画矩形

代码

 public partial class MainWindow : Window
    {
        static Point startPoint;
        static Point endPoint;
        public MainWindow()
        {
            InitializeComponent();
            this.MouseLeftButtonDown += MainWindow_MouseLeftButtonDown;
            this.MouseMove += MainWindow_MouseMove;
        }
        private void MainWindow_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                endPoint = e.GetPosition(sender as Window);
                DrawRectangle(sender);
            }
        }
        private void MainWindow_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            var win = sender as Window;
            startPoint = e.GetPosition(win);
        }
        static void DrawRectangle(object sender)
        {
            var tmpPoint = startPoint;
            if (endPoint.X < startPoint.X)
            {
                startPoint.X = endPoint.X;
                endPoint.X = tmpPoint.X;
            }
            if (endPoint.Y < startPoint.Y)
            {
                startPoint.Y = endPoint.Y;
                endPoint.Y = tmpPoint.Y;
            }
            var win = sender as Window;
            var rect = new Rectangle();
            rect.Stroke = Brushes.Black;
            rect.Fill = Brushes.SkyBlue;
            rect.HorizontalAlignment = HorizontalAlignment.Left;
            rect.VerticalAlignment = VerticalAlignment.Center;
            rect.Height = Math.Abs(endPoint.Y - startPoint.Y);
            rect.Width = Math.Abs(endPoint.X - startPoint.X);
            Canvas canvas = new Canvas();
            Canvas.SetLeft(rect, startPoint.X);
            Canvas.SetTop(rect, startPoint.Y);
            canvas.Children.Add(rect);
            win.Content = canvas;
        }

效果