How can I get data binding to work properly with a DependencyProperty having an ObservableCollection property?

[SetTrend]

My WPF window's view model resembles the following:

public class ViewModel : INotifyPropertyInfo
{
  private ObservableCollection<MyItems> _myItems;

  public  ObservableCollection<MyItems> MyItemCollection
  {
    get => _myItems;

    set =>
    {
      _myItems = value;
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(MyItemCollection)));
    }
  }
}

My WPF window's code-behind class resembles this:

public partial class MainWindow : Window
{
  public static readonly DependencyProperty VMProperty = DependencyProperty.Register("VM"
                                                                                    , typeof(ViewModel)
                                                                                    , typeof(MainWindow)
                                                                                    );

  public ViewModel VM
  {
    get => (ViewModel)GetValue(VMProperty);
    set => SetValue(VMProperty, value);
  }
}

Now, wenn I bind an ItemsControl.ItemSource to VM.MyItemCollection – nothing happens when items in the ObservableCollection member of my view model are updated.

What am I doing wrong?

Here's an excerpt of my MainWindow.xaml:

<Window x:Class="GaqlReplApplication.WpfWindows.Main.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" x:Name="MainWin"
				DataContext="{Binding RelativeSource={RelativeSource Mode=Self}}"
				>
  <ListView ItemsSource="{Binding Path=VM.MyItemCollection}"/>
</Window>

[miloush]

Do you have a small repro project?

[miloush]

For 2., the command needs to be set on the container, so you can do:

<MenuItem.ItemContainerStyle>
	<Style TargetType="MenuItem">
		<Setter Property="Command" Value="{StaticResource ResourceKey=FileMruCmd}" />
	</Style>
</MenuItem.ItemContainerStyle>

This will not affect your ClearCmd because local values have precedence over values from style.

[SetTrend]

Thanks so much for enlightening me! 👍

OK, my issues 2 and 3 are successfully solved now. Yet, I still have problems understanding issue 1 …

1. Because you are updating different collection than the controls are bound to. In Window_Loaded, you are setting Settings.Preferences to a new instance, so FileMruList => Settings.Preferences.FileMruList now points to a brand new collection. You either need to set Settings.Preferences before the InitializeComponent() call, or, you will have to make sure changes to FileMruList instance are announced via property change notification.

As far as I can see, all involved classes support INotifyPropertyChanged:

public class FullConfig : _NotifyPropertyChanged
{
  public Options Preferences
  {
    get => _options;

    set
    {
      _options = value;
      OnPropertyChanged();
    }
  }
}

public class Options : _NotifyPropertyChanged
{
  public FileMruList FileMruList
  {
    get => _fileMruList;

    private init
    {
      _fileMruList = value;
      OnPropertyChanged();
    }
  }
}

public class FileMruList : ObservableCollection<MruItem>
{}

[miloush]

not the MainWindow though where you take the FileMruList from in the XAML binding

[miloush]

And you silently update the underlying storage, so you don't have a way of notifying it anyway. If Settings is INotifyPropertyChanged, then it might work if you bind to Settings.Preferences.FileMruList instead.

[miloush]

Alternatively, if you make the MainWindow INotifyPropertyChanged, then after you set the Settings.Preferences in Window_Loaded, you could fire OnProperyChanged for the FileMruList (and possibly the Settings as well)

[SetTrend]

Marvellous! Thank you so much for your excellent help! 👍❤️❤️❤️👍

So, from what I learned today, data binding doesn't work by barely inspecting a property's object for implementing INotifyPropertyChanged, but requires a dependency property as a starting point when swinging from that property to the property's property and so on. Am I right?

[SetTrend]

Yet, @miloush, there are two more data binding errors now appearing in the XAML Binding Failures panel:

ItemTemplate and ItemTemplateSelector are ignored for items already of the ItemsControl's container type Separator
ItemTemplate and ItemTemplateSelector are ignored for items already of the ItemsControl's container type MenuItem

These errors are bothering me. And they are distracting when examining my own data binding errors. Do you have these XAML binding failures on your backlog?

PS: I uploaded the amended version to the MRE now.

[miloush]

The starting point does not need to be a dependency property. It can notify using other ways including INotifyPropertyChanged.

The problem was that your property never changed. Your binding was simply reading MainWindow.FileMruList. It doesn't know anything about settings and preferences, you simply provided an instance to the binding and that instance never changed. It was still pointing to the list in the old preferences.

When databinding does the "swinging" itself, it looks at the property path and monitors all subproperties for changes. However, you shortcut it - you did the "swinging" yourself in MainWindow without monitoring for the changes.