ASP.NET 依赖注入 HTTP 模块（MS 企业库）

Question

我一直按照“Microsoft Enterprise Library 5.0”文档中的步骤创建一个 HTTP 模块，以将 Enterprise Library 容器的引用注入到 ASP.NET Web 应用程序的页面中。

它包含以下代码（也出现在网上here）：

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using Microsoft.Practices.Unity;

namespace Unity.Web
{
  public class UnityHttpModule : IHttpModule
  {
    public void Init(HttpApplication context)
    {
      context.PreRequestHandlerExecute += OnPreRequestHandlerExecute;
    }

    public void Dispose() { }

    private void OnPreRequestHandlerExecute(object sender, EventArgs e)
    {
      IHttpHandler currentHandler = HttpContext.Current.Handler;
      HttpContext.Current.Application.GetContainer().BuildUp(
                          currentHandler.GetType(), currentHandler);

      // User Controls are ready to be built up after page initialization is complete
      var currentPage = HttpContext.Current.Handler as Page;
      if (currentPage != null)
      {
        currentPage.InitComplete += OnPageInitComplete;
      }
    }

    // Build up each control in the page's control tree
    private void OnPageInitComplete(object sender, EventArgs e)
    {
      var currentPage = (Page)sender;
      IUnityContainer container = HttpContext.Current.Application.GetContainer();
      foreach (Control c in GetControlTree(currentPage))
      {
        container.BuildUp(c.GetType(), c);
      }
      context.PreRequestHandlerExecute -= OnPreRequestHandlerExecute;
    }

    // Get the controls in the page's control tree excluding the page itself
    private IEnumerable<Control> GetControlTree(Control root)
    {
      foreach (Control child in root.Controls)
      {
        yield return child;
        foreach (Control c in GetControlTree(child))
        {
          yield return c;
        }
      }
    }
  }
}

此代码及其附带的说明存在许多问题。

1) 说明中没有提到该代码的放置位置。由于它是一个类，所以我将它放在我的 ASP.NET 网站项目的 App_Code 文件夹中。

其实，下面是这段代码的说明：

创建一个新的 ASP.NET HTTP 模块类（例如，命名为 UnityHttpModule ) 在您的项目中捕获 PreRequestHandlerExecute 事件并执行遍历 当前页面请求的完整控制树，应用Unity 每个控件的 BuildUp 方法。

2) HttpContext.Current.Application.GetContainer() 方法对我来说不存在，即使我使用了相同的 DLL 引用（我正在编码在 .NET 4.0 中）。

3) OnPageInitComplete 事件引用了一个“上下文”变量...在此上下文中似乎不存在。

关于我在这里缺少什么的任何想法？

Answer 1

似乎文档组织不当。

针对(2)，没有说明的是HttpContext.Current.Application.GetContainer()方法其实是一个扩展方法，实现方式如here所示的代码。

要使用此扩展方法，您只需导入“Unity.Web”命名空间。

这里是扩展方法的一个副本：

using System.Web;
using Microsoft.Practices.Unity;

namespace Unity.Web
{
  public static class HttpApplicationStateExtensions
  {
    private const string GlobalContainerKey = "EntLibContainer";

    public static IUnityContainer GetContainer(this HttpApplicationState appState)
    {
      appState.Lock();
      try
      {
        var myContainer = appState[GlobalContainerKey] as IUnityContainer;
        if (myContainer == null)
        {
          myContainer = new UnityContainer();
          appState[GlobalContainerKey] = myContainer;
        }
        return myContainer;
      }
      finally
      {
          appState.UnLock();
      }
    }
  }
}

关于依赖注入模块代码，我实际上只是使用basic method 来获取容器的一个实例，就我而言，它也可以正常工作。文档说依赖注入 HTTP 模块代码提高了“可测试性”和“可发现性”，这有点模糊。

无论如何，这是基本方法的代码：

protected void Application_Start(object sender, EventArgs e)
{
  Application.Lock();
  try
  {
    var myContainer = Application["EntLibContainer"] as IUnityContainer;
    if (myContainer == null)
    {
      myContainer = new UnityContainer();
      myContainer.AddExtension(new EnterpriseLibraryCoreExtension());
      // Add your own custom registrations and mappings here as required
      Application["EntLibContainer"] = myContainer;
    }
  }
  finally
  {
    Application.UnLock();
  }
}          

因此，有了扩展代码和我的 global.asax 文件中的代码来创建企业库容器的实例，剩下要做的就是编写代码以根据需要获取容器的实例。所以当我想获取 LogWriter 类的实例时，我会这样写：

using Unity.Web;

public LogWriter getLogWriter()
{
    var container = HttpContext.Current.Application.GetContainer();
    return container.Resolve<LogWriter>();
}

