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POWhy is my ListBox throwing an exception?
 

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  1. POWhy is my ListBox throwing an exception?
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    <p>Okay, I was able to create a simple Windows Forms project that reproduces some strange behavior I found. In the designer, make a form with a ListBox (named lbx) anchored Top, Left, Right, and Bottom, and a button (button1). Now, the Form's code is here:</p> <pre><code>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 &lt; this.lbx.Items.Count) { this.lbx.Items.RemoveAt(this.lbx.Items.Count - 1); } while (lines &gt; this.lbx.Items.Count) { this.lbx.Items.Add("Item " + (this.lbx.Items.Count + 1).ToString()); } this.ResumeLayout(); } base.OnLayout(e); } } } </code></pre> <p>PLEASE NOTE THE FOLLOWING INSTRUCTIONS:</p> <p><strong>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</strong>. Kaboom.</p> <p>Exception (sometimes NullReferenceException and 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 Windows Forms, or am I doing something wrong?</p> <p>Stack trace:</p> <blockquote> <p>at System.Windows.Forms.ListBox.NativeUpdateSelection() <br/></p> <p>at System.Windows.Forms.ListBox.SelectedObjectCollection.EnsureUpToDate() <br/></p> <p>at System.Windows.Forms.ListBox.SelectedObjectCollection.get_InnerArray() <br/></p> <p>at System.Windows.Forms.ListBox.SelectedObjectCollection.get_Item(Int32 index) <br/></p> <p>at System.Windows.Forms.ListBox.get_SelectedItem() <br/></p> </blockquote>
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    1. USPeter Mortensen
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      1. POWhy is my ListBox throwing an exception?
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      1. VTDownMod
    1. COplease post the exception with stack info.
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      1. POWhy is my ListBox throwing an exception?
      1. USPreet Sangha
    2. COI ran the code but don't get an Exception. What kind of Exception are you getting and where is it thrown?
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      1. POWhy is my ListBox throwing an exception?
      1. USEric J.
    3. COWithout the exception, I doubt we can help.
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      1. POWhy is my ListBox throwing an exception?
      1. USJeff Yates
 

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