. net自定义UserControl'的内容会在父控件调整大小时消失

我已经派生了。net UserControl，来创建我自己的PictureList类。但是，当我调整控件的父控件的大小时，控件的内容就消失了。没有发出Resize事件之类的。我省略了与问题无关的代码。

    public ImageList(int width, int height)
    {
        ClientSize = new Size(width, height);
        ResizeRedraw = true;
    }

    // Ensure background transparency will be handled corretly.
    protected override CreateParams CreateParams
    {
        get {
            CreateParams cp = base.CreateParams;
            cp.ExStyle |= 0x20; // WS_EX_TRANSPARENT
            return cp;
        }
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        MessageBox.Show("This messagebox is only shown upon the first drawing, but not after the magically disappearing of the contents");
        img = /*the image to draw*/
        rect = /*the size of the image*/
        e.Graphics.DrawImage(img, rect);
    }