Why Does Setting DataContext to `this` Break DependencyProperty Binding in a UserControl?

Troubleshooting DependencyProperty Binding in UserControls

Setting a UserControl's DataContext to this can unexpectedly disrupt data bindings to parent control properties. This occurs because the binding loses the inheritance of the DataContext property.

The Problem Explained

When the DataContext of a UserControl is set to itself, bindings within the UserControl attempting to access properties from a parent control's view model will fail. The binding mechanism can't find the target property because it's looking in the wrong context (the UserControl itself, not its parent).

The Solution: Leverage RelativeSource

The solution is to avoid setting DataContext = this in the UserControl's constructor. Instead, utilize RelativeSource within the XAML binding to explicitly target the parent control's DataContext.

Here's how to adjust the XAML binding:

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

This binding now correctly searches up the visual tree to find the DataContext of the nearest ancestor of type UserControl, effectively accessing the parent's view model.

To complete the setup, bind the SelectedFile property of the UserControl to the corresponding property in the parent's view model:

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

This approach ensures a proper connection between the UserControl's SelectedFile DependencyProperty and the parent's view model property, resolving the binding issue.

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