WPF: Update Dependency Property without INotifyPropertyChanged (UserControl)

Question

Scenario: UserControl that has a read-only TextBox and a Button. TextBox.Text is modified and updated whenever the Button is pressed.

Problem: TextControl.Text property is bound to UserControl.Message dependency property but does not update when UserControl.Message is modified from within UserControl. However, target does update when INotifyPropertyChanged is implemented.

I don't actually need to implement INotifyPropertyChanged on a dependency property do I? What am I missing? Please see demonstration code here.

Thanks.

Message Property Declaration

    public static readonly DependencyProperty MessageProperty = 
        DependencyProperty.Register("Message", typeof (string), 
        typeof (TextControl), new FrameworkPropertyMetadata("[WPFApp]" + 
        Environment.NewLine, OnMessageChanged, OnMessageCoerce));

    public string Message
    {
        get { return (string) GetValue(MessageProperty); }
        set { SetValue(MessageProperty, value); }
    }

    private static object OnMessageCoerce(DependencyObject obj, 
        object baseValue)
    {
        return (string) obj.GetValue(MessageProperty) + (string) baseValue;
    }

    private static void OnMessageChanged(DependencyObject d,
        DependencyPropertyChangedEventArgs e)
    {
         // do i need to do this?
         ((TextControl) d).NotifyPropertyChanged("Message");
    }

UserControl abbreviated XAML

<UserControl x:Class="WPFApp.TextControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" d:DesignHeight="64" d:DesignWidth="355"
         DataContext="{Binding RelativeSource={RelativeSource Self}}">
<Grid>
    <TextBox Text="{Binding Message, Mode=OneWay}" ... />
    <Button ... />
</Grid>
</UserControl>

Answer 1

1) No, you don't have to call NotifyPropertyChanged for DependencyProperties.
2) Use a relative source for the binding:

<TextBox Text="{Binding Message, Mode=OneWay,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=UserControl}}" ... />

Additional information:

To find binding related errors, look in the Visual Studio outut window for binding error messages. They are mostly very clear and lead you quickly to the problem.