WPF DataBinding to control properties

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WPF

MVVM

Binding

I have an issue whereby I am not receiving updates through my bindings.

I have a label which is bound to the ExtentWidth of the TextBox property via the DataContext.

My binding initially works and displays the value of 0 in the label however it does not update after this.

ExtentWidth is a read only property, I'm not sure if this affects the binding in any way but I have a label the binds to the text when it is set so I know it can receive updates. (button updates text and label is updated)

below is some code to demonstrate my issue.

<Window x:Class="TestHarnesses.Views.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window1" Height="300" Width="300">
    <Grid>
        <StackPanel>
            <ContentPresenter x:Name="ContentPresenter" Content="{Binding}"></ContentPresenter>
            <Label x:Name="lblExtentWidth" 
                   Content="{Binding ExtentWidth, Mode=OneWay}"/>
            <Label x:Name="lblText" 
                   Content="{Binding Text, Mode=OneWay}"/>
            <Button Content="Different Jibber Jabber" Click="ButtonBase_OnClick"/>
        </StackPanel>
    </Grid>
</Window>

using System.Windows;
using System.Windows.Controls;

namespace TestHarnesses.Views
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
            TextBox tb = new TextBox(){Text = "Jibber Jabber"};
            this.TestTextBox = tb;
        }



        public TextBox TestTextBox
        {
            get { return (TextBox)GetValue(TestTextBoxProperty); }
            set { SetValue(TestTextBoxProperty, value); }
        }

        // Using a DependencyProperty as the backing store for TestTextBox.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty TestTextBoxProperty =
            DependencyProperty.Register("TestTextBox", typeof(TextBox), typeof(Window1), new PropertyMetadata(OnTestTextBoxProperty));

        private static void OnTestTextBoxProperty(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ((Window1) d).DataContext = (TextBox) e.NewValue;
        }

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            TestTextBox.Text = "Different Jibber Jabber";
        }
    }
}