Trying to implement a class using System.Windows DependencyProperty (windowsbase.dll) in a C# Avalonia UI project and failing

Question

I am using the latest version of Avalonia UI and I need to size the drop-down contents of a ComboBox to it's widest member. So I found this solution and I'm trying to implement it. Here's the code for reference:

public static class ComboBoxWidthFromItemsBehavior
{
    public static readonly DependencyProperty ComboBoxWidthFromItemsProperty =
        DependencyProperty.RegisterAttached
        (
            "ComboBoxWidthFromItems",
            typeof(bool),
            typeof(ComboBoxWidthFromItemsBehavior),
            new UIPropertyMetadata(false, OnComboBoxWidthFromItemsPropertyChanged)
        );
    public static bool GetComboBoxWidthFromItems(DependencyObject obj)
    {
        return (bool)obj.GetValue(ComboBoxWidthFromItemsProperty);
    }
    public static void SetComboBoxWidthFromItems(DependencyObject obj, bool value)
    {
        obj.SetValue(ComboBoxWidthFromItemsProperty, value);
    }
    private static void OnComboBoxWidthFromItemsPropertyChanged(DependencyObject dpo,
                                                                DependencyPropertyChangedEventArgs e)
    {
        ComboBox comboBox = dpo as ComboBox;
        if (comboBox != null)
        {
            if ((bool)e.NewValue == true)
            {
                comboBox.Loaded += OnComboBoxLoaded;
            }
            else
            {
                comboBox.Loaded -= OnComboBoxLoaded;
            }
        }
    }
    private static void OnComboBoxLoaded(object sender, RoutedEventArgs e)
    {
        ComboBox comboBox = sender as ComboBox;
        Action action = () => { comboBox.SetWidthFromItems(); };
        comboBox.Dispatcher.BeginInvoke(action, DispatcherPriority.ContextIdle);
    }
}

Unfortunately when I create the class, VS2022 is telling me that The type or namespace name 'DependencyProperty' could not be found.

After looking it up, DependencyProperty is in the System.Windows namespace and the assembly is WindowsBase.dll.

So I added using System.Windows; to the top of my class, but that didn't resolve the problem. I read a couple other solutions on here and they instruct to add windowsbase.dll as an assembly reference.

Thing is, according to my solution, it's already referenced (I think):

The path is C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Ref\7.0.4\ref\net7.0\WindowsBase.dll.

It shows up in my solution's Dependencies under Frameworks > Microsoft.NETCore.App

So how can I get VS2022 to stop complaining that the namespace DependencyProperty and DependencyObject can't be found?

Any help would be massively appreciated.

score 0 · Answer 1 · answered Apr 17 '23 at 06:33

DependencyProperty is for WPF and does not work in Avalonia. You are looking for StyledProperty in AvaloniaUI.

public static readonly StyledProperty<Brush> BackgroundProperty =
    AvaloniaProperty.Register<Border, Brush>(nameof(Background));

public Brush Background
{
    get { return GetValue(BackgroundProperty); }
    set { SetValue(BackgroundProperty, value); }
}

You can read more in the documentation here.

Trying to implement a class using System.Windows DependencyProperty (windowsbase.dll) in a C# Avalonia UI project and failing

1 Answers1