C# HttpWebRequest 呼叫 Web Service (ASMX) 完整範例：手工構造 SOAP 封包

雖然現代多用 WCF / Web API / gRPC，但維護舊系統時仍常需呼叫傳統 ASMX Web Service。若不想引入 Service Reference 產生的 Proxy 類別，可直接用 HttpWebRequest 手工發送 SOAP 訊息。本文示範完整流程：載入 XML 請求檔、設定 HTTP Header (SOAPAction)、發送並接收回應。

範例場景

• 目標服務：https://localhost/websrv/dsmp.asmx
• 方法：SyncOrderRelation (CMPP 3.0 訂購關係同步)
• 請求/回應皆為 XML (SOAP 1.1)

1. SOAP 請求 XML 範本 (SyncOrderRelationReq.xml)

  
    00110318384464
  
  
    
      1.5.0
      SyncOrderRelationReq
      
        0
        0011
      
      
        400
        0
      
      
        1
        13456781234
        
      
      
        1
        13456781234
        
      
      SP
      1
      1
      419000
      -YYXXYYXX
      3
      MTA2NjIxNDQgREE=
    
  

2. C# 呼叫程式碼

using System;
using System.IO;
using System.Net;
using System.Xml;

public class AsmxCaller
{
    public XmlDocument CallSyncOrderRelation(string requestXmlPath, string serviceUrl)
    {
        // 1. 載入請求 XML
        var requestDoc = new XmlDocument();
        requestDoc.Load(requestXmlPath);

        // 2. 序列化到 MemoryStream 取得位元組陣列
        using var ms = new MemoryStream();
        requestDoc.Save(ms);
        byte[] payload = ms.ToArray();

        // 3. 建立 HttpWebRequest
        var request = (HttpWebRequest)WebRequest.Create(serviceUrl);
        request.Method = "POST";
        request.ContentType = "text/xml; charset=utf-8";
        request.Headers.Add("SOAPAction", "\"sim.SyncOrderRelation\""); // 必須雙引號包裹
        request.Credentials = CredentialCache.DefaultCredentials; // 整合驗證
        request.ContentLength = payload.Length;
        request.Timeout = 30000; // 30 秒
        request.ReadWriteTimeout = 30000;

        // 4. 寫入請求流
        using (var reqStream = request.GetRequestStream())
        {
            reqStream.Write(payload, 0, payload.Length);
        }

        // 5. 取得回應並解析
        using var response = (HttpWebResponse)request.GetResponse();
        using var respStream = response.GetResponseStream();
        using var reader = new StreamReader(respStream, System.Text.Encoding.UTF8);
        string responseXml = reader.ReadToEnd();

        var responseDoc = new XmlDocument();
        responseDoc.LoadXml(responseXml);

        // 6. 可選：存檔留證
        responseDoc.Save("SyncOrderRelationResp.xml");

        return responseDoc;
    }
}

關鍵細節說明

現代替代方案

• HttpClient (.NET 4.5+)：HttpClient.PostAsync + StringContent，支援 async/await、連線池、更易測試。
• WCF Client Channel：ChannelFactory<T> 動態建立 Proxy，不需專案加入 Service Reference。
• SoapHttpClientProtocol (舊版) / Service Reference：Visual Studio 自動產生強型別 Proxy，開發效率最高但耦合度高。

HttpClient 版本範例

using var client = new HttpClient();
client.DefaultRequestHeaders.Add("SOAPAction", "\"sim.SyncOrderRelation\"");
var content = new StringContent(File.ReadAllText(requestXmlPath), Encoding.UTF8, "text/xml");
var response = await client.PostAsync(serviceUrl, content);
string responseXml = await response.Content.ReadAsStringAsync();