How can I force a DropDownList style ComboBox to only open when the user clicks the drop-down button?

Question

In C# .NET 2.0, I have a WinForms ComboBox with ComboBoxStyle DropDownList. However, the drop down appears whenever the user clicks anywhere on the combo box. Instead, I'd like to have it only open when the user explicitly clicks on the drop down button. When the user clicks on the rest of the combo box, I'd like to just assign it the keyboard focus so he or she can use some keyboard commands on the selected item. What's the best way to do this?

Answer 1

After some help from the other answers, I arrived at this quick solution:

public class MyComboBox : ComboBox
{
    public MyComboBox()
    {
        FlatStyle = FlatStyle.Popup;
        DropDownStyle = ComboBoxStyle.DropDownList;
    }

    protected override void WndProc(ref Message m)
    {
        if (m.Msg == 0x0201 /* WM_LBUTTONDOWN */ || m.Msg == 0x0203 /* WM_LBUTTONDBLCLK */)
        {
            int x = m.LParam.ToInt32() & 0xFFFF;
            if (x >= Width - SystemInformation.VerticalScrollBarWidth)
                base.WndProc(ref m);
            else
            {
                Focus();
                Invalidate();
            }
        }
        else
            base.WndProc(ref m);
    }
}

Answer 2

You have two issues to consider. The first is rather simple: determine whether the dropdown should be opened or closed. This code can do that:

    void comboBox1_MouseClick(object sender, MouseEventArgs e)
    {
        ComboBox combo = sender as ComboBox;
        int left = combo.Width - (SystemInformation.HorizontalScrollBarThumbWidth + SystemInformation.HorizontalResizeBorderThickness);
        if (e.X >= left)
        {
            // They did click the button, so let it happen.
        }
        else
        {
            // They didn't click the button, so prevent the dropdown.
        }
    }

The second issue is more significant -- actually preventing the dropdown from appearing. The simplest approach is:

comboBox1.DropDownStyle = ComboBoxStyle.DropDown;

But, that allows typing into the box, which you may not want.

I spent about 15 minutes looking at options, and it appears that to prevent the dropdown from appearing and simultaneously prevent the user from typing into the dropdown, you would need to subclass the control. That way, you can override OnMouseClick(), and only call the base.OnMouseClick() when they did click on button. It would look something like this (untested):

public class CustomComboBox : ComboBox
{
    protected override void OnMouseClick(MouseEventArgs e)
    {
        base.OnMouseClick(e);

        int left = this.Width - (SystemInformation.HorizontalScrollBarThumbWidth + SystemInformation.HorizontalResizeBorderThickness);
        if (e.X >= left)
        {
            // They did click the button, so let it happen.
            base.OnMouseClick(e);
        }
        else
        {
            // They didn't click the button, so prevent the dropdown.
            // Just do nothing.
        }
    }
}