Question

Create a .net 2.0 Windows form app, add a splitContainer docked Fill, with this in its form constructor:

public Form1()
{
    InitializeComponent();

    for (int i = 1; i <= 300; i++)
    {
        FlowLayoutPanel f = new FlowLayoutPanel();
        f.Dock = DockStyle.Fill;
        Button b = new Button();
        f.Controls.Add(b);
        splitContainer1.Panel2.Controls.Add(f);
    }
}

Press F5. Grab the form by the lower right-hand-edge and drag quickly to expand the form. The form expands rather jerkily due to the work being done in all the controls.

I have an app with a form in it with some slow controls which has similarly become slow. The difference is that in my app, I get ugly black space appearing for a fraction of a second in the gaps as I drag out the form with my mouse. The window is not re-drawing properly. This ugly black space does not appear in the sample code above.

Any ideas what might cause black space to appear when expanding a slow form?

I have tried double-buffered but that makes no difference.

EDIT: I have dismantled the form down to its basics so that this can be reproduced. Start a new C# Windows Form application called WindowsFormsApplication_SampleFault. Paste the code below into the Form1.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Reflection;
using System.Windows.Forms;
using System.Diagnostics;
using System.Security.Principal;
using System.Runtime.InteropServices;


namespace WindowsFormsApplication_SampleFault
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            for (int i = 1; i <= 200; i++)
            {
                Button b = new Button();
                b.Dock = DockStyle.Fill;
                this.Controls.Add(b);
            } 
        }
        }
}

then paste this into Form1.Designer.cs

namespace WindowsFormsApplication_SampleFault
{
    partial class Form1
    {
            /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        /// 

        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.statusStrip1 = new System.Windows.Forms.StatusStrip();
            this.SuspendLayout();
            // 
            // statusStrip1
            // 
            this.statusStrip1.Location = new System.Drawing.Point(0, 472);
            this.statusStrip1.Name = "statusStrip1";
            this.statusStrip1.Size = new System.Drawing.Size(756, 22);
            this.statusStrip1.TabIndex = 6;
            this.statusStrip1.Text = "statusStrip1";
            // 
            // frmOptions
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(756, 494);
            this.Controls.Add(this.statusStrip1);
            this.DoubleBuffered = true;
            this.ImeMode = System.Windows.Forms.ImeMode.Off;
            this.MinimizeBox = false;
            this.MinimumSize = new System.Drawing.Size(750, 500);
            this.Name = "frmOptions";
            this.ShowIcon = false;
            this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private System.Windows.Forms.StatusStrip statusStrip1;

    }
}

Press F5 and attempt to drag-out the form. Note I am deliberately overloading the redraw mechanism to produce the black areas.

I believe that something is wrong with the form designer code but I am not sure what.

Answer 1

This particular case can be easily fixed by WS_EX_COMPOSITED.

const int WS_EX_COMPOSITED = 0x02000000;
protected override CreateParams CreateParams
{
    get
    {
        CreateParams cp = base.CreateParams;
        cp.ExStyle |= WS_EX_COMPOSITED;
        return cp;
    }
}

P.S. 200 buttons is too much for any GUI :)

IMHO

EDITED: here I post what I managed to do to fix black boxes on Win7. The code is a bit ugly but the idea is clear I hope.

public Form1()
{
    InitializeComponent();

    for (int i = 1; i <= 200; i++)
    {
        Button b = new Button();
        b.Dock = DockStyle.Fill;
        this.Controls.Add(b);
    }
    _layoutWorker.Tick += new EventHandler(LayoutWorker_Tick);

}

private void LayoutWorker_Tick(object sender, EventArgs e)
{
    _layoutWorker.Stop();
    this.PerformLayout();
}

protected override void OnResize(EventArgs e)
{
    this.SuspendLayout();
    base.OnResize(e);
    this.ResumeLayout(false);
    _layoutWorker.Start();
}

private Timer _layoutWorker = new Timer { Enabled = false, Interval = 1 };

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