It seems that add and remove cannot activate ObservableCollection to notify changes. How to solve this problem?

[IronEaterBeast]

My view model:

public partial class MainWindowViewModel : ViewModelBase
{
    public ObservableCollection<OneLineClass> _allContentOC { get; set; } = [];
    public ObservableCollection<OneLineClass> AllContentOC
    {
        get
        {
            return _allContentOC;
        }
        set
        {
            _allContentOC = value;
            this.RaisePropertyChanged(nameOf(AllContentOC));
        }
    }
}

Can activate:

var test = new MainWindowViewModel();
test.AllContentOC = [];

Cannot activate
test.AllContentOC.Add(new OneLineClass("test"));

Did I miss something?

[maxkatz6]

Which changes do you want to activate? Adding a new item to the array won't notify parent view model about any changes. Instead you need to subscribe on AllContentOC.CollectionChanged event.

[IronEaterBeast]

In my imagination, adding a new item should be the same as assigning and should enter the set method, then use RaisePropertyChanged to update the window display, but it doesn't seem to work that way, so I don't know how to handle this situation.

[IronEaterBeast]

if OneLineClass is another ViewModelBase like public class OneLineClass : ViewModelBase then AllContentOC.CollectionChanged will not work too.

[maxkatz6]

In my imagination, adding a new item should be the same as assigning and should enter the set method

It's not how it works. Normally "Property changed" event only is raised when any actual property is changed.
But since you implement your own ViewModelBase, that's up to you. Still - you wrote no code to call RaisePropertyChanged when collection items are changed. Setter won't be executed in this situation obviously.

if OneLineClass is another ViewModelBase like public class OneLineClass : ViewModelBase then AllContentOC.CollectionChanged will not work too.

AllContentOC.CollectionChanged still will work, when collection is changed.
But if you want to subscribe on property changes of the OneLineClass, you need to subscribe on each item.

PropertyChanged and CollectionChanged are standard .NET events, and that's how they work. Avalonia just follows the same rules.

PropertyChanged is about property of this class, where event is defined.
And CollectionChanged is about collection changes of this collection/class, where event is defined.