求助：绘图显示需要按钮按两次问题

简单例子：在窗体上放置两个完全一样并重叠的Panel控件，Panel1里面有一PictureBox控件，开始显示Panel2，Panel1不显示，按下button按钮，在PictureBox上绘图。代码如下：
问题：需要button按两次才能显示所画的线，为什么按下button不能显示啊？
private void button1_Click(object sender, EventArgs e)
{
panel2.Visible = false;
panel1.Visible = true;
//panel1.BringToFront();
Graphics g = pictureBox1.CreateGraphics();
Pen p = new Pen(Color.Red, 1);
g.DrawLine(p, 10, 50, 200, 100);
p.Dispose();
g.Dispose();

}

private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
Graphics g = pictureBox1.CreateGraphics();
Pen p = new Pen(Color.Red, 1);
g.DrawLine(p, 10, 50, 200, 100);
p.Dispose();
g.Dispose();
}