HttpContext.Current在异步Callback中为null

Question

试图访问HttpContext.Current一个方法调用回来,我可以修改一个Session变量,但我收到的异常HttpContext.Current是null.当_anAgent对象触发它时,异步触发回调方法.

在查看SO上的类似 问题之后,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班级是这样的:

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,在此内部_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参考相同的内容SessionID).收到后,它会调用BackendReceived()方法,该方法返回到.aspx.cs页面调用该Callback()方法.

问:
当WorkAgent触发该Callback()方法尝试访问HttpContext.Current它null.为什么如果我继续,忽略异常,我仍然可以使用ajax返回的方法获得相同SessionID和Session变量GetSessionVar().

我应该启用aspNetCompatibilityEnabled设置吗？
我应该创建某种异步模块处理程序吗？
这与集成/经典模式有关吗？

Answer 1

这是一个基于类的解决方案,适用于MVC5中迄今为止的简单案例(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();
    }

然后可以使用它来代替HttpContext.Current:

    HttpContextProvider.Current

可能存在问题,因为我目前不理解这个相关的答案.请评论.

参考: AsyncLocal (需要.NET 4.6)

Answer 2

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; 
}

一旦设置了current正确的实例，代码的其余部分都是独立的，无论是从线程调用还是直接从WebRequest.

Answer 3

请参阅以下文章，了解 Session 变量为何为 null 的说明以及可能的解决方法

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

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引自文章；

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当前的HttpContext实际上位于线程本地存储中，这解释了为什么子线程无法访问它

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正如作者提出的一项工作所说

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在您的子线程中传递对它的引用。在回调方法的 \xe2\x80\x9cstate\xe2\x80\x9d 对象中包含对的引用HttpContext，然后可以将其存储到HttpContext.Current该线程上

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