How to implement two-way binding on a property?

Question

I know there are lots of questions about dependency properties, and I have looked at many of them, but none of them seems to solve my problem.

I have a Window like t

Answer 1

Remove the assignment

DataContext="{Binding RelativeSource={RelativeSource Self}}"

from the UserControl's XAML. Instead, set the RelativeSource of the "internal" binding to the control instance:

<UserControl x:Class="WpfBindingTest.TextInputWrapper" ...>
    <TextBox Text="{Binding MyText,
                    RelativeSource={RelativeSource AncestorType=UserControl}}"/>
</UserControl>

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Explicitly setting the DataContext of a UserControl prevents inheriting the DataContext from its parent control, i.e. a Binding like

<local:TextInputWrapper MyText="{Binding MyTextValue, Mode=TwoWay}" />

would use the UserControl as source object, instead of the current DataContext.

---

As a general rule, never explicitly set the DataContext of a UserControl.

Answer 2

Try changing this line:

public static readonly DependencyProperty MyTextProperty = DependencyProperty.Register("MyText",
typeof(string), typeof(TextInputWrapper), new PropertyMetadata("Empty"));

To this:

public static readonly DependencyProperty MyTextProperty = DependencyProperty.Register("MyText",
    typeof(string), typeof(TextInputWrapper), new FrameworkPropertyMetadata("Empty", FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));