StyledProperty Setter Not Called in Custom Control

[ArtemRazin]

I've noticed an unusual behavior in a large project and created a small sample to demonstrate the issue.

When a custom control has a StyledProperty and is styled in XAML, the setter of the StyledProperty is never called.

Code Example

internal class CustomControl : Control
{
    public static readonly StyledProperty<Color> BackgroundColorProperty =
        AvaloniaProperty.Register<CustomControl, Color>(nameof(BackgroundColor), Colors.Transparent);

    public Color BackgroundColor
    {
        get => GetValue(BackgroundColorProperty);
        set
        {
            SetValue(BackgroundColorProperty, value);
            Debug.Assert(false);
        }
    }

    public CustomControl()
    {
        this.GetObservable(BackgroundColorProperty)
            .Subscribe(_ => InvalidateVisual());
    }

    public override void Render(DrawingContext context)
    {
        context.DrawRectangle(
            new SolidColorBrush(BackgroundColor),
            null,
            Bounds);
    }
}

To verify this, I added Debug.Assert(false) inside the setter.

Here's how the control is used in XAML:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AvaloniaCustomControlStyling.MainWindow"
        Title="AvaloniaCustomControlStyling"
        xmlns:local="using:AvaloniaCustomControlStyling"
        xmlns:controls="using:AvaloniaCustomControlStyling.Controls">

    <Window.Styles>
        <Style Selector="controls|CustomControl#StyledCustomControl">
            <Setter Property="BackgroundColor" Value="Green" />
        </Style>
    </Window.Styles>

    <controls:CustomControl x:Name="StyledCustomControl" />

</Window>

The control renders with a green background, but the setter is never called.

Is this expected behavior?

[timunie]

Yeah this is by design. The Property is just for convenience.

If you want to handle some values set, I'd suggest to override OnPropertyChanged or use a ClassHandler in static constructor.