Drop files behavior for WinUI 3.

DropFilesBehavior.cs

using Microsoft.UI.Xaml;
using Windows.ApplicationModel.DataTransfer;

namespace App1.Helpers;

// based on https://stackoverflow.com/a/65266427/1200847
public interface IFilesDropped
{
    void OnFilesDropped(string[] files);
}

public class DropFilesBehavior
{
    public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.RegisterAttached(
        "IsEnabled", typeof(bool), typeof(DropFilesBehavior), PropertyMetadata.Create(default(bool), OnIsEnabledChanged));

    public static readonly DependencyProperty FileDropTargetProperty = DependencyProperty.RegisterAttached(
        "FileDropTarget", typeof(IFilesDropped), typeof(DropFilesBehavior), null);

    public static void SetIsEnabled(DependencyObject element, bool value)
    {
        element.SetValue(IsEnabledProperty, value);
    }

    public static bool GetIsEnabled(DependencyObject element)
    {
        return (bool)element.GetValue(IsEnabledProperty);
    }

    public static void SetFileDropTarget(DependencyObject obj, IFilesDropped value)
    {
        obj.SetValue(FileDropTargetProperty, value);
    }

    public static IFilesDropped GetFileDropTarget(DependencyObject obj)
    {
        return (IFilesDropped)obj.GetValue(FileDropTargetProperty);
    }

    private static void OnIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var fe = d as FrameworkElement ?? throw new InvalidOperationException();
        
        if ((bool)e.NewValue)
        {
            fe.AllowDrop = true;
            fe.Drop += OnDrop;
            fe.DragOver += OnDragOver;
        }
        else
        {
            fe.AllowDrop = false;
            fe.Drop -= OnDrop;
            fe.DragOver -= OnDragOver;
        }
    }

    private static void OnDragOver(object sender, DragEventArgs e)
    {
        e.AcceptedOperation = DataPackageOperation.Move; // or Link/Copy
        e.Handled = true;
    }

    private static void OnDrop(object sender, DragEventArgs e)
    {
        var dobj = (DependencyObject)sender;
        var target = dobj.GetValue(FileDropTargetProperty);
        var filesDropped = target switch
        {
            IFilesDropped fd => fd,
            null => throw new InvalidOperationException("File drop target is not set."),
            _ => throw new InvalidOperationException($"Binding error, '{target.GetType().Name}' doesn't implement '{nameof(IFilesDropped)}'."),
        };

        if (filesDropped == null)
        {
            return;
        }

        var files = e.DataView.GetStorageItemsAsync().GetAwaiter().GetResult();
        if (files.Count == 0)
        {
            return;
        }

        filesDropped.OnFilesDropped(files.Select(f => f.Path).ToArray());
    }
}

View.xaml

<Page
    x:Class="App1.Views.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    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:helpers="using:App1.Helpers" 
    Background="{ThemeResource SolidBackgroundFillColorBaseBrush}"
    mc:Ignorable="d">

    <Grid x:Name="ContentArea" helpers:DropFilesBehavior.IsEnabled="True" helpers:DropFilesBehavior.FileDropTarget="{x:Bind ViewModel}">
                
    </Grid>
</Page>

ViewModel.cs

using App1.Helpers;

namespace App1.ViewModels;

public partial class ViewModel : ObservableRecipient, IFilesDropped
{
    public ViewModel()
    {
    }

    public void OnFilesDropped(string[] files)
    {
        Debug.Print("---");
        foreach (var file in files)
        {
            Debug.Print($"{file}");
        }
    }
}