C#窗体最小化后的图形重绘

使用C#绘图，在执行点击鼠标事件后绘制圆，最小化窗体之后再点开窗体绘制好的圆消失，如何重绘所绘制的圆

画好的圆要用数据保存起来，在onpaint消息里面绘制出来

亲测可用，最小化后再点开依然有。

 private void button1_Click(object sender, EventArgs e)
        {
            Bitmap rtbCurrent = new Bitmap((int)pictureBox_Yuan.Width, (int)pictureBox_Yuan.Height, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
            Graphics g_Bitmap = Graphics.FromImage(rtbCurrent);
            g_Bitmap.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
            g_Bitmap.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            g_Bitmap.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
            Color oColor = this.GetColor(100);
            g_Bitmap.DrawEllipse(new Pen(oColor,2), 0, 0, 250, 250);
            this.pictureBox_Yuan.Image = rtbCurrent; // 显示绘制的图像
        }