Code in filtered exception handler throws NullReferenceException when accessing exception

Question

When I compile a UWP app with the .NET Native compiler and turn on code optimizations (essentially release mode), then I get a NullReferenceException when I try to access the actual exception in the catch block.

Code Sample:

try
{
    throw new ArgumentNullException("Param");
}
catch (ArgumentNullException ex) when (ex.ParamName == "Param")
{
    ErrorBlock.Text = ex.ParamName; // ErrorBlock is a TextBlock in the xaml
}
catch (Exception)
{
}

It goes into the correct catch block, and throws a NullReferenceException when I access ex. This only fails if both .Net Native and code optimizations are on.

What causes this issue?

@Pan why remove the tags? It seems related to this build mode and thus possibly a compiler problem with .NET native. — Patrick Hofman, Apr 05 '16 at 14:27
Because they are irrelevant. `exc.Message` is null. This is a plain NulLReferenceException. The OP called the constructor that only accepts a parameter name — Panagiotis Kanavos, Apr 05 '16 at 14:27
No, it is not... The message is defaulted. Please try this code yourself. — Patrick Hofman, Apr 05 '16 at 14:27
Yeah this is not a plain NRE... The exception can be de-referenced fine on the line above it. — FUR10N, Apr 05 '16 at 14:29
_"The exception can be dereferenced fine on the line above it"_ - well yes and no, exception filters aren't that straightforward. — CodeCaster, Apr 05 '16 at 14:30

score 5 · Answer 1 · answered Apr 05 '16 at 14:20

5

I am not exactly sure why it is going wrong (have been debugging for quite some time now), but the lack of await made me curious.

If you do await the ShowAsync method the code runs without a problem (obviously you need to make the method async if you didn't do that yet):

await new MessageDialog("Argument null exception: " + argEx.Message).ShowAsync();

While the code block without the await failed. Not sure if this is a bug or something you should have fixed...

answered Apr 05 '16 at 14:20

Patrick Hofman

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Hmm, that worked for me as well! I didn't realize it was related to async/await. The actual code I found this in is not awaiting a particular result by design (it's all in ICommands anyway so they'd be async void). I don't think you should be required to await this anyway, right? – FUR10N Apr 05 '16 at 14:39
Yes, every `async` should be `await`ed. – Patrick Hofman Apr 05 '16 at 14:39
This is a long running task and I don't need to schedule a continuation afterwards. I want the fire-and-forget behavior here. Awaiting it isn't really an option. – FUR10N Apr 05 '16 at 14:55
I did some more tests, this doesn't look related to async/await after all. – FUR10N Apr 05 '16 at 15:08
Can you tell us what you've found? – Patrick Hofman Apr 05 '16 at 15:09
I've updated the code. Instead of using MessageDialog, I added a textblock to the page and tried just updating that. I still get a NRE but weirdly I can access the exception in the debugger. – FUR10N Apr 05 '16 at 15:11

score 2 · Accepted Answer · edited Apr 05 '16 at 19:12

2

I work on the .NET Native runtime and compiler team.

This is a bug inside of our compiler. You can think of each exception handling region (try, catch, finally, when) as a small function or "funclet". We lose track of the exception object when setting up the stack for the "when" (aka filter block). This bug is corrected in Windows Tools 1.3 which, given no major setbacks, should be shipping in another week or two. It'll show up as an update for folks that have installed VS 2015 Update 2.

Let me know if you have any other questions.

edited Apr 05 '16 at 19:12

svick

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answered Apr 05 '16 at 18:47

MattWhilden

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Thanks @Matt! I saw slightly different behaviors in 3 scenarios around this: non-async, async but not awaited, and async + awaited. I'll give all 3 a shot when the update comes out. – FUR10N Apr 05 '16 at 19:23
Excellent. Please let us know how it goes. We love hearing from folks at dotnetnative@microsoft.com. – MattWhilden Apr 06 '16 at 01:14

Code in filtered exception handler throws NullReferenceException when accessing exception

2 Answers2