HttpContext.Current 在异步回调中为空

Question

尝试在方法回调中访问HttpContext.Current，以便我可以修改Session 变量，但是我收到HttpContext.Current 是null 的异常。回调方法在_anAgent 对象触发时异步触发。

在 SO 上查看 similar questions 后，我仍然不确定解决方案。

我的代码的简化版本如下所示：

public partial class Index : System.Web.UI.Page

  protected void Page_Load()
  {
    // aCallback is an Action<string>, triggered when a callback is received
    _anAgent = new WorkAgent(...,
                             aCallback: Callback);
    ...
    HttpContext.Current.Session["str_var"] = _someStrVariable;
  }

  protected void SendData() // Called on button click
  {
    ...
    var some_str_variable = HttpContext.Current.Session["str_var"];

    // The agent sends a message to another server and waits for a call back
    // which triggers a method, asynchronously.
    _anAgent.DispatchMessage(some_str_variable, some_string_event)
  }

  // This method is triggered by the _webAgent
  protected void Callback(string aStr)
  {
    // ** This culprit throws the null exception **
    HttpContext.Current.Session["str_var"] = aStr;
  }

  [WebMethod(EnableSession = true)]
  public static string GetSessionVar()
  {
    return HttpContext.Current.Session["str_var"]
  }
}

不确定是否有必要，但我的 WorkAgent 课程如下所示：

public class WorkAgent
{
  public Action<string> OnCallbackReceived { get; private set; }

  public WorkAgent(...,
                   Action<string> aCallback = null)
  {
    ...
    OnCallbackReceived = aCallback;
  }

  ...

  // This method is triggered when a response is received from another server
  public BackendReceived(...)
  {
    ...
    OnCallbackReceived(some_string);
  }
}

代码中发生了什么：
单击按钮调用SendData() 方法，其中_webAgent 将消息发送到另一个服务器并等待回复（同时用户仍然可以与此页面交互并引用相同的SessionID）。一旦收到它就会调用BackendReceived() 方法，该方法在.aspx.cs 页面中调用Callback() 方法。

问题：
当WorkAgent 触发Callback() 方法时，它会尝试访问HttpContext.Current，即null。为什么会出现这种情况，如果我继续忽略异常，我仍然可以使用 ajax 返回的GetSessionVar() 方法获得相同的SessionID 和Session 变量。

我应该启用aspNetCompatibilityEnabled 设置吗？
我应该创建某种asynchronous module handler 吗？
这与Integrated/Classic mode 有关吗？

Answer 1

这是一个基于类的解决方案，目前在 MVC5 中适用于简单的情况（MVC6 支持基于 DI 的上下文）。

using System.Threading;
using System.Web;

namespace SomeNamespace.Server.ServerCommon.Utility
{
    /// <summary>
    /// Preserve HttpContext.Current across async/await calls.  
    /// Usage: Set it at beginning of request and clear at end of request.
    /// </summary>
    static public class HttpContextProvider
    {
        /// <summary>
        /// Property to help ensure a non-null HttpContext.Current.
        /// Accessing the property will also set the original HttpContext.Current if it was null.
        /// </summary>
        static public HttpContext Current => HttpContext.Current ?? (HttpContext.Current = __httpContextAsyncLocal?.Value);

        /// <summary>
        /// MVC5 does not preserve HttpContext across async/await calls.  This can be used as a fallback when it is null.
        /// It is initialzed/cleared within BeginRequest()/EndRequest()
        /// MVC6 may have resolved this issue since constructor DI can pass in an HttpContextAccessor.
        /// </summary>
        static private AsyncLocal<HttpContext> __httpContextAsyncLocal = new AsyncLocal<HttpContext>();

        /// <summary>
        /// Make the current HttpContext.Current available across async/await boundaries.
        /// </summary>
        static public void OnBeginRequest()
        {
            __httpContextAsyncLocal.Value = HttpContext.Current;
        }

        /// <summary>
        /// Stops referencing the current httpcontext
        /// </summary>
        static public void OnEndRequest()
        {
            __httpContextAsyncLocal.Value = null;
        }
    }
}

要使用它可以从 Global.asax.cs 挂钩：

    public MvcApplication() // constructor
    {            
        PreRequestHandlerExecute += new EventHandler(OnPreRequestHandlerExecute);
        EndRequest += new EventHandler(OnEndRequest);
    } 

    protected void OnPreRequestHandlerExecute(object sender, EventArgs e)
    {
        HttpContextProvider.OnBeginRequest();   // preserves HttpContext.Current for use across async/await boundaries.            
    }

    protected void OnEndRequest(object sender, EventArgs e)
    {
        HttpContextProvider.OnEndRequest();
    }

然后可以用这个代替HttpContext.Current：

    HttpContextProvider.Current

可能有问题，因为我目前不明白这个related answer。请发表评论。

参考：AsyncLocal（需要 .NET 4.6）

Answer 2

请参阅以下文章，了解为什么 Session 变量为空，以及可能的解决方法

http://adventuresdotnet.blogspot.com/2010/10/httpcontextcurrent-and-threads-with.html

引用自文章；

当前的HttpContext实际上是在线程本地存储中，这就解释了为什么子线程无权访问它

正如作者所说的一项拟议工作

在您的子线程中传递对它的引用。在回调方法的“状态”对象中包含对HttpContext 的引用，然后您可以将其存储到该线程上的HttpContext.Current

Answer 3

当使用线程或async 函数时，HttpContext.Current 不可用。

尝试使用：

HttpContext current;
if(HttpContext != null && HttpContext.Current != null)
{
  current = HttpContext.Current;
}
else
{
    current = this.CurrentContext; 
    //**OR** current = threadInstance.CurrentContext; 
}

一旦您使用适当的实例设置current，您的其余代码都是独立的，无论是从线程调用还是直接从WebRequest 调用。