private void ScreenBody_Load(object sender, EventArgs e)
{
    this.WindowState = FormWindowState.Maximized;
    MainPainter = this.CreateGraphics();
    pen = new Pen(Brushes.Blue);
    isDowned = false;
    baseImage = this.BackgroundImage;
    Rect = new Rectangle();
    RectReady = false;
}

辅助函数
本来应该写更多的辅助函数的,将窗体响应函数里面的代码放到里面来,不过本人很懒,就这样将就了.呵呵

private void DrawRect(Graphics Painter, int Mouse_x, int Mouse_y)
{
    int width = 0;
    int heigth = 0;
    if (Mouse_y < Rect.Y)
    {
        Rect.Y = Mouse_y;
        heigth = downPoint.Y - Mouse_y;
    }
    else
    {
        heigth = Mouse_y - downPoint.Y;
    }
    if (Mouse_x < Rect.X)
    {
        Rect.X = Mouse_x;
        width = downPoint.X - Mouse_x;
    }
    else
    {
        width = Mouse_x - downPoint.X;
    }
    Rect.Size = new Size(width, heigth);
    Painter.DrawRectangle(pen, Rect);
}

private Image DrawScreen(Image back, int Mouse_x, int Mouse_y)
{
    Graphics Painter = Graphics.FromImage(back);
    DrawRect(Painter, Mouse_x, Mouse_y);
    return back;
}
private void MoveRect(Image image, Rectangle Rect)
{
    Graphics Painter = Graphics.FromImage(image);
    Painter.DrawRectangle(pen, Rect.X, Rect.Y, Rect.Width, Rect.Height);