GDI+发生一般性错误

Question

        int flag = 0;
        Point pa1, pa2;
        Point pb1, pb2;               
                if (flag == 1)
                {
                    pa2.X = e.X;
                    pa2.Y = pa1.Y + pa2.X - pa1.X;
                    this.Show();
                    Pen pen = new Pen(Color.Red, 1);
                    dc.DrawEllipse(pen, pa1.X, pa1.Y, pa2.X - pa1.X, pa2.Y - pa1.Y);
                    pen.Dispose();
                }
                if (flag == 2)
                {
                    pb2.X = e.X;
                    pb2.Y = pb1.Y + pb2.X - pb1.X;
                    this.Show();
                    Pen pen = new Pen(Color.Blue, 1);
                    dc.DrawEllipse(pen, pb1.X, pb1.Y, pb2.X - pb1.X, pb2.Y - pb1.Y);
                    pen.Dispose();
                }此段代码为什么在单独的一个Form里面就可以运行，但是当这个Form是另一个Form的子窗体时就会发生：GDI+发生一般性的错误。发生错误的地方是：dc.DrawEllipse(pen, pa1.X, pa1.Y, pa2.X - pa1.X, pa2.Y - pa1.Y);
dc.DrawEllipse(pen, pb1.X, pb1.Y, pb2.X - pb1.X, pb2.Y - pb1.Y);请各位高手指点。

Answer 1

public partial class draw1 : Form
    {
        Graphics dc = null;
        int flag = 0;
        Point pa1, pa2;
        Point pb1, pb2;
        public draw1()
        {
            InitializeComponent();
            dc = this.CreateGraphics();
        }        private void draw1_MouseDown(object sender, MouseEventArgs e)
        {
            flag++;
            if (flag == 1)
            {
                pa1.X = e.X;
                pa1.Y = e.Y;
            }
            if (flag == 2)
            {
                pb1.X = e.X;
                pb1.Y = e.Y;
            }
        }        private void draw1_MouseUp(object sender, MouseEventArgs e)
        {
            try
            {
                if (flag == 1)
                {
                    pa2.X = e.X;
                    pa2.Y = pa1.Y + pa2.X - pa1.X;
                    this.Show();
                    Pen pen = new Pen(Color.Red, 1);
                    dc.DrawEllipse(pen, pa1.X, pa1.Y, pa2.X - pa1.X, pa2.Y - pa1.Y);
                    pen.Dispose();
                }
                if (flag == 2)
                {
                    pb2.X = e.X;
                    pb2.Y = pb1.Y + pb2.X - pb1.X;
                    this.Show();
                    Pen pen = new Pen(Color.Blue, 1);
                    dc.DrawEllipse(pen, pb1.X, pb1.Y, pb2.X - pb1.X, pb2.Y - pb1.Y);
                    pen.Dispose();
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
}
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