& isClipping) {
    rectSelected.Width = e.X - rectSelected.X;
    rectSelected.Height = e.Y - rectSelected.Y;
    this.Invalidate();
    }
    }
    protected override void OnMouseUp(MouseEventArgs e) {
    if (e.Button == MouseButtons.Left && isClipping) {
    rectSelected.Width = e.X - rectSelected.X;
    rectSelected.Height = e.Y - rectSelected.Y;
    this.Invalidate();
    resultBmp = new Bitmap(rectSelected.Width, rectSelected.Height);
    using (Graphics g = Graphics.FromImage(resultBmp)) {
    g.DrawImage(screen,new Rectangle(0, 0, rectSelected.Width, rectSelected.Height), rectSelected, GraphicsUnit.Pixel);
    }
    this.DialogResult = DialogResult.OK;
    }
    }
    protected override void OnPaint(PaintEventArgs e) {
    Graphics g = e.Graphics;
    g.DrawImage(screen, 0, 0);
    g.DrawImage(coverLayer, 0, 0);
    PaintRectangle();
    }
    protected override void OnPaintBackground(PaintEventArgs e) {
    }
    protected override void OnKeyDown(KeyEventArgs e) {
    if (e.KeyCode == Keys.Escape) {
    this.DialogResult = DialogResult.Cancel;
    }
    }
    private void PaintRectangle() {
    using (Graphics g = Graphics.FromImage(coverLayer)) {
    g.Clear(coverColor);
    GraphicsPath path = new GraphicsPath();
    path.AddRectangle(this.Bounds);
    path.AddRectangle(rectSelected);
    g.FillPath(rectBrush, path);
    g.DrawRectangle(Pens.Blue, rectSelected);
    }
    }
    public Bitmap ResultBitmap {
    get { return resultBmp; }
    }
    }
    上面的代码都很容易看明白，这里有一个技巧就是GraphicsPath，它自动会形成一个中空的区域。上面的实现很容易扩展：多区域截图，多裁判截图等都很容易实现。