C# 拖放到网格中未正确拖放

我在这里找到了代码：

看起来我真正需要的是拖放到一个有列定义的网格中

所以我有：

<Window x:Class="WpfApplication1.MainWindow"
    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:local="clr-namespace:WpfApplication1"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
    <Grid PreviewMouseLeftButtonDown="Grid_PreviewMouseLeftButtonDown" Background="Transparent"
      PreviewMouseMove="Grid_PreviewMouseMove"
      DragEnter="Grid_DragEnter"
      Drop="Grid_Drop"
      AllowDrop="True">
      <Grid.ColumnDefinitions>
        <ColumnDefinition/>
        <ColumnDefinition/>
      </Grid.ColumnDefinitions>
      <Grid.RowDefinitions>
        <RowDefinition/>
        <RowDefinition/>
      </Grid.RowDefinitions>
      <Image x:Name="Ld" Margin="5" Grid.Row="0" Grid.Column="0" Source="greenBall.ico"/>
      <Image x:Name="iar" Margin="5" Grid.Row="1" Grid.Column="0" Source="greenBall.ico"/>
      <Image x:Name="ry" Margin="5" Grid.Row="0" Grid.Column="1" Source="greenBall.ico"/>
      <Image x:Name="dia" Margin="5" Grid.Row="1" Grid.Column="1" Source="greenBall.ico"/>
   </Grid>

现在，当我拖放红色球时，效果是什么都没有出现

我不是WPF专家，但我看到dropImage.source为空

没有必要保留相同的代码，我所需要的只是将图像拖放到网格列和行定义中 提前谢谢

public partial class MainWindow : Window
{

  private Point startDragPoint;
  Image dragImage = new Image();

  public MainWindow()
  {
    InitializeComponent();
  }

  private void Grid_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
  {
    dragImage = e.Source as Image;
    startDragPoint = e.GetPosition(null);
  }

  private void Grid_PreviewMouseMove(object sender, MouseEventArgs e)
  {
    if (e.LeftButton == MouseButtonState.Pressed)
    {
      Point position = e.GetPosition(null);

      if (Math.Abs(position.X - startDragPoint.X) >
             SystemParameters.MinimumHorizontalDragDistance ||
          Math.Abs(position.Y - startDragPoint.Y) >
             SystemParameters.MinimumVerticalDragDistance)

      {
        DataObject data = new DataObject(typeof(ImageSource), dragImage.Source);
        DragDrop.DoDragDrop(dragImage, data, DragDropEffects.Move);
      }
    }
  }

  private void Grid_DragEnter(object sender, DragEventArgs e)
  {
    if (e.Data.GetDataPresent(typeof(DataObject)))
    {
      e.Effects = DragDropEffects.Copy;
    }
    else
    {
      e.Effects = DragDropEffects.None;
    }
}

  private void Grid_Drop(object sender, DragEventArgs e)
  {
    Image dropImage = e.Source as Image;
    double x = dropImage.ActualWidth;

    dragImage.Source = dropImage.Source;
    dropImage.Source = e.Data.GetData(typeof(ImageSource)) as ImageSource;

  }
 }