HttpContext.在异步回调中Current为null

本文关键字：Current null 回调 异步 HttpContext | 更新日期: 2023-09-27 18:11:17

试图在方法回调中访问HttpContext.Current，以便我可以修改Session变量，但是我收到HttpContext.Current是null的异常。当_anAgent对象触发回调方法时，将异步触发回调方法。

在SO上看到类似的问题后，我仍然不确定这个问题的解决方案。

我的代码的简化版本如下:

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类看起来像这样:

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_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)。一旦收到它调用BackendReceived()方法，回到.aspx.cs页面调用Callback()方法。

问题:
当WorkAgent触发Callback()方法时，它尝试访问HttpContext.Current，即null。为什么是这种情况下，如果我继续，忽略异常，我仍然可以获得相同的SessionID和Session变量使用ajax返回的GetSessionVar()方法。

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?

HttpContext.在异步回调中Current为null

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

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:

    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();
    }

然后可以用它来代替HttpContext。当前:

    HttpContextProvider.Current

可能有问题，因为我目前不理解这个相关的答案。请发表评论。

参考:AsyncLocal(需要。net 4.6)

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; 
}

一旦你为current设置了一个合适的实例，你的代码的其余部分是独立的，无论是从线程调用还是直接从WebRequest。

请参阅下面的文章，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

从文章中引用;

当前的HttpContext实际上是在线程本地存储，这解释了为什么子线程不能访问它

作为一个建议的工作围绕作者说

在子线程中传递对它的引用。在回调方法的" state "对象中包含对HttpContext的引用，然后您可以将其存储到该线程的HttpContext.Current

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