How to fix the flickering in User controls

It is not the kind of flicker that double-buffering can solve. Nor BeginUpdate or SuspendLayout. You've got too many controls, the BackgroundImage can make it a lot worse.

It starts when the UserControl paints itself. It draws the BackgroundImage, leaving holes where the child control windows go. Each child control then gets a message to paint itself, they'll fill in the hole with their window content. When you have a lot of controls, those holes are visible to the user for a while. They are normally white, contrasting badly with the BackgroundImage when it is dark. Or they can be black if the form has its Opacity or TransparencyKey property set, contrasting badly with just about anything.

This is a pretty fundamental limitation of Windows Forms, it is stuck with the way Windows renders windows. Fixed by WPF btw, it doesn't use windows for child controls. What you'd want is double-buffering the entire form, including the child controls. That's possible, check my code in this thread for the solution. It has side-effects though, and doesn't actually increase painting speed. The code is simple, paste this in your form (not the user control):

protected override CreateParams CreateParams {
  get {
    CreateParams cp = base.CreateParams;
    cp.ExStyle |= 0x02000000;  // Turn on WS_EX_COMPOSITED
    return cp;
  }
} 

There are many things you can do to improve painting speed, to the point that the flicker isn't noticeable anymore. Start by tackling the BackgroundImage. They can be really expensive when the source image is large and needs to be shrunk to fit the control. Change the BackgroundImageLayout property to "Tile". If that gives a noticeable speed-up, go back to your painting program and resize the image to be a better match with the typical control size. Or write code in the UC's OnResize() method to create a properly sized copy of the image so that it doesn't have to be resized every time the control repaints. Use the Format32bppPArgb pixel format for that copy, it renders about 10 times faster than any other pixel format.

Next thing you can do is prevent the holes from being so noticeable and contrasting badly with the image. You can turn off the WS_CLIPCHILDREN style flag for the UC, the flag that prevents the UC from painting in the area where the child controls go. Paste this code in the UserControl's code:

protected override CreateParams CreateParams {
  get {
    var parms = base.CreateParams;
    parms.Style &= ~0x02000000;  // Turn off WS_CLIPCHILDREN
    return parms;
  }
}

The child controls will now paint themselves on top of the background image. You might still see them painting themselves one by one, but the ugly intermediate white or black hole won't be visible.

Last but not least, reducing the number of child controls is always a good approach to solve slow painting problems. Override the UC's OnPaint() event and draw what is now shown in a child. Particular Label and PictureBox are very wasteful. Convenient for point and click but their light-weight alternative (drawing a string or an image) takes only a single line of code in your OnPaint() method.

This is a real issue, and the answer Hans Passant gave is great for saving the flicker. However, there are side effects as he mentioned, and they can be ugly (UI ugly). As stated, "You can turn off the WS_CLIPCHILDREN style flag for the UC", but that only turns it off for a UC. The components on the main form still have issues.

Example, a panel scroll bar doesn't paint, because it is technically in the child area. However the child component doesn't draw the scroll bar, so it doesn't get painted until mouse over (or another event triggers it).

Also, animated icons (changing icons in a wait loop) doesn't work. Removing icons on a tabPage.ImageKey doesn't resize/repaint the other tabPages appropriately.

So I was looking for a way to turn off the WS_CLIPCHILDREN on initial painting so my Form will load nicely painted, or better yet only turn it on while resizing my form with a lot of components.

The trick is to get the application to call CreateParams with the desired WS_EX_COMPOSITED/WS_CLIPCHILDREN style. I found a hack here (https://web.archive.org/web/20161026205944/http://www.angryhacker.com/blog/archive/2010/07/21/how-to-get-rid-of-flicker-on-windows-forms-applications.aspx) and it works great. Thanks AngryHacker!

I put the TurnOnFormLevelDoubleBuffering() call in the form ResizeBegin event and TurnOffFormLevelDoubleBuffering() call in the form ResizeEnd event (or just leave it WS_CLIPCHILDREN after it is initially painted properly.)

    int originalExStyle = -1;
    bool enableFormLevelDoubleBuffering = true;

    protected override CreateParams CreateParams
    {
        get
        {
            if (originalExStyle == -1)
                originalExStyle = base.CreateParams.ExStyle;

            CreateParams cp = base.CreateParams;
            if (enableFormLevelDoubleBuffering)
                cp.ExStyle |= 0x02000000;   // WS_EX_COMPOSITED
            else
                cp.ExStyle = originalExStyle;

            return cp;
        }
    }

    public void TurnOffFormLevelDoubleBuffering()
    {
        enableFormLevelDoubleBuffering = false;
        this.MaximizeBox = true;
    }

If you are doing any custom painting in the control (i.e. overriding OnPaint) you can try the double buffering yourself.

Image image;
protected override OnPaint(...) {
    if (image == null || needRepaint) {
        image = new Bitmap(Width, Height);
        using (Graphics g = Graphics.FromImage(image)) {
            // do any painting in image instead of control
        }
        needRepaint = false;
    }
    e.Graphics.DrawImage(image, 0, 0);
}

And invalidate your control with a property NeedRepaint

Otherwise the above answer with SuspendLayout and ResumeLayout is probably what you want.