ComboBox selection breaks when Items is updated (two bugs? check comments)

[Kermalis]

The following xaml:

<ComboBox Items="{Binding MyItems}" SelectedItem="{Binding MySelection}"/>

The following code:

public IEnumerable<object> MyItems { get; }
private object _mySelection;
public object MySelection
{
	get => _mySelection;
	set
	{
		if (_mySelection != value)
		{
			_mySelection = value;
			UpdateMyItems(); // MyItems gets set
			OnPropertyChanged(nameof(MySelection)); // Fire MySelection first with INotifyPropertyChanged
			OnCollectionChanged(nameof(MyItems)); // Fire MyItems second with INotifyCollectionChanged
		}
	}
}

You get the following ComboBox:

If you swap the order, it works as intended.

Due to the order my code runs in, I cannot simply swap the order without breaking my programs, so I have been forced to make hacky workarounds in them. It should work both ways since SelectedItem is still the correct value. Even if SelectedItem does not change, but Items does change, it still breaks until a propertychanged event is fired even though the SelectedItem is in the new collection. A fix for this would greatly reduce the amount of redundant extra code being run on my side.

[Kermalis]

Just to be clear, I'm using INotifyCollectionChanged, but I think if you use INotifyPropertyChanged to change the MyItems object for another object, it still will not adapt to the SelectedItem. I think this affects all ItemsControl but I have only been using ComboBox in my programs where I've had to deal with this

[Kermalis]

I know this isn't helpful but this is probably the most annoying bug in Avalonia and it's annoyed me for years now. I'm begging you to fix this before 0.10

Right now I'm making a model that has a ComboBox of string objects. If I update the items of the ComboBox, then the selection is hidden no matter what order I fire events and no matter if the old index is available in range of the new items. I have to manually click the empty ComboBox and select my item in the UI because it's not working in code-behind at all (I have no idea why, this is the first time). It's driving me crazy that something so simple does not work. I really don't know what to do about it other than come here and complain.

<ComboBox Items="{Binding SelectableItems}" SelectedIndex="{Binding Index}" />

private int _index = 0; // Default to first item works
public int Index
{
  get => _index;
  private set
  {
    if (value == -1)
    {
      _index = 0; // No -1 please I beg you, why would I ever want -1
      OnPropertyChanged(nameof(Index));
    }
    else if (value != _index)
    {
      _index = value;
      OnPropertyChanged(nameof(Index));
    }
  }
}
private IEnumerable<string> _selectableItems = new[] { "Default" };
public IEnumerable<string> SelectableItems
{
  get => _selectableItems;
  private set
  {
    _selectableItems = value;
    OnPropertyChanged(nameof(SelectableItems));
  }
}

private void UpdateItems(object[] items)
{
  // Setting selectable items sets the selected index to -1 for no reason
  if (items.Length == 0)
  {
    SelectableItems = new[] { "Default" };
  }
  else
  {
    SelectableItems = items.Select(o => o.ToString());
  }
  Index = 0; // No selection even though I'm telling it to select 0 here and in the setter
  /* Same nonsense if you fire in reverse order
  Index = 0;
  SelectableItems = items;
  */
}

Honestly what can I do?

[Kermalis]

The (different) bug in the above comment does not occur if you let the ComboBox successfully set your index to -1 and you change the index from -1 afterwards to what you want. Really, this is unnecessary (and will not work if you are using unsigned types to retrieve your value like I am in my real code), but I have yet another hacky workaround for this now (which is to have a conditional in my getter to return -1 instead of my unsigned value when this bug would occur).

[Gillibald]

I think we might be able to get this to work.

The ItemsControl could use SelectedIndex = IndexOf(SelectedItem) when Items changes. That way one can add items and the selection isn't changing if the currently selected item is present in the Items collection.

[Kermalis]

Something to that effect would definitely fix both of these issues, as long as it works that way for both CollectionChanged and PropertyChanged

[Kermalis]

Something with Items is really wrong. This may be related to what's causing #4461

[grokys]

@Kermalis taking a look at this now. Got a few questions:

1. Which version of Avalonia?
2. What does OnCollectionChanged(nameof(MyItems)); do? Is MyItems a custom collection class that implements INotifyCollectionChanged and OnCollectionChanged is raising the CollectionChanged event on that? If so, with what Action?
3. ComboBox selection breaks when Items is updated (two bugs? check comments) #4048 (comment) is a different problem, correct? It would be better to open a separate issue for that

I really don't know what to do about it other than come here and complain.

Opening an issue here is definitely the correct thing to do! You're not complaining ;) We can't fix issues if we don't know about them

One this that would really help me would be to create a repository which demonstrates the problem with the last code possible. I'll try to work out the exact problem from the information you've provided here in the meantime.

[grokys]

Maybe @Gillibald has understood the problem better than me. Is the issue just that:

• You assign Items and SelectedItem
• You change Items to a new collection which also contains SelectedItem
• SelectedItem isn't preserved?

[JaggerJo]

Maybe @Gillibald has understood the problem better than me. Is the issue just that:

• You assign Items and SelectedItem
• You change Items to a new collection which also contains SelectedItem
• SelectedItem isn't preserved?

I think so :)

[Gillibald]

Y that is how I understood it

[Kermalis]

Yes, that's the issue. SelectedIndex is set to -1 when the collection changes. Also my other comment you linked to is indeed another issue, #4472.