Double exception throwing in a try / finally block - vb.net

Question

Here's the code example :
Try
Throw New FirstException()
Finally
Throw New SecondException()
End Try
I figured out it only throws SecondException out and FirstException just vanishes.
I thought FirstException would be inside InnerException property of SecondException but it appears it is not.
I'm not blocked on anything as I don't really need the FirstException to show up, I'm just rather intrigued about this behaviour.
Is there a way to know SecondException did get thrown first when
catching it all at upper level ?
If the first exception really is overriden by the second, what is the
reason ?
Does it happen in every other language ? Is it logical ?

Answer 1

I guess the primary explanation for why this works this way is that you are never catching your first exception and passing it along the chain. If you have a situation like the above where you may be throwing several exceptions on the way back to the original caller then you have to either catch them as they are thrown (and include them as an inner exception when creating the next one) :
Dim ex1 As Exception = Nothing
Try
Throw New Exception("first exception")
Catch ex As Exception
ex1 = ex
Finally
Throw New Exception("second exception", ex1)
End Try
Or, probably better - just don't throw until you have all of the exceptions figured out:
Dim ex1 As Exception = Nothing
Try
ex1 = New Exception("first exception")
Finally
Throw New Exception("second exception", ex1)
End Try
Throwing and catching exceptions is expensive, so it's probably best to not throw until you're ready to return and just log along the way.

Answer 2

One of the limitations of exception handling in .net is that there is no nice way for code in a Finally block to know what exception, if any, caused the code in the Try block to exit, nor is there any normal way for code in a finally block which does have such information to make it available to code which might throw an exception.
In vb.net, it's possible to kludge things in a manner that works pretty well, even though it looks a bit ugly.
Module ExceptionDemo
Function CopySecondArgToFirstAndReturnFalse(Of T)(ByRef dest As T, src As T) As Boolean
dest = src
Return False
End Function
Function AnnotateExceptionAndReturnFalse(ex As Exception, TryBlockException As Exception) As Boolean
If ex Is Nothing Then Return False ' Should never occur
If TryBlockException Is Nothing Then Return False ' No annotation is required
ex.Data("TryBlockException") = TryBlockException
Return False
End Function
Sub ExceptionTest(MainAction As Action, CleanupAction As Action)
Dim TryBlockException As Exception = Nothing
Try
MainAction()
Catch ex As Exception When CopySecondArgToFirstAndReturnFalse(TryBlockException, ex)
' This block never executes, but above grabs a ref to any exception that occurs
Finally
Try
CleanupAction()
Catch ex As Exception When AnnotateExceptionAndReturnFalse(ex, TryBlockException)
' This block never executes, but above performs necessary annotations
End Try
End Try
End Sub
Sub ExceptionTest2(Message As String, MainAction As Action, CleanupAction As Action)
Debug.Print("Exception test: {0}", Message)
Try
ExceptionTest(MainAction, CleanupAction)
Catch ex As Exception
Dim TryBlockException As Exception = Nothing
Debug.Print("Exception occurred:{0}", ex.ToString)
If ex.Data.Contains("TryBlockException") Then TryBlockException = TryCast(ex.Data("TryBlockException"), Exception)
If TryBlockException IsNot Nothing Then Debug.Print("TryBlockException was:{0}", TryBlockException.ToString)
End Try
Debug.Print("End test: {0}", Message)
End Sub
Sub ExceptionDemo()
Dim SuccessfulAction As Action = Sub()
Debug.Print("Successful action")
End Sub
Dim SuccessfulCleanup As Action = Sub()
Debug.Print("Cleanup is successful")
End Sub
Dim ThrowingAction As Action = Sub()
Debug.Print("Throwing in action")
Throw New InvalidOperationException("Can't make two plus two equal seven")
End Sub
Dim ThrowingCleanup As Action = Sub()
Debug.Print("Throwing in cleanup")
Throw New ArgumentException("That's not an argument--that's just contradiction")
End Sub
ExceptionTest2("Non-exception case", SuccessfulAction, SuccessfulCleanup)
ExceptionTest2("Exception in main; none in cleanup", ThrowingAction, SuccessfulCleanup)
ExceptionTest2("Exception in cleanup only", SuccessfulAction, ThrowingCleanup)
ExceptionTest2("Exception in main and cleanup", ThrowingAction, ThrowingCleanup)
End Sub
End Module
The module above starts with a couple helper modules which should probably be in their own "Exception helpers" module. The ExceptionTest method shows the pattern for code which might throw an exception in both the Try and Finally block. The ExceptionTest2 method calls ExceptionTest and reports what exception if any comes back from it. ExceptionDemo calls ExceptionTest2 in such a way as to cause exceptions in different combinations of the Try and Finally blocks.
As shown, if an exception occurs during cleanup, that exception will be returned to the caller, with the original exception being an item in its Data dictionary. An alternative pattern would be to catch the exception that occurs on cleanup and include it in the data of the original exception (which would be left uncaught). My general inclination is that it's probably better in many cases to propagate the exception that occurs during cleanup, since any code which was planning to deal with the original exception will probably expect that cleanup succeeded; if such an expectation cannot be met, the exception that escapes should probably not be the one the caller was expecting. Note also that the latter approach would require a slightly different method of adding information to the original exception, since an exception which is thrown in a nested Try block might need to hold information about multiple exceptions that were thrown in nested Finally blocks.