WPF controls are built on the foundation of dependency objects - the DependencyObject is at the bottom of their inheritance chain. But they also add additional functionality on top of that through another common base class, FrameworkElement. The FrameworkElement is involved in the layout algorithm, as well as helping to define the elements tree. Let’s add a second dependency property to our <NumberBox>, a Value property that will represent the value the <NumberBox> currently represents, which will be displayed in the <TextBox>.

We register this dependency property in much the same way as our Step. But instead of supplying the DependencyProperty.Register() method a PropertyMetadata, we’ll instead supply a FrameworkPropertyMetadata, which extends PropertyMetadata to include additional data about how the property interacts with the WPF rendering and layout algorithms. This additional data is in the form of a bitmask defined in FrameworkPropertyMetadataOptions enumeration.

Some of the possible options are:

• FrameworkPropertyMetadataOptions.AffectsMeasure - changes to the property may affect the size of the control
• FrameworkPropertyMetadataOptions.AffectsArrange - changes to the property may affect the layout of the control
• FrameworkPropertyMetadataOptions.AffectsRender - changes to the property may affect the appearance of the control
• FrameworkPropertyMetadataOptions.BindsTwoWayByDefault - This property uses two-way bindings by default (i.e. the control is an editable control)
• FrameworkPropertyMetadataOptions.NotDataBindable - This property does not allow data binding

In this case, we want a two-way binding by default, so we’ll include that flag, and also we’ll note that it affects the rendering process. Multiple flags can be combined with a bitwise OR. Constructing our FrameworkPropertyMetadata object would then look like:

new FrameworkPropertyMetadata(0, FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.BindsTwoWayByDefault)

And registering the dependency property would be:

/// <summary>
/// Identifies the NumberBox.Value XAML attached property
/// </summary>
public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(nameof(Value), typeof(double), typeof(NumberBox), new FrameworkPropertyMetadata(0, FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

As with the Step, we also want to declare a traditional property with the name “Value”. But instead of declaring a backing field, we will use the key/value pair stored in our DependencyObject using GetValue() and SetValue():

/// <summary>
/// The NumberBox's displayed value
/// </summary>
public double Value {
    get { return (double)GetValue(ValueProperty); }
    set { SetValue(ValueProperty, value); }
}

If we want to display the current value of Value in the textbox of our NumberBox control, we’ll need to bind the <TextBox> element’s Text property. This is accomplished in a similar fashion to the other bindings we’ve done previously, only we need to specify a RelativeSource. This is a source relative to the control in the elements tree. We’ll specify two properties on the RelativeSource: the Mode which we set to FindAncestor to search up the tree, and the AncestorType which we set to our NumberBox. Thus, instead of binding to the DataContext, we’ll bind to the NumberBox the <TextBox> is located within. The full declaration would be:

<TextBox Grid.Column="1" Text="{Binding Path=Value, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=local:NumberBox}}"/>

Now a two-way binding exists between the Value of the <NumberBox> and the Text value of the textbox. Updating either one will update the other. We’ve in effect made an editable control!