Why Doesn't My XAML Binding Work on a Dependency Property, But Works in Code-Behind?

XAML binding fails on dependency properties

Data binding for dependency properties has no effect in XAML, but works fine in code-behind. The following code snippet demonstrates the problem:

<code class="language-xml"><UserControl ...="" x:Class="WpfTest.MyControl">
    <TextBlock Text="{Binding Test}"/>
</UserControl></code>

Dependency properties are defined as follows:

<code class="language-csharp">public static readonly DependencyProperty TestProperty =
    DependencyProperty.Register("Test", typeof(string), typeof(MyControl), new PropertyMetadata("DEFAULT"));

public string Test
{
    get { return (string)GetValue(TestProperty); }
    set { SetValue(TestProperty, value); }
}</code>

In the main window, binding to normal properties works perfectly:

<code class="language-xml"><TextBlock Text="{Binding MyText}"/></code>

However, the same binding in the user control does not update the text:

<code class="language-xml"><MyControl Test="{Binding MyText}" x:Name="TheControl"/></code>

It's worth noting that the binding works fine when implemented in code-behind:

<code class="language-csharp">TheControl.SetBinding(MyControl.TestProperty, new Binding
{
    Source = DataContext,
    Path = new PropertyPath("MyText"),
    Mode = BindingMode.TwoWay
});</code>

Solution:

Correct dependency property declaration:

<code class="language-csharp">public static readonly DependencyProperty TestProperty =
    DependencyProperty.Register(
        nameof(Test),
        typeof(string),
        typeof(MyControl),
        new PropertyMetadata("DEFAULT"));</code>

Binding in UserControl XAML:

<code class="language-xml"><UserControl ...="" x:Class="WpfTest.MyControl">
    <TextBlock Text="{Binding Test, RelativeSource={RelativeSource AncestorType=UserControl}}"/>
</UserControl></code>

Avoid setting DataContext in UserControl constructor:

Never set the DataContext in the UserControl's constructor. This prevents the UserControl from inheriting its parent's DataContext.

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