异步回调中的HttpContext.Current为空
尝试在方法回调中访问HttpContext.Current以便可以修改Session变量，但是我收到HttpContext.Current为null的异常。当_anAgent对象触发回调方法时，它将被异步激发。
在SO上查看 之后，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类如下所示：
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);
}
}代码中发生的事情：
单击按钮将调用SendData()方法，在该方法中，_webAgent将向另一个服务器发送一条消息并等待回复(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，它将调用BackendReceived()方法，在.aspx.cs页面中，该方法将调用Callback()方法。
问题：
当WorkAgent触发Callback()方法时，它会尝试访问HttpContext.Current，即null。为什么在忽略异常的情况下继续操作，仍然可以使用ajax返回的GetSessionVar()方法获得相同的SessionID和Session变量。
我是否应该启用aspNetCompatibilityEnabled设置？
我是否应该创建某种类型的asynchronous module handler
回答 3
Stack Overflow用户
发布于 2016-07-13 05:19:01
这是一个基于类的解决方案，到目前为止，它在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连接：
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可能会有问题，因为我目前不理解这个related answer。请发表意见。
参考：AsyncLocal (需要.NET 4.6)
Stack Overflow用户
发布于 2014-07-03 13:13:06
有关会话变量为null的原因以及可能的解决方法，请参阅以下文章
http://adventuresdotnet.blogspot.com/2010/10/httpcontextcurrent-and-threads-with.html
引用自文章中的；
当前的
HttpContext实际上在线程本地存储中，这解释了为什么子线程无法访问它
正如作者提出的那样，作者说
在你的子线程中传递一个对它的引用。在回调方法的“
HttpContext”对象中包含对状态的引用，然后可以将其存储到线程
上的HttpContext.Current中
Stack Overflow用户
发布于 2014-07-03 13:20:58
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;
}一旦使用适当的实例设置了current，剩下的代码就是独立的，无论是从线程调用还是直接从WebRequest调用。
https://stackoverflow.com/questions/24544272
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