C# - Why is VS 2017 greying out null check on event object?

Question

When doing a null check on an event object, Visual Studio is changing the code color to light grey. If I'm not mistaken , it normally implies that this line is unnecessary. Obviously, it is not the case in my situation (or is it ?). See my example below:

 public event PropertyChangedEventHandler PropertyChanged;

    string test; 

    protected void OnPropertyChanged (string propertyName)
    {
        if (PropertyChanged != null) {    //This line is grey
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }  //This line is grey
        if (test != null) {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

Here is the screen shot:

It means that the code can be simplified. Probably something like `PropertyChanged?.Invoke(...)` — Konrad, Jun 05 '18 at 12:56
Yes or click on the grey line and press `Ctrl+.` or `Alt+Enter` — Konrad, Jun 05 '18 at 12:57
As it stands, there's a race condition that disappears with the use of `?.Invoke`. — spender, Jun 05 '18 at 12:58
@bommelding that's strange, I restarted my VS and now it appears. I guess it was indeed broken. Couldn't see the glorious light bulb... — scharette, Jun 05 '18 at 12:59
@spender - true but VS (or R#) is not clever enough to detect that. The hint is just about the simplification. — bommelding, Jun 05 '18 at 13:00
@spender So it has to do with more than just simplification ? — scharette, Jun 05 '18 at 13:02
I don't know what the warning pertains to but there's certainly a problem with your original approach. Maybe [this question](https://stackoverflow.com/questions/282653/checking-for-null-before-event-dispatching-thread-safe) I asked (almost in SO's pre-history) will help you understand. — spender, Jun 05 '18 at 13:05
@scharette And [another article](https://codeblog.jonskeet.uk/2015/01/30/clean-event-handlers-invocation-with-c-6/) by the [Skeet](https://stackoverflow.com/users/22656/jon-skeet) about why the info in that old question is now stale... — spender, Jun 05 '18 at 13:08
@spender Kinda sad I can't call it has PropertyChanged?(...), but oh well... — scharette, Jun 05 '18 at 13:20
@scharette I'd second that, but at least it's a good deal less pre-requisite knowledge that was previously required when trying to make a robust event system. — spender, Jun 05 '18 at 13:34
@spender true... Anyway, thanks a lot for the articles really helped. — scharette, Jun 05 '18 at 13:37

score 5 · Accepted Answer · edited Jun 05 '18 at 13:00

5

If you right click and view (Quick Actions and Refactorings) the suggested fix you will see that the null reference check can be performed on the same line as the raising of the event through the use of the ? operator (Safe Navigation Operator).

The action it will suggest is the following:

this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

edited Jun 05 '18 at 13:00

CodeNotFound

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answered Jun 05 '18 at 12:57

Bijington

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Wow, that was simple. Thanks a lot for the explanation. – scharette Jun 05 '18 at 13:00

score 1 · Answer 2 · answered Jun 05 '18 at 12:58

1

It's a code suggestion. If you hover your cursor over it, you will see a little "Quick Actions" icon in the margin, and if you click it you will see that it's suggesting you could improve your statement -- make it more succinct -- by using a null conditional operator:

PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

answered Jun 05 '18 at 12:58

rory.ap

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+1 for ss. After restarting my VS that's exactly how I could see it. It even suggest potential fixes. Thanks for your insight – scharette Jun 05 '18 at 13:05

C# - Why is VS 2017 greying out null check on event object?

2 Answers2