Binding using a specified source fails

[eriklimakc]

Describe the bug

When attempting to bind data from the ViewModel to a Custom UserControl created entirely in C# the binding fails and no data appears.

Steps to reproduce the bug

Repro app: FrameControl.zip

MainWindow.xaml

<StackPanel x:Name="RootElement" 
            Orientation="Horizontal"
            HorizontalAlignment="Center"
            VerticalAlignment="Center" />

MainWindow.xaml.cs

public MainWindow()
{
	this.InitializeComponent();

	RootElement.DataContext = new MainPageViewModel();

	var frameControl = new FrameControl();
	frameControl.SetBinding(FrameControl.CaptionProperty, new Binding
	{
		Path = new PropertyPath(nameof(MainPageViewModel.Title))
	});

	RootElement.Children.Add(frameControl);
}

MainPageViewModel

public partial class MainPageViewModel : ObservableObject
{
	[ObservableProperty]
	private string _title = "Hello!";
}

FrameControl

public partial class FrameControl : UserControl
{
	public string Caption
	{
		get { return (string)GetValue(CaptionProperty); }
		set { SetValue(CaptionProperty, value); }
	}

	public static readonly DependencyProperty CaptionProperty =
		DependencyProperty.Register(nameof(Caption), typeof(string), typeof(FrameControl), new PropertyMetadata(""));

	public FrameControl()
	{
		var grid = new Grid();
		grid.ColumnDefinitions.Add(new ColumnDefinition() { Width = GridLength.Auto });
		grid.ColumnDefinitions.Add(new ColumnDefinition() { Width = new GridLength(1, GridUnitType.Star) });

		var border = new Border
		{
			Width = 300,
			Background = new SolidColorBrush(Colors.Blue)
		};
		Grid.SetColumn(border, 0);

		var textBlock = new TextBlock
		{
			FontSize = 30,
			Foreground = new SolidColorBrush(Colors.White)
		};
		textBlock.SetBinding(TextBlock.TextProperty, new Binding
		{
			Path = new PropertyPath(nameof(Caption)),
			Source = this,
			Mode = BindingMode.OneWay
		});
		Grid.SetColumn(textBlock, 1);

		grid.Children.Add(border);
		grid.Children.Add(textBlock);

		Content = grid;
	}
}

Expected behavior

The source binding should work.

Screenshots

Current

Expected

NuGet package version

None

Packaging type

No response

Windows version

No response

IDE

No response

Additional context

No response

[evelynwu-msft]

@eriklimakc Because FrameControl ultimately inherits from DependencyObject it needs to follow the rules for a DependencyObject. Specifically, it needs to have type information that can be located through the app's (generated by default) IXamlMetadataProvider implementation. If FrameControl were referenced directly in XAML markup then this type information would be automatically included in the IXamlMetadataProvider implementation but because it is not you need to decorate the type with the [Bindable] attribute in order to force its inclusion. Note that MainPageViewModel does not have the same requirement because it does not inherit from DependencyObject.