CODEWITHSUNDEEP
c#winforms.net-2.0combobox
Eric
Eric

Reputation: 6446

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

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?

Upvotes: 6

Views: 6048

Answers (3)

Eric
Eric

Reputation: 6446

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);
    }
}

Upvotes: 4

John Fisher
John Fisher

Reputation: 22727

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.
        }
    }
}

Upvotes: 1

CodeMonkey1313
CodeMonkey1313

Reputation: 16031

You may be able to get the X,Y position of the mouse click, and from there you could force it collapsed if it isn't on the drop down "icon" (for lack of a better word).

Upvotes: 0

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