WPF usercontrol Twoway binding Dependency Property

Bind the TextBox.Text property in the UserControl to its SampleProperty like this:

<TextBox Text="{Binding SampleProperty,
                RelativeSource={RelativeSource AncestorType=UserControl}}"/>

Now you could simply remove your OnSamplePropertyChanged callback.

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You might also register SampleProperty to bind two-way by default like this:

public static readonly DependencyProperty
    SamplePropertyProperty = DependencyProperty.Register(
        "SampleProperty", typeof(string), typeof(UserControl1),
        new FrameworkPropertyMetadata(
            null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

An alternative way to do this is an ElementName Binding. First assign the x:Name attribute to the UserControl (for example x:Name="MyUC"), then change the binding to:

<TextBox Text="{Binding ElementName=MyUC, Path=SampleProperty}"/>