c# - 填充橡皮筋在Winforms应用程序中

如何在不透明度为0.3的windows窗体上绘制零不透明度橡皮筋？
（橡皮筋是在Microsoft示例之后制作的
更新
我需要那个橡皮筋来做面具之类的工作。如果您使用jing或任何其他屏幕截图工具，您将确切地看到当您尝试制作屏幕截图时我需要做的事情：屏幕变为半不透明，当您进行选择时，您将看到0不透明的选择

最佳答案

这就是你要找的机器人吗？

    public Form1()
    {
        InitializeComponent();
        DoubleBuffered = true;
    }

    bool mouseDown = false;
    Point mouseDownPoint = Point.Empty;
    Point mousePoint = Point.Empty;

    protected override void OnMouseDown(MouseEventArgs e)
    {
        base.OnMouseDown(e);
        mouseDown = true;
        mousePoint = mouseDownPoint = e.Location;
    }

    protected override void OnMouseUp(MouseEventArgs e)
    {
        base.OnMouseUp(e);
        mouseDown = false;
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        base.OnMouseMove(e);
        mousePoint = e.Location;
        Invalidate();
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);

        if (mouseDown)
        {
            Region r = new Region(this.ClientRectangle);
            Rectangle window = new Rectangle(
                Math.Min(mouseDownPoint.X, mousePoint.X),
                Math.Min(mouseDownPoint.Y, mousePoint.Y),
                Math.Abs(mouseDownPoint.X - mousePoint.X),
                Math.Abs(mouseDownPoint.Y - mousePoint.Y));
            r.Xor(window);
            e.Graphics.FillRegion(Brushes.Red, r);
            Console.WriteLine("Painted: " + window);
        }
    }