Bug 52698 - Event handler with capturing lambda not hooked up
Summary: Event handler with capturing lambda not hooked up
Alias: None
Product: iOS
Classification: Xamarin
Component: General (show other bugs)
Version: master
Hardware: PC Windows
: Normal normal
Target Milestone: Future Cycle (TBD)
Assignee: Bugzilla
Depends on:
Reported: 2017-02-23 05:54 UTC by Kent
Modified: 2017-03-06 00:06 UTC (History)
7 users (show)

See Also:
Is this bug a regression?: ---
Last known good build:

Repro solution (165.83 KB, application/x-zip-compressed)
2017-02-23 05:54 UTC, Kent

Description Kent 2017-02-23 05:54:10 UTC
Created attachment 19918 [details]
Repro solution

I apologize if I didn't get the categorization of this bug correct. I'm quite sure it's a compiler/runtime problem, but wasn't sure exactly where to stick it.

Consider the following:

    public static void Hook(this INotifyPropertyChanged @this)
        var somethingToCapure = "hello";

        @this.PropertyChanged += (s, e) =>
            var dummy = 42;

        @this.PropertyChanged += (s, e) =>
            var dummy = 42;
            var captured = somethingToCapure;

        PropertyChangedEventHandler handler1 = (s, e) =>
            var dummy = 42;
        @this.PropertyChanged += handler1;

        PropertyChangedEventHandler handler2 = (s, e) =>
            var dummy = 42;
            var captured = somethingToCapure;
        @this.PropertyChanged += handler2;

When hooking a component that implements INotifyPropertyChanged directly, all four handlers are executed as expected. However, when hooking a component that implements INotifyPropertyChanged indirectly through ReactiveUI, only the non-capturing lambdas are executed.

This is a regression, but I can't tell you exactly when this occurred. We have code in production that works fine, and it was whilst trying to re-use said code in a new project that I noticed it no longer works.

I've attached a repro. Have tried both Cycle 8 and 9 (via a colleague) and the result was the same.
Comment 1 Zoltan Varga 2017-02-23 20:19:44 UTC
I can also reproduce this with cycle6, i.e.:

Version: (Xamarin Enterprise)
Hash: f37444a
Branch: cycle7-sr1
Build date: 2016-07-28 12:17:02-0400

	Mono 4.6.2 (mono-4.6.0-branch/ac9e222) (64-bit)
	GTK+ 2.24.23 (Raleigh theme)

	Package version: 406020016
Comment 2 Vincent Dondain [MSFT] 2017-02-27 06:14:41 UTC
I could also reproduce this with the following environment: https://gist.github.com/VincentDondain/3a32f2ec661c9d8a3d3d0dc37749410b
Comment 3 Zoltan Varga 2017-02-27 06:19:08 UTC
Since this is also reproducible with cycle6, what version is this worked ?
Comment 4 Kent 2017-02-27 06:22:45 UTC
I believe it was just an earlier version of cycle 8. The last production build we have is dated Jan 4, 2017. I'm guessing whatever cycle 8 was out at the time should work. If not, I'm at a loss.
Comment 5 Kent 2017-03-06 00:06:55 UTC
Sorry, forget everything I said about this working in a prior build. I had written the code in a previous project, unit tested it (tests run against full framework just fine), but I never actually used the code in the product. I must have concocted an alternative solution, probably because I couldn't get it to work!

Really happy to see this confirmed and at least targeted for a future cycle. I have a feeling it's also root cause of other insidious problems, such as that reported by #31415.

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