<TextBox Grid.ColumnSpan="2" Grid.Row="1" x:Name="textBoxFolder" Margin="2,4">
<TextBox.Text>
<Binding Path="this.MovieFolder" UpdateSourceTrigger="PropertyChanged">
  <Binding.ValidationRules>
    <!--  Validation rule set to run when binding target is updated. -->
    <Rules:MandatoryInputRule ValidatesOnTargetUpdated="True" />
  </Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>