<Grid x:Name="Grid">
    <Image Source="TIM截圖20180811150831.png"></Image>
    <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">歡迎訪問我博客 http://lindexi.oschina.io </TextBlock>
    <Rectangle x:Name="Rectangle" StrokeThickness="2" Stroke="Black" HorizontalAlignment="Left" VerticalAlignment="Top"></Rectangle>
  </Grid>

MouseDown += MainWindow_MouseDown;
      MouseMove += MainWindow_MouseMove;
      MouseUp += MainWindow_MouseUp;

private void MainWindow_MouseDown(object sender, MouseButtonEventArgs e)
    {
      _started = true;

      _downPoint = e.GetPosition(Grid);
    }

    private bool _started;

    private Point _downPoint;

private void MainWindow_MouseUp(object sender, MouseButtonEventArgs e)
    {
      _started = false;
    }

    private void MainWindow_MouseMove(object sender, MouseEventArgs e)
    {
      if (_started)
      {
        var point = e.GetPosition(Grid);

        var rect = new Rect(_downPoint, point);
        Rectangle.Margin = new Thickness(rect.Left, rect.Top, 0, 0);
        Rectangle.Width = rect.Width;
        Rectangle.Height = rect.Height;
      }
    }