Why Is My XAML Binding to a Dependency Property Failing, But Code-Behind Works?

XAML binding dependency properties failed

Question

Binding dependency properties fails in XAML but works fine in code-behind.

Reason

1. Dependency property declaration error: TestProperty The declaration of the dependency property lacks the nameof() operator, resulting in a mismatch between the property name and the XAML binding path.
2. Binding source error in XAML: The binding source (DataContext) is not explicitly set in XAML binding, so it defaults to the UserControl itself, rather than the DataContext inherited from the parent window.
3. Set DataContext in the constructor: Setting the DataContext property in the constructor of UserControl will prevent the inheritance of the parent's DataContext, causing the binding source to be invalid.

Solution

1. Correctly declare dependency attributes

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

2. Set Bindings.RelativeSource in XAML

<code class="language-xml"><TextBlock Text="{Binding Test,
    RelativeSource={RelativeSource AncestorType=UserControl}}"></TextBlock></code>

3. Delete the DataContext assignment in the constructor

Remove the DataContext = this; line of code from the UserControl constructor.

Additional Notes

Explicitly set the binding source in code-behind:

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

Set binding source implicitly in XAML:

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

In XAML, the implicit binding source defaults to the current DataContext and should be set correctly by the parent window. Setting the DataContext in the constructor of UserControl will overwrite the inherited DataContext, causing binding to fail. By setting Source = DataContext in the code-behind binding, the binding source is explicitly set to the inherited DataContext, ensuring that the binding is valid.

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