how do I make a WinForms ListBox who's items toggle on and off

The functionality I'm after is a cross between a check list box and a list box in multi selection mode.

For list box items A and B

• A then B results in a single selection that moves from A to B.

• A then control-click B results in a multi-selection of A and B.

(What I want is):

• A then A results in an A toggling on and off.

I thought this would be easy but I can't figure it out. Maybe I'm missing something obvious or maybe I'm thinking the wrong way and nobody really wants a listbox whos开发者_JAVA百科 items toggle on/off.

---

If you set SelectionMode to MultiSimple this gets you the control-click multi-selection and the toggling on and off.

To get the moving selection to work you could handle the SelectedIndexChanged event and add some logic to de-select the other items if control isn't pressed. If I have more time later I could try to create some basic code for it, but this should be somewhere to start.

---

You already have the behavior you want if you set the ListBox.SelectionMode to MultiExtended and hold down control when making a selection.

---

If I understood correctly your problem, you want a ListBox with SelectionMode.One that can toggle selections with CTLR modifier similarly to SelectionMode.MultiSimple and SelectionMode.MultiExtented. Below my answer, just set ToggleSingleSelection to true. Also as a bonus it provides items click events that fires also when clicking already selected items.

using System;
using System.Drawing;
using System.Windows.Forms;

namespace Common
{
    public class ListBoxEx : ListBox
    {
        public event ItemClickedEventHandler ItemClick;
        public event ItemClickedEventHandler ItemDoubleClick;

        /// <summary>
        /// Toggle selections when list has SelectionMode.One
        /// </summary>
        public bool ToggleSingleSelection { get; set; }

        int _PreSelectedIndex = -1;
        int _PrevClickedItem = -1;

        protected override void OnSelectedIndexChanged(EventArgs e)
        {
            base.OnSelectedIndexChanged(e);
        }

        protected override void WndProc(ref Message m)
        {
            const int WM_LBUTTONDOWN = 0x201;
            switch (m.Msg)
            {
                case WM_LBUTTONDOWN:
                    // NOTE: Unfortunately SelectedIndex is already setted before OnMouseDown,
                    // so we must intercept mouse click even before to compare
                    _PreSelectedIndex = SelectedIndex;
                    break;
            }

            base.WndProc(ref m);
        }

        protected override void OnMouseClick(MouseEventArgs e)
        {
            base.OnMouseClick(e);

            // Reset clicked event, also for double click
            _PrevClickedItem = -1;
            int selectedIndex = SelectedIndex;
            if (selectedIndex != -1)
            {
                Rectangle selectedItemRectangle = GetItemRectangle(selectedIndex);
                if (selectedItemRectangle.Contains(e.Location))
                {
                    _PrevClickedItem = selectedIndex;

                    // Check when to toggle selection
                    if (SelectionMode == SelectionMode.One && ToggleSingleSelection && ModifierKeys.HasFlag(Keys.Control)
                        && _PreSelectedIndex != -1 && selectedIndex == _PreSelectedIndex)
                    {
                        SelectedIndex = -1;
                    }

                    if (_PrevClickedItem != -1)
                        OnItemClick(new ItemClickedEventArgs() { ItemIndex = _PrevClickedItem });
                }
            }
        }

        protected override void OnMouseDoubleClick(MouseEventArgs e)
        {
            base.OnMouseDoubleClick(e);

            if (_PrevClickedItem != -1)
                OnItemDoubleClick(new ItemClickedEventArgs() { ItemIndex = _PrevClickedItem });
        }

        protected virtual void OnItemDoubleClick(ItemClickedEventArgs args)
        {
            ItemDoubleClick?.Invoke(this, args);
        }

        protected virtual void OnItemClick(ItemClickedEventArgs args)
        {
            ItemClick?.Invoke(this, args);
        }
    }

    public class ItemClickedEventArgs : EventArgs
    {
        public int ItemIndex { get; set; }
    }

    public delegate void ItemClickedEventHandler(Control sender, ItemClickedEventArgs args);
}