/1.1 " + statusCode + " " + 

     HttpWorkerRequest.GetStatusDescription(statusCode) + "\r\n");

    sb.Append("Server: Microsoft-Cassini/"

        +Messages.VersionString+"\r\n");

    sb.Append("Date: " + DateTime.Now.ToUniversalTime().

        ToString("R", DateTimeFormatInfo.InvariantInfo) + "\r\n");

    if (contentLength >= 0)

        sb.Append("Content-Length: " + contentLength + "\r\n");

    if (moreHeaders != null)

        sb.Append(moreHeaders);

    if (!keepAlive)

        sb.Append("Connection: Close\r\n");

    sb.Append("\r\n");

    return sb.ToString();

}

public void WriteEntireResponseFromString(

int statusCode, String extraHeaders, String body, 

bool keepAlive) {

    try {

        int bodyLength = (body != null) ? 

            Encoding.UTF8.GetByteCount(body) : 0;

        String headers = 

            MakeResponseHeaders(statusCode, extraHeaders, 

              bodyLength, keepAlive);

        _socket.Send(Encoding.UTF8.GetBytes(headers + body));

    }

    finally {

        if (!keepAlive)

            Close();

    }

}