TPL Exception - How to attach to the main thread - .net-4.0

Question

I am planning to implement TPL in my existing asp.net application to improve the performance.
I am implementing them in the selective places (like calls to write data to one table and some kind of irrelevant and non sync operations ), and we are processing all the unhandled exception in the application_error event in the global.asax file.
But when I get an error the TPL its not throwing that error to the application_error event.
When I tried with Task.WaitAll(t1) then its throwing to app_error event, but its taking a while to process the written code.
But when I try with the following method (to take some time only when fault is occurred) then also its not throwing the exception to the main thread.
Task.Factory.StartNew(Sub(mainContext)
HttpContext.Current = mainContext
LookUpRepository.AddItems(CurrentContext.LoggedInUser.UserID, ClientID)
End Sub, HttpContext.Current).ContinueWith(Sub(tplException)
For Each ex In tplException.Exception.InnerExceptions
Throw ex
Next
End Sub, TaskContinuationOptions.OnlyOnFaulted Or TaskContinuationOptions.ExecuteSynchronously)
I tried with ConcurrentQueue to catch the exceotion and sometimes it halts the applcation and the control is not proceeding further line of code.
So how can I attach the the TPL exceotion to the main thread which is hosting the asp.net application.

Answer 1

There is an observable collection ObservableCollection that is available in .Net. If there is an error in the thread that we opened and if we add them in this ObservableCollection , then it will raise an event, From that event we can handle this error
Task.Factory.StartNew(Sub(mainContext)
'Using sw As StreamWriter = File.AppendText("c:/logs/t7.txt")
' sw.WriteLine("thread=" + Thread.CurrentThread.ManagedThreadId.ToString())
'End Using
HttpContext.Current = mainContext
'Do some operation
End Sub, HttpContext.Current).ContinueWith(Sub(tplException, subContext)
HttpContext.Current = subContext
If IsNothing(tplException.Exception) Then Exit Sub
For Each ex In tplException.Exception.InnerExceptions
MyAppState.Instance.ObservableException.Add(ex)
Next
End Sub, HttpContext.Current)
Collection needs to declared
Public WithEvents ObservableException As ObservableCollection(Of Exception)
Event that will get raised by .Net framework to handle the exception
Private Sub CollectionChangedHandler(sender As Object, e As NotifyCollectionChangedEventArgs) Handles ObservableException.CollectionChanged
Dim ex = DirectCast(e.NewItems(0), Exception)
ExceptionLog.SaveFile(ExceptionLog.ReadExceptionInformationFromException(ex))
End Sub