ReadOnly functionality for TexBox-like controls

The TextBox control (Microsoft.UI.Xaml.Controls.TextBox) exposes an "IsReadonly" property, which seems to differ from !IsEnabled in the following ways:

• The content of a ReadOnly TextBox can be selected
• A ReadOnly TextBox is a tab stop by default
• ReadOnly TextBoxes are initially visually indistinct from Non-ReadOnly TextBoxes

Other textbox-like controls, such as the NumberBox and AutoSuggestBox, have no such property. Is there any way to convince other controls that are visually similar to a TextBox to have ReadOnly behaviour?

AutoSuggestBox and NumberBox have an inner TextBox. You can access it with the CommunityToolkit.WinUI.Extensions's FindDescendant().

For example, you can create an AttachedProperty like this:

public static class Extensions
{
    public static readonly DependencyProperty IsReadOnlyProperty =
        DependencyProperty.RegisterAttached(
            "IsReadOnly",
            typeof(bool),
            typeof(Extensions),
            new PropertyMetadata(default, OnIsReadOnlyPropertyChanged));

    public static bool GetIsReadOnly(DependencyObject obj) => (bool)obj.GetValue(IsReadOnlyProperty);

    public static void SetIsReadOnly(DependencyObject obj, bool value) => obj.SetValue(IsReadOnlyProperty, value);

    private static void OnIsReadOnlyPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not FrameworkElement element)
        {
            return;
        }

        element.Loaded -= Element_Loaded;

        if (element.IsLoaded is false)
        {
            element.Loaded += Element_Loaded;
            return;
        }

        _ = TryUpdateIsReadOnly(element);
    }

    private static void Element_Loaded(object sender, RoutedEventArgs e)
    {
        _ = TryUpdateIsReadOnly(sender);
    }

    private static bool TryUpdateIsReadOnly(object sender)
    {
        if (sender is not FrameworkElement element ||
            element.FindDescendant<TextBox>() is not TextBox innerTextBox)
        {
            return false;
        }

        innerTextBox.IsReadOnly = GetIsReadOnly(element);
        System.Diagnostics.Debug.WriteLine($"{element.GetType()} IsReadOnly: {innerTextBox.IsReadOnly}");
        return true;
    }
}

Then use it like this:

<StackPanel Spacing="8">
    <ToggleSwitch
        x:Name="IsReadOnlyToggleSwitch"
        Header="IsReadOnly"
        OffContent="False"
        OnContent="True" />
    <TextBox
        IsReadOnly="{x:Bind IsReadOnlyToggleSwitch.IsOn, Mode=OneWay}"
        Text="TextBox" />
    <AutoSuggestBox
        local:Extensions.IsReadOnly="{x:Bind IsReadOnlyToggleSwitch.IsOn, Mode=OneWay}"
        Text="AutoSuggestBox" />
    <NumberBox
        local:Extensions.IsReadOnly="{x:Bind IsReadOnlyToggleSwitch.IsOn, Mode=OneWay}"
        Value="12345" />
</StackPanel>