File Drag and Drop Not Working

[maguro1kan]

I have set DragDrop.AllowDrop to True on a Window or Border, but the Drop method does not respond when I drag a file onto the application window. Could you please provide a solution?

MainWindow.axaml

<Window x:Class="DragDropAvalonia.Views.MainWindow"
        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"
        xmlns:vm="using:DragDropAvalonia.ViewModels"
        Title="DragDropAvalonia"
        d:DesignHeight="450"
        d:DesignWidth="800"
        x:DataType="vm:MainWindowViewModel"
        Icon="/Assets/avalonia-logo.ico"
        mc:Ignorable="d">

    <Design.DataContext>
        <vm:MainWindowViewModel />
    </Design.DataContext>

    <Border Background="Transparent" DragDrop.AllowDrop="True">
        <TextBlock HorizontalAlignment="Center"
                   VerticalAlignment="Center"
                   Text="{Binding Greeting}" />
    </Border>

</Window>

MainWindow.axaml.cs

namespace DragDropAvalonia.Views
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            if (OperatingSystem.IsWindows())
            {
                Thread.CurrentThread.SetApartmentState(ApartmentState.Unknown);
                Thread.CurrentThread.SetApartmentState(ApartmentState.STA);
            }

            AddHandler(DragDrop.DragOverEvent, DragOver);
            AddHandler(DragDrop.DropEvent, Drop);
            AddHandler(DragDrop.DragLeaveEvent, DragLeave);
        }

        private void DragOver(object? sender, DragEventArgs e)
        {
            Debug.WriteLine("DragOver");
        }

        private void Drop(object? sender, DragEventArgs e)
        {
            Debug.WriteLine("Drop");
        }

        private void DragLeave(object? sender, DragEventArgs e)
        {
            Debug.WriteLine("Drag leave");
        }
    }
}

Desktop.2024.09.25.-.16.22.13.33.mp4

[maxkatz6]

You can't call SetApartmentState on already running thread.
Instead you should have apartment state attribute on the Main method.

[maguro1kan]

    internal class Program
    {
        // Initialization code. Don't use any Avalonia, third-party APIs or any
        // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
        // yet and stuff might break.
        [STAThread]
        public static void Main(string[] args)
        {
            if (OperatingSystem.IsWindows())
            {
                Thread.CurrentThread.SetApartmentState(ApartmentState.Unknown);
                Thread.CurrentThread.SetApartmentState(ApartmentState.STA);
            }

            BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
        }

        // Avalonia configuration, don't remove; also used by visual designer.
        public static AppBuilder BuildAvaloniaApp()
        {
            IconProvider.Current
                .Register<FontAwesomeIconProvider>()
                .Register<MaterialDesignIconProvider>();

            return AppBuilder.Configure<App>()
                .UsePlatformDetect()
                .LogToTrace()
                .UseReactiveUI();
        }
    }

I moved the code to Program.cs, but DragDrop still doesn't work.
Are there any other places I need to change?

[stevemonaco]

You likely need to set the DragEventArgs to let Avalonia know if a certain operation is valid. See https://github.com/wieslawsoltes/HexView/blob/4dd5c91c79fb6f8070e14f7787c6db9f5e9f9e40/HexView/Views/DiffView.axaml.cs#L53-L83 for an example.

[maguro1kan]

Resolved.

The issue was that Drag and Drop did not work when opening the project from the “Recent items” list in Visual Studio 2022. I discovered that Drag and Drop becomes enabled if I open the project by right-clicking the .sln file and selecting "Open with" -> "Visual Studio 2022".