WinForms ComboBox SelectedIndexChanged not firing when typing few chars followed by Alt+Down

Question

In short

When I type a character in a ComboBox, press Alt+Down followed by Enter or Tab, the SelectedIndexChanged event doesn\'t fire, even though the S

Answer 1

I've tried several google searches in order to find a definitive answer on this but didn't find one before. Just now I found a thread that actually refers to a Microsoft knowledge base article about the problem. Article KB948869 describes the problem.

The knowledge base article suggest to create your own combobox and override the ProcessDialogKey method.

using System.Windows.Forms;

public class MyComboBox : ComboBox
{
    protected override bool ProcessDialogKey(Keys keyData)
    {
        if (keyData == Keys.Tab)
            this.DroppedDown = false;
        return base.ProcessDialogKey(keyData);
    }
}

I've tried it, but unfortunately, it doesn't seem to have any effect. Which is a bit strange. I would expect a workaround described in a knowledge base article to be accurate.

I found another workaround though, which is to use the DropDownClosed event in stead.

private void comboBox1_DropDownClosed(object sender, EventArgs e)
{
    label1.Text = "DroDownClosed Selected index: " + comboBox1.SelectedIndex;
}

This does seem to work, but only when using DropDownStyle.DropDown. When you set the DropDownStyle to DropDownList, typing a character does not fire the DropDownClosed (as there is no actual drop down in that case). Only if you actually open up the drop down list and select a value the DropDownClosed event is fired.

So, both options are not really a good answer.

Update I've even tried overriding property SelectedIndex in MyComboBox, having it call OnSelectedIndexChanged(EventArgs.Empty). After typing a character and pressing Alt+Down, the setter is executed, but it's setting the value to -1, which it already is. After pressing Tab, the setter isn't executed again, although somehow the SelectedIndex value does change. It looks like the ComboBox is directly changing the backing field for SelectedIndex, bypassing the setting. I believe something like this probably also happens in the real ComboBox.