Why is my ListBox throwing an exception?

Question

Okay, I was able to create a simple WinForms project that reproduces some strange behavior I found. In the designer, make a Form with a ListBox (named lbx) Anchored Top,Left,Right,Bottom, and a Button (button1). Now, the Form's code is here:

using System;
using System.Windows.Forms;

namespace ListBoxKaboom
{
    public partial class Form1 : Form
    {
        private bool _initFinished = false;

        public Form1()
        {
            InitializeComponent();

            this._initFinished = true;

            this.Height += 100;
            this.Height -= 50;
            this.Height += 50;
        }

        private void lbx_SelectedIndexChanged(object sender, EventArgs e)
        {
            this.button1.Enabled = (this.lbx.SelectedItem != null);
        }

        protected override void OnLayout(LayoutEventArgs e)
        {
            if (_initFinished)
            {
                int lines = (this.lbx.Height - 4) / this.lbx.ItemHeight;

                this.SuspendLayout();
                while (lines < this.lbx.Items.Count)
                {
                    this.lbx.Items.RemoveAt(this.lbx.Items.Count - 1);
                }

                while (lines > this.lbx.Items.Count)
                {
                    this.lbx.Items.Add("Item " + (this.lbx.Items.Count + 1).ToString());
                }
                this.ResumeLayout();
            }

            base.OnLayout(e);
        }
    }
}

PLEASE NOTE THE FOLLOWING INSTRUCTIONS: Run this, click any of the items in the list box, and use the arrow keys to move down far enough to cause the list box to scroll. Kaboom. Exception (sometimes NullReferenceException, sometimes IndexOutOfBoundsException). Any ideas why? Also, I would think that the items would be in order, but they're not. Is this just a goofy corner case that didn't get handled properly by WinForms or am I doing something wrong?

Stack Trace:

at System.Windows.Forms.ListBox.NativeUpdateSelection()

at System.Windows.Forms.ListBox.SelectedObjectCollection.EnsureUpToDate()

at System.Windows.Forms.ListBox.SelectedObjectCollection.get_InnerArray()

at System.Windows.Forms.ListBox.SelectedObjectCollection.get_Item(Int32 index)

at System.Windows.Forms.ListBox.get_SelectedItem()

Answer 1

You should not manipulate any GUI elements in the constructor, e.g. this.Height += 100; in your example. Strange things can happen. I have been bitten by this several times in legacy code.

Wait until form load time - handle the base.Load event and do the height manipulation there.

---

From "When does Form.Load event get raised?":

Q: "... I need to find out basically what the difference is between putting code in Load event's handler, versus putting code in the Form's constructor after the InitializeComponents() line ..."

A: "The Load event is fired once the control/form has been fully initialized and has a window handle created. Therefore once this event has fired it is a fully usable user interface control. Remember that inside the constructor the actual window handle for the control/form has not yet been created, you are only creating C# objects here and inside the InitializeComponent call."