asp.net - Global_asax_BeginRequest - 此行如何出错？

Question

我在 Global.asax.vb 中定义了以下内容...

Private Sub Global_asax_BeginRequest(sender As Object, e As EventArgs) Handles Me.BeginRequest
    Try
        If Request IsNot Nothing Then 'this line throws an exception...
            With Request
                ...

错误是...

ERROR - Global_asax:System.NullReferenceException: Object reference not set to an instance of an object.

我对这个特定的行如何出错感到有点困惑。我想做的就是测试该对象是否为 null/Nothing。

我猜当请求开始时，幕后一定发生了其他事情，但我不知道如何进一步调试它。

此错误并非每次都会发生。我只是偶尔在日志中看到这些错误，但我不知道它们是如何发生的。我无法重现它。由于无法访问 Request 对象，我无法获得有关导致该对象的请求类型的任何其他信息。

更新...

我尝试更改访问 Request 属性的方式，看看是否会产生任何影响...

Public Sub Application_BeginRequest(sender As Object, e As EventArgs)
    Dim app As HttpApplication = TryCast(sender, HttpApplication)
    If app IsNot Nothing Then
        Dim _request = app.Request
...

这次，有趣的是，异常发生在这一行...

Dim app As HttpApplication = TryCast(sender, HttpApplication)

这看起来很奇怪，因为 TryCast 是专门为了不抛出异常而设计的。

这是我得到的完整堆栈跟踪...

System.NullReferenceException: Object reference not set to an instance of an object.
     at Global_asax.Application_BeginRequest(Object sender, EventArgs e) in C:\vs_agent\_work\4\s\...\Global.asax.vb:line 97
     at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
     at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
     at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

第 97 行对应于 TryCast 行。

我目前的理论是，可能与 Owin 中间件有关

Answer 1

所描述的情况似乎不可能(我稍后会解释)，例如我会推荐更多的防弹诊断

Try
  Dim locReq As HttpRequest = Request
Catch ex As Exception
   'record Exception with the complete stack trace
    Throw
End Try 

并使用 HttpRequest 变量 (locReq) 而不是 Request 属性。该代码片段还可以防止更改 Request 属性返回值。

为什么我认为你的假设不可能:

无法在该行中获取 NullReference

If Request IsNot Nothing ...

但它可以来自 Request 属性的深度。它(翻译)source code是

Public Class HttpApplication
'...
    Public ReadOnly Property Request As HttpRequest
        Get
            Dim request As HttpRequest = Nothing
            If _context IsNot Nothing AndAlso
                 Not _hideRequestResponse Then request = _context.Request

            If request Is Nothing Then Throw New HttpException(SR.GetString(SR.Request_not_available))
            Return request
        End Get
    End Property

这里又是一个属性请求(已翻译)source code

Public NotInheritable Class HttpContext
    '...
    Private _response As HttpResponse
    '...
    Public ReadOnly Property Response As HttpResponse
        Get
            If HideRequestResponse OrElse
               HasWebSocketRequestTransitionCompleted Then Throw New 
                  HttpException(SR.GetString(SR.Response_not_available))
            Return _response
        End Get
    End Property

当我阅读代码时，我发现没有空引用的空间，但可能存在 HttpException。所以我的结论是日志系统错误地记录了该行或异常。

请查看IIS7 Integrated mode: Request is not available in this context exception in Application_Start这与 Response_not_available 异常有关。

编辑

我试图解释所呈现的堆栈跟踪。我从最深的过程中得出的结论(ExecuteStep(IExecutionStep step, ref bool completedSynchronously)是异常只能发生在它的catch block 中(用原始注释翻译)。

Catch e As Exception
        [error] = e
        'Since we will leave the context later, we need to remember if we are impersonating
         'before we lose that info - VSWhidbey 494476

        If ImpersonationContext.CurrentThreadTokenExists Then
            e.Data(System.Web.Management.WebThreadInformation.IsImpersonatingKey) = String.Empty
        End If
        'This might force ThreadAbortException to be thrown
        'automatically, because we consumed an exception that was
        'hiding ThreadAbortException behind it

        If TypeOf e Is ThreadAbortException AndAlso ((Thread.CurrentThread.ThreadState And ThreadState.AbortRequested) = 0) Then
            [error] = Nothing
            'Response.End from a COM+ component that re-throws ThreadAbortException
            'It is not a real ThreadAbort
             'VSWhidbey 178556
            _stepManager.CompleteRequest()
        End If
    End Try 

由此看来，问题确实与中止线程有关。我的推测是 _stepManager 没什么，但我不知道为什么。