OnPropertyChanging not called for generated property

[marhja]

Describe the bug

OnPropertyChanging is not called for a generated property when using the NotifyPropertyChangedFor attribute.

Steps to reproduce

Using this class:

public partial class MyObject : ObservableObject
{
  [ObservableProperty]
  [NotifyPropertyChangedFor(nameof(FullName))]
  private string name;

  [ObservableProperty]
  [NotifyPropertyChangedFor(nameof(FullName))]
  private string surname;

  public string FullName => $"{Name} {Surname}";

  protected override void OnPropertyChanging(PropertyChangingEventArgs e)
  {
    base.OnPropertyChanging(e);

    // This is never called for FullName
    Debug.Assert(e.PropertyName != nameof(FullName)); 
  }
}

The following code should cause OnPropertyChanging to be called for property FullName:

var o = new MyObject();
o.Name = "Jack";

Expected behavior

I expected OnPropertyChanging to be called also for property FulleName when Name is assigned a new value.

IDE and version

VS 2022

IDE version

17.2.12

Nuget packages

• [ ] CommunityToolkit.Common
• [ ] CommunityToolkit.Diagnostics
• [ ] CommunityToolkit.HighPerformance
• [X] CommunityToolkit.Mvvm (aka MVVM Toolkit)

Nuget package version(s)

8.2.0

Help us help you

No, just wanted to report this

[Sergio0694]

Oooh good catch! Thank you for reporting this! 😄

EDIT: actually on second thought, this was by design. The rationale is that OnPropertyChanging is only raised for cases where an actual property is being set, right before assigning the field. That is not the case for dependent properties. Those are only notified as having changed due to some external event, such as in this case, a dependent property having changed 🤔

[marhja]

OK, I can understand that this is by design, but then the following paragraph should probably be deleted from the doc comment for NotifyPropertyChangedForAttribute, because that was what made me believe OnPropertyChanging() would be called:

If the containing type also implements the INotifyPropertyChanging interface and exposes a method with the same signature as ObservableObject.OnPropertyChanging(string), then this method will be invoked as well by the property setter.

[Sergio0694]

Thinking about this more, I think it makes sense to fix this. I've implemented it, but changed target to 9.0 instead of 8.2.1, as this is arguably a small breaking change, functionally speaking. Will want to validate this more with a preview release 🙂