How to handle drag/drop without violating MVVM principals?

Question

Currently I have in my XAML

All of

Answer 1

Here is a solution a bit more generic, out-of-the-box and easier than Mustafa's one, with a single DependencyProperty

1. Copy this interface in your projet and

public interface IFilesDropped
{
    void OnFilesDropped(string[] files);
}

2. Make you ViewModel implement the interface

public class SomeViewModel : IFilesDropped
{
    public void OnFilesDropped(string[] files)
    {
        // Implement some logic here
    }
}

3. Copy this generic extension in your project

public class DropFilesBehaviorExtension
{
    public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.RegisterAttached(
        "IsEnabled", typeof(bool), typeof(DropFilesBehaviorExtension), new FrameworkPropertyMetadata(default(bool), OnPropChanged)
        {
            BindsTwoWayByDefault = false,
        });

    private static void OnPropChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (!(d is FrameworkElement fe))
            throw new InvalidOperationException();
        if ((bool)e.NewValue)
        {
            fe.AllowDrop = true;
            fe.Drop += OnDrop;
            fe.PreviewDragOver += OnPreviewDragOver;
        }
        else
        {
            fe.AllowDrop = false;
            fe.Drop -= OnDrop;
            fe.PreviewDragOver -= OnPreviewDragOver;
        }
    }

    private static void OnPreviewDragOver(object sender, DragEventArgs e)
    {
        // NOTE: PreviewDragOver subscription is required at least when FrameworkElement is a TextBox
        // because it appears that TextBox by default prevent Drag on preview...
        e.Effects = DragDropEffects.Move;
        e.Handled = true;
    }

    private static void OnDrop(object sender, DragEventArgs e)
    {
        var dataContext = ((FrameworkElement)sender).DataContext;
        if (!(dataContext is IFilesDropped filesDropped))
        {
            if (dataContext != null)
                Trace.TraceError($"Binding error, '{dataContext.GetType().Name}' doesn't implement '{nameof(IFilesDropped)}'.");
            return;
        }

        if (!e.Data.GetDataPresent(DataFormats.FileDrop))
            return;

        if (e.Data.GetData(DataFormats.FileDrop) is string[] files)
            filesDropped.OnFilesDropped(files);
    }

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

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

4. Enable the Drop files behavior to the UI components of you choice (here a TextBox)

Happy drops !