Why Doesn't My DependencyProperty Binding Update the Parent ViewModel?

Troubleshooting DependencyProperty Bindings in a File Browser Control

While building a custom file browser control, a common issue arises where selecting a file updates the control's internal textbox but fails to propagate the change to the SelectedFile property in the parent view model. This happens even when using a DependencyProperty and standard binding.

The root cause often stems from setting the DataContext of the UserControl to itself within its constructor:

DataContext = this;

This self-referencing DataContext overrides the inherited DataContext from the parent, breaking the binding to the parent view model.

Solution: Utilizing RelativeSource Binding

The solution involves modifying the binding within the UserControl's XAML to explicitly target the parent view model. Use RelativeSource to traverse the visual tree:

<TextBox Text="{Binding SelectedFile, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}" />

This revised binding ensures that the SelectedFile property in the UserControl binds correctly to the SelectedFile property of the parent view model, even when the DataContext of the UserControl is set internally.

Now, when embedding the UserControl:

<FileBrowserControl SelectedFile="{Binding SelectedFile}" />

The binding will function as expected, updating the parent view model's SelectedFile property upon file selection.

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