Threading.Thread.Sleep（） 抛出意外异常

问：

在一个大容量（每天 1-200 万个 http 请求）的 Web 应用程序（SCORM 播放器）中，我们有一个队列来优雅地处理峰值负载 - 在峰值负载下，请求可能需要等待 20-50 毫秒才能在应用程序实例之间共享的队列中得到服务。这是下面代码中对 Sleep（） 的调用。奇怪的是，偶尔（每天 100-150 次）调用 Sleep（） 会抛出一个空引用异常，该异常不会被 try/catch 捕获。我们将其记录到 Application Insights。

• Sleep（） 如何抛出这个未记录的异常？Threding.Thread 可以为 null 吗？
• try/catch 怎么会无法捕获异常？

{
   "Message":"Object reference not set to an instance of an object.",
   "Data":[
      
   ],
   "InnerException":null,
   "TargetSite":"Void WaitInQueue(System.Guid, SCORM.LmsInstance)",
   "StackTrace":"   at SCORM.API.WaitInQueue(Guid key, LmsInstance instance) in D:\\a\\1\\s\\SCORM\\API.ashx.vb:line 57",
   "HelpLink":null,
   "Source":"SCORM",
   "HResult":-2147467261,
   "_typeTag":"NullReferenceException"
}

Private Shared Lock As New Object

Sub ProcessRequest(context As HttpContext) Implements IHttpHandler.ProcessRequest
    Dim response As HttpResponse = context.Response
    response.ContentType = "text/plain"
    Dim myLmsInstance As New LmsInstance With {
        .RequestData = New ApiRequestData(context.Request)
    }
    myLmsInstance.InitializeSessionData()

    Dim key As Guid = Guid.NewGuid
    MaintainQueue(True, key, myLmsInstance)
    WaitInQueue(key, myLmsInstance)
    myLmsInstance.HandleApiCall()
    MaintainQueue(False, key, myLmsInstance)
    response.Write(myLmsInstance.GetResponseData)
End Sub

Private Sub MaintainQueue(addToQueue As Boolean, key As Guid, instance As LmsInstance)
    SyncLock Lock
        If instance.SessionData IsNot Nothing Then
            If addToQueue Then
                instance.SessionData.RequestQueue.Add(key)
            Else
                instance.SessionData.RequestQueue.Remove(key)
            End If
            Dim NextInQueue = instance.SessionData.RequestQueue.FirstOrDefault
            If NextInQueue <> Guid.Empty Then
                instance.SessionData.NextInQueue = NextInQueue
            Else
                instance.SessionData.NextInQueue = Nothing
            End If
        End If
    End SyncLock
End Sub

Private Sub WaitInQueue(key As Guid, instance As LmsInstance)
    If instance.SessionData IsNot Nothing Then
        Try
            Dim NextInQueue = instance.SessionData.NextInQueue
            While NextInQueue.HasValue AndAlso NextInQueue.Value <> key
                ' Sleep as briefly as possible. The default on a Windows server is 15 ms.
                Threading.Thread.Sleep(1)
                NextInQueue = instance.SessionData.NextInQueue
            End While
        Catch ex As Exception
            Serilog.Log.Logger.Warning(ex, "Exception in SCORM Player, WaitInQueue():  {@ex}", ex)
        End Try
    End If
End Sub

最近添加了 try/catch，期望处理异常并为我们提供日志记录。但是异常仍未处理，永远不会到达 Logger.Warning（） 代码。