Where does the default TwoWay binding comes from ?

I got a comment on my post about a very simple MVVM application about the fact that removing the TwoWay mode on a binding did not change the behavior of the application. This is a quick occasion for me to share a quick explanation about this.

Actually and as you already know if you can write XAML like Text={Binding …} it is only because Text is a Dependency Property. Also, dependency properties are defined in a static way (so that if you have 50 textboxes you don’t have to instantiate 50 times the Text property). The default behavior for the mode of the binding (TwoWay, OneWay, etc.) can be found in the static declaration of the dependency property. For example, in the case of the Text property of the TextBox we have:

TextProperty = DependencyProperty.Register(
	"Text", 
	typeof(string), 
	typeof(TextBox), 
	new FrameworkPropertyMetadata(
		string.Empty, 
		FrameworkPropertyMetadataOptions.Journal 
		| FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
	new PropertyChangedCallback(TextBox.OnTextPropertyChanged), 
	new CoerceValueCallback(TextBox.CoerceText),
	true, 
	UpdateSourceTrigger.LostFocus));

The interesting part here if of course the BindsTwoWayByDefault option. Note that this is the only default option available (we can’t have a OneWayToSource binding by default).

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