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POWhy does Listbox.Drawitem get called many times?
 

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  1. POWhy does Listbox.Drawitem get called many times?
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    <p>I have a databound listbox on my C# WinForm that holds strings that are links to image file locations. I want to display the image as a thumbnail for the user to click on and view. I got it working correctly by setting DrawMode=OwnerDrawVariable and handling the DrawItem and MeasureItem events. </p> <p>However I noticed that I have to click exit 2 times to get out of the application (looks like it's called selectedIndexChanged on first click, then exits on second). On further inspection I noticed that the DrawItem event is fired a multitude of times when I click an item in the listbox (like 15+ times). There is only 1-2 items EVER in the listbox at a time! Why is it getting called soo many times? </p> <p>I tested this with a non databound simple listbox and it does the same thing. I am only curious as I have to read the image from disk and get a thumbnail of it to put into the listbox, which if it did that 15-20 times could affect performance (and is TOTALLY unnecessary).</p> <pre><code>private void listBox1_MeasureItem(object sender, MeasureItemEventArgs e) { MessageBox.Show("listBox1_MeasureItem"); // Cast the sender object back to ListBox type. ListBox theListBox = (ListBox)sender; // Get the file path contained in each item. DataRowView drv = (DataRowView)theListBox.Items[e.Index]; string fileString = drv.Row["fullpath"].ToString(); // Create an image object and load image file into it Image img = Image.FromFile(fileString); e.ItemHeight = Convert.ToInt32(img.Height * 0.15); e.ItemWidth = Convert.ToInt32(img.Width * 0.15); } private void listBox1_DrawItem(object sender, DrawItemEventArgs e) { MessageBox.Show("listBox1_DrawItem"); // If the item is the selected item, then draw the rectangle // filled in blue. The item is selected when a bitwise And // of the State property and the DrawItemState.Selected // property is true. if ((e.State &amp; DrawItemState.Selected) == DrawItemState.Selected) { e.Graphics.FillRectangle(Brushes.CornflowerBlue, e.Bounds); } else { // Otherwise, draw the rectangle filled in beige. e.Graphics.FillRectangle(Brushes.Beige, e.Bounds); } DataRowView drv = (DataRowView)listBox1.Items[e.Index]; Image img = Image.FromFile(drv.Row["fullpath"].ToString()); img = img.GetThumbnailImage(e.Bounds.Width, e.Bounds.Height, null, IntPtr.Zero); e.Graphics.DrawImage(img, e.Bounds.X, e.Bounds.Y); // Draw the focus rectangle around the selected item. e.DrawFocusRectangle(); } </code></pre>
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    1. USSwDevMan81
    1. USCorey Blair
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    1. COI wouldn't use `MessageBoxes` in painting/drawing events — it interferes with the painting/drawing event.
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      1. POWhy does Listbox.Drawitem get called many times?
      1. USLarsTech
    2. COI would also cache those images so that you aren't doing an `Image.FromFile(...)` from within a drawing / measuring event. That really would slow down your performance.
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      1. POWhy does Listbox.Drawitem get called many times?
      1. USLarsTech
    3. COOnce again, despite its flaws, I'm reminded of the awesomeness of WPF.
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      1. USRitch Melton
 

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Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

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