How to prevent controls from capturing KeyDown events while a modal form is open?

Question

I have a form with CancelButton and AcceptButton (named btnCancel and btnOK). And I have some ComboBoxes as input fields.

ComboBoxes prevent my AcceptButton and CancelBu

Answer 1

While I have no idea about the main reason behind this behavior.

But in this situation obviously KeyDown event triggers 2 times. (Set a breakpoint and you will see.)

Since you need to handle it in code, You can try this to neglect one of Enter keys:

bool handled = true;
private void comboBox1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Enter)
    {
        /*Prevent handling the Enter key twice*/
        handled = !handled;
        if(handled)
            return;

        //Rest of logic
        //OkButton.PerformClick();
    }
}

Answer 2

You cannot correctly process Escape and Enter key in KeyDown event because they are handled during the keyboard preprocessing phase - Control.IsInputKey and Control.ProcessDialogKey. Normally controls do that for you, but looks like there is a bug in ComboBox implementation when DropDownStyle is Simple.

To get the desired behavior, create and use your own ComboBox subclass like this

public class MyComboBox : ComboBox
{
    protected override bool IsInputKey(Keys keyData)
    {
        if (DropDownStyle == ComboBoxStyle.Simple)
        {
            switch (keyData & (Keys.KeyCode | Keys.Alt))
            {
                case Keys.Return:
                case Keys.Escape:
                    return false;
            }
        }
        return base.IsInputKey(keyData);
    }
}

P.S. And of course don't forget to remove your KeyDown event handlers.