I want the user to drag and drop a .txt file into a field. I created a Border for this. I wrote a helpful c# code that checks for drag and drop states. The following application codes understand when the user drags and drops the file.

I can understand that the user brought the file to the field. So how can I tell if the user removed the file from that area?

Xaml Code

 <Border BorderThickness="1,1,1,1"
            HorizontalAlignment="Center"
            VerticalAlignment="Center"
            Grid.ColumnSpan="5"
            Grid.RowSpan="5"
            Width="500"
            Opacity="0.5"
            Background="LightGray"
            Height="200"
            helper:DropFilesBehaviorExtension.IsEnabled ="True">
        <Border.Style>
            <Style TargetType="Border">
                <Setter Property="Visibility" Value="Collapsed"/>
                <Style.Triggers>
                    <DataTrigger Binding="{Binding DragDropVisibility}" Value="True">
                        <Setter Property="Visibility" Value="Visible"/>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </Border.Style>
        <Border.BorderBrush>
            <DrawingBrush Viewport="0,0,8,8" ViewportUnits="Absolute" TileMode="Tile">
                <DrawingBrush.Drawing>
                    <DrawingGroup>
                        <GeometryDrawing Brush="Black">
                            <GeometryDrawing.Geometry>
                                <GeometryGroup>
                                    <RectangleGeometry Rect="0,0,50,50" />
                                    <RectangleGeometry Rect="50,50,50,50" />
                                </GeometryGroup>
                            </GeometryDrawing.Geometry>
                        </GeometryDrawing>
                    </DrawingGroup>
                </DrawingBrush.Drawing>
            </DrawingBrush>
        </Border.BorderBrush>

        <TextBlock Text="Drag and drop the file you want to view onto the screen!"
                   FontSize="18"
                   HorizontalAlignment="Center"
                   VerticalAlignment="Center"/>
    </Border>

DropFilesBehavisorExtension

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)
    {
        // Border opacity will be raised on drag.
        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);
    }
}

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

CodePudding user response：

DragLeave function solved my problem. The function is triggered as soon as the mouse drag leaves the desired area.

fe.DragLeave  = Fe_DragLeave;

private static void Fe_DragLeave(object sender, DragEventArgs e)
{
           
}