需要 Unity.Web 命名空间来允许我们调用 GetContainer() 扩展方法。

Answer 2

MSDN 文章提供的代码实际上非常令人震惊。首先，它不会编译，因为你会在这一行得到一个未声明的变量错误：

context.PreRequestHandlerExecute -= OnPreRequestHandlerExecute;

因为上下文被传递到 Init 方法中，而不是存储在任何地方。如果您确实捕获该参数并将其存储在一个字段中，那么您将获得运行时异常：

事件处理程序只能在期间绑定到 HttpApplication 事件 IHttpModule 初始化。

因此，以下似乎可行：

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using Microsoft.Practices.Unity;

namespace Unity.Web
{
    /// <summary>
    /// An <see cref="IHttpModule" /> that automatically injects dependencies into ASP.NET WebForms pages.
    /// </summary>
    /// <remarks>
    /// Since the pages have already been constructed by the time the module is called, constructor injection cannot be used. However,
    /// property injection can be used instead.
    /// </remarks>
    public class UnityHttpModule : IHttpModule
    {
        private HttpApplication _context;
        private bool _disposed;

        /// <summary>
        /// Initializes a module and prepares it to handle requests.
        /// </summary>
        /// <param name="context">An <see cref="T:System.Web.HttpApplication"/> that provides access to the methods, properties, and events common to all application objects within an ASP.NET application </param>
        public void Init(HttpApplication context)
        {
            _context = context;
            _context.PreRequestHandlerExecute += OnPreRequestHandlerExecute;
        }

        /// <summary>
        /// Disposes of the resources (other than memory) used by the module that implements <see cref="T:System.Web.IHttpModule"/>.
        /// </summary>
        public void Dispose()
        {
            GC.SuppressFinalize(this);
            Dispose(true);
        }

        /// <summary>
        /// Releases unmanaged and - optionally - managed resources.
        /// </summary>
        /// <param name="disposing"><c>true</c> to release both managed and unmanaged resources; <c>false</c> to release only unmanaged resources.</param>
        protected virtual void Dispose(bool disposing)
    {
        if (_disposed)
        {
            return;
        }

        if (disposing)
        {
            if (_context != null)
            {
                _context.PreRequestHandlerExecute -= OnPreRequestHandlerExecute;
            }
        }

        _disposed = true;
    }

    /// <summary>
    /// Handles the <see cref="E:PreRequestHandlerExecute" /> event.
    /// </summary>
    /// <param name="sender">The sender.</param>
    /// <param name="eventArgs">The <see cref="EventArgs"/> instance containing the event data.</param>
    private void OnPreRequestHandlerExecute(object sender, EventArgs eventArgs)
    {
        var currentHandler = HttpContext.Current.Handler;
        if (currentHandler != null)
        {
            HttpContext.Current.Application.GetContainer().BuildUp(currentHandler.GetType(), currentHandler);
        }

        // User Controls are ready to be built up after page initialization is complete
        var currentPage = HttpContext.Current.Handler as Page;
        if (currentPage != null)
        {
            currentPage.InitComplete += OnPageInitComplete;
        }
    }

    /// <summary>
    /// Handles the <see cref="E:PageInitComplete" /> event.
    /// </summary>
    /// <param name="sender">The sender.</param>
    /// <param name="e">The <see cref="EventArgs"/> instance containing the event data.</param>
    private void OnPageInitComplete(object sender, EventArgs e)
    {
        var currentPage = (Page)sender;
        var container = HttpContext.Current.Application.GetContainer();
        foreach (var c in GetControlTree(currentPage))
        {
            container.BuildUp(c.GetType(), c);
        }
    }

    /// <summary>
    /// Gets the controls in the page's control tree, excluding the page itself.
    /// </summary>
    /// <param name="root">The root control.</param>
    /// <returns>The child controls of the <paramref name="root" /> control.</returns>
    private static IEnumerable<Control> GetControlTree(Control root)
    {
        foreach (Control child in root.Controls)
        {
            yield return child;
            foreach (var control in GetControlTree(child))
            {
                yield return control;
            }
        }
    }
}

您需要@CiaranGallagher 在他的回答中提到的其余基础设施代码来完成管道，尽管我更喜欢使用项目注入器，所以在他的示例中，代码将是：

using Unity.Web;

[Dependency]
public LogWriter Writer { get; set; }

您不能在 WebForms 中使用构造函数注入，因为模块在现有控件上使用 BuildUp，但属性注入和方法注入一样工作正常。