C# HttpWebRequest 完整指南：POST/GET、SOAP、串流處理與錯誤處理

基本請求流程

var request = (HttpWebRequest)WebRequest.Create(url);
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
request.Credentials = CredentialCache.DefaultCredentials;

byte[] payload = Encoding.UTF8.GetBytes(postData);
request.ContentLength = payload.Length;

using (var reqStream = request.GetRequestStream())
    reqStream.Write(payload, 0, payload.Length);

using (var response = (HttpWebResponse)request.GetResponse())
using (var respStream = response.GetResponseStream())
using (var reader = new StreamReader(respStream, Encoding.UTF8)) {
    string result = reader.ReadToEnd();
    // 處理結果
}

關鍵屬性與設定

SOAP 請求範例

var req = (HttpWebRequest)WebRequest.Create(soapEndpoint);
req.Method = "POST";
req.ContentType = "text/xml; charset=utf-8";
req.Headers.Add("SOAPAction", "https://tempuri.org/MyMethod");

var soapXml = @"...";
byte[] body = Encoding.UTF8.GetBytes(soapXml);
req.ContentLength = body.Length;

using (var stm = req.GetRequestStream()) stm.Write(body, 0, body.Length);

using (var resp = (HttpWebResponse)req.GetResponse())
using (var stm = resp.GetResponseStream())
using (var reader = new StreamReader(stm, Encoding.UTF8)) {
    var soapResponse = reader.ReadToEnd();
    // 解析 XML 回應
}

串流儲存二進位檔案

using (var resp = (HttpWebResponse)request.GetResponse())
using (var respStream = resp.GetResponseStream())
using (var fileStream = File.Create(localPath)) {
    var buffer = new byte[8192];
    int read;
    while ((read = respStream.Read(buffer, 0, buffer.Length)) > 0)
        fileStream.Write(buffer, 0, read);
}

錯誤處理與重試策略

• 捕獲 WebException，檢查 Response 屬性取得 HTTP 狀態碼
• 網路暫時性錯誤（5xx、408、Timeout）實作指數退避重試（Polly 套件）
• 驗證 HttpWebResponse.StatusCode == HttpStatusCode.OK 才處理內容
• 釋放回應串流：using 確保 GetResponseStream() 被正確關閉，避免連線洩漏

現代替代：HttpClient

.NET Core / .NET 5+ 建議使用 HttpClient（單例模式、非同步優先、自動解壓縮、支援 HttpRequestMessage 完整控制）。HttpWebRequest 仍適用於 .NET Framework 老專案維護。

ASP 經典 XMLHTTP 參考

舊版 ASP 使用 Server.CreateObject("MSXML2.XMLHTTP") 同步發送，現已淘汰，僅供維舊參考。