C#•2mo ago

✅ Possible null reference assignment warning on event unsubscribe

On the following code gives me a CS8601: Possible null reference assignment. on build.

  // View.razor.cs
  internal EventHandler<FocusSectionEvent> FocusSectionRequested = delegate { };

  // Section.razor.cs
  [CascadingParameter] public View View { get; set; } = default!;

  protected override void OnInitialized() => View.FocusSectionRequested += OnFocusSection; // No warning
  public void Dispose() => View.FocusSectionRequested -= OnFocusSection; // warning CS8601: Possible null reference assignment.

  private void OnFocusSection(object? sender, FocusSectionEvent e) { /* stuff */ }

  // View.razor.cs
  internal EventHandler<FocusSectionEvent> FocusSectionRequested = delegate { };

  // Section.razor.cs
  [CascadingParameter] public View View { get; set; } = default!;

  protected override void OnInitialized() => View.FocusSectionRequested += OnFocusSection; // No warning
  public void Dispose() => View.FocusSectionRequested -= OnFocusSection; // warning CS8601: Possible null reference assignment.

  private void OnFocusSection(object? sender, FocusSectionEvent e) { /* stuff */ }

From what I can gather, it thinks that View.FocusSectionRequested is null, but checking that with an if statement just gives me an 'expression is always true' warning.

PS $> dotnet --info
.NET SDK:
 Version:           8.0.206
 Commit:            bb12410699
 Workload version:  8.0.200-manifests.afa40ca5

PS $> dotnet --info
.NET SDK:
 Version:           8.0.206
 Commit:            bb12410699
 Workload version:  8.0.200-manifests.afa40ca5

How can I get rid of this warning? Just pragma?

8 Replies

sibber•2mo ago

when nothing is subscribed to the event it will be null you didnt define it as nullable, but when you unsubscribe there, it could be null i see you tried to work around this by initializing it to a delegate just define it as nullable and null check when you invoke thats the standard way of doing ut

sneki•2mo ago

just define it as nullable and null check when you invoke

that works, thank you. but isn't that a bug in the compiler? FocusSectionRequested is never null, because there will always be the empty delegate..

sibber•2mo ago

when you unsubscribe from all events its null

333fred•2mo ago

No, it is not a bug We did debate this topic actually, repeatedly. Ultimately, it is possible that unsubscribing from an event will turn that event null. In your particular case, a more complicated analysis would reveal that it can't be, but that's a particularly complex analysis, and is effectively equivalent to the halting problem

sneki•2mo ago

ok, that makes sense. It's a bit more verbose but i can get rid of the warning like that pretty elegantly. thank you! @sibber 🇵🇸 thank you too 🙂

sibber•2mo ago

you mean trying to analyze at each invokation of the event whether there are subscribers or not? is equivalent to the halting problem?

333fred•2mo ago

Yes

sibber•2mo ago

ah ok oh i just understood your question i dont know what i was drinking earlier lmao

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✅ Possible null reference assignment warning on event unsubscribe