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掌握 System.Web.Abstractions：打造可測試、可擴充的 ASP.NET MVC 應用

編輯：關於ASP.NET

ASP.NET 內建 HttpContext、HttpRequest、HttpResponse 皆為具象密封類別，難以 Mock、難以擴充、綁死 Web 宿主環境。.NET 3.5 SP1 引入 System.Web.Abstractions.dll，提供 HttpContextBase、HttpRequestBase 等抽象基類與 HttpContextWrapper 適配器，徹底解耦框架與宿主，是 MVC 可測試性的基石。

核心類別架構

HttpContextBase (abstract)
    ├─ HttpContextWrapper : HttpContextBase  // 適配真實 HttpContext
    └─ MockHttpContext : HttpContextBase     // 測試用實作

HttpRequestBase  → HttpRequestWrapper / MockHttpRequest
HttpResponseBase → HttpResponseWrapper / MockHttpResponse
HttpSessionStateBase → HttpSessionStateWrapper / MockHttpSession
HttpServerUtilityBase → HttpServerUtilityWrapper / MockHttpServer

為何要避免直接使用具象類別？

具象類別缺點	抽象基類優勢
無法在單元測試中模擬	輕鬆建立 Mock/Stub 驗證邏輯
綁定 IIS/ASP.NET 管線	可在 Console App、Service、Azure Functions 執行
方法多為 non-virtual，無法覆寫	設計為可覆寫，支援擴充行為
靜態 `HttpContext.Current` 造成隱性依賴	透過建構子注入 `HttpContextBase` 明確相依

實戰：Kooboo CMS 替換請求上下文

需求：在路由解析之前修改請求環境（如重寫路徑、注入租戶資訊）。

1. 攔截點分析

UrlRoutingModule.PostResolveRequestCache 為路由解析入口，首先建立 HttpContextWrapper：

private void OnApplicationPostResolveRequestCache(object sender, EventArgs e)
{
    var context = new HttpContextWrapper(((HttpApplication)sender).Context);
    this.PostResolveRequestCache(context); // 此處開始路由解析
}

2. 自訂 HttpModule 提前介入

public class ContextReplacementModule : IHttpModule
{
    public void Init(HttpApplication app)
    {
        // 比 UrlRoutingModule 更早：PreRequestHandlerExecute
        app.PreRequestHandlerExecute += OnPreRequestHandlerExecute;
    }

    private void OnPreRequestHandlerExecute(object sender, EventArgs e)
    {
        var app = (HttpApplication)sender;
        var originalContext = app.Context;
        
        // 建立自訂 HttpContextBase 實作
        var customContext = new CustomHttpContext(originalContext);
        
        // 替換內部 Context（需反射或繼承 HttpApplication）
        // 此處示意：實際需透過 HttpApplication.Context 属性設定器
        // 或實作自訂 HttpApplication 類別
    }
}

3. 自訂 HttpContextBase 實作


public class CustomHttpContext : HttpContextBase
{
    private readonly HttpContext _inner;
    private HttpRequestBase _request;
    private HttpResponseBase _response;

    public CustomHttpContext(HttpContext inner) { _inner = inner; }

    public override HttpRequestBase Request 
    { 
        get { return _request ?? (_request = new CustomHttpRequest(_inner.Request)); } 
    }

    public override HttpResponseBase Response 
    { 
        get { return _response ?? (_response = new CustomHttpResponse(_inner.Response)); } 
    }

    // 其他屬性直接委派
    public override IDictionary Items => _inner.Items;
    public override IPrincipal User => _inner.User;
    // ...
}

public class CustomHttpRequest : HttpRequestBase
{
    private readonly HttpRequest _inner;
    public CustomHttpRequest(HttpRequest inner) { _inner = inner; }

    public override string Path 
    { 
        get => _inner.Path; 
        // 可攔截修改：return ModifiedPath; 
    }
    
    public override NameValueCollection QueryString => _inner.QueryString;
    // ...
}

Controller 中正確使用抽象類別


public class ProductController : Controller
{
    // 悪例：直接用 HttpContext.Current
    public ActionResult Bad() 
    {
        var url = HttpContext.Current.Request.Url; // 難測試
        return View();
    }

    // 佳例：使用基類屬性（內部已為 HttpContextBase）
    public ActionResult Good() 
    {
        var url = Request.Url; // Controller.Request 為 HttpRequestBase
        var userAgent = Request.UserAgent;
        return View();
    }

    // 最佳：建構子注入（適用於非 Controller 服務類別）
    public class ProductService
    {
        private readonly HttpContextBase _context;
        public ProductService(HttpContextBase context) { _context = context; }
        
        public string GetClientIp() => _context.Request.UserHostAddress;
    }
}

單元測試範例


[Test]
public void ProductService_GetClientIp_ReturnsCorrectIp()
{
    // Arrange
    var mockContext = new Mock();
    var mockRequest = new Mock();
    mockRequest.Setup(r => r.UserHostAddress).Returns("192.168.1.100");
    mockContext.Setup(c => c.Request).Returns(mockRequest.Object);

    var service = new ProductService(mockContext.Object);

    // Act
    var ip = service.GetClientIp();

    // Assert
    Assert.AreEqual("192.168.1.100", ip);
}