C# 使用Graphics绘制图片时发生闪烁的问题

    public partial class Form1 : Form
    {
        int width, height;
        string path = "C:\\Users\\Public\\Pictures\\Sample Pictures\\Penguins.jpg";
        Image img = null;

        public Form1()
        {
            InitializeComponent();
            this.DoubleBuffered = true;
            img = Image.FromFile(path);
            width = img.Width;
            height = img.Height;
            this.MouseWheel += new MouseEventHandler(Form1_MouseWheel);
        }

        void Form1_MouseWheel(object sender, MouseEventArgs e)
        {
            width += (e.Delta / 5);
            height += (e.Delta / 5);
            this.Invalidate();
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            Rectangle rect = new Rectangle(0, 0, width, height);
            e.Graphics.Clear(SystemColors.Control);
            e.Graphics.DrawImage(img, rect);
        }
    }