how to choose multiply choices from list box by click

I've used this in the past, and modified it to handle left-click-Alt: it let's you select/de-select all with either alt-down and left click, or control-A/a: if all are selected it de-selects all.

To build this: Add a new Project Item to your WinForm Project of Type Component named : ListBoxWithSelectAll, and paste in the following code:

using System;
using System.Windows.Forms;

namespace CustomControls
{
    public partial class ListBoxWithSelectAll : ListBox
    {
        public ListBoxWithSelectAll()
        {
            InitializeComponent();
        }

        protected override void OnPaint(PaintEventArgs pe)
        {
            base.OnPaint(pe);
        }

        private bool MouseIsDown = false;
        
        private bool AllLBItemsSelected = false;

        private void ListBoxWithSelectAll_KeyDown(object sender, KeyEventArgs e)
        {
            if ((MouseIsDown && e.Alt && MouseButtons == MouseButtons.Left) || (e.Control && e.KeyCode == Keys.A))
            {
                AllLBItemsSelected = !AllLBItemsSelected;

                for (int i = 0; i < this.Items.Count; i++)
                {
                    this.SetSelected(i, AllLBItemsSelected);
                }

                // use this to prevent flicker from multiple whatever
                MouseIsDown = false;
            }
        }

        private void ListBoxWithSelectAll_MouseDown(object sender, MouseEventArgs e)
        {
            MouseIsDown = true;
        }

        private void ListBoxWithSelectAll_MouseUp(object sender, MouseEventArgs e)
        {
            MouseIsDown = false;
        }

        // why we're doing this ... like this ... is ... too long a story
        private void ListBoxWithSelectAll_Enter(object sender, EventArgs e)
        {
            // guard against incorrect SelectionMode setting
            if (this.SelectionMode == SelectionMode.One || this.SelectionMode == SelectionMode.None)
            {
                this.SelectionMode = SelectionMode.MultiSimple;
            }
        }
    }
}

What I changed: this was adapted from some code that also implemented an 'Undo mechanism for selections, but I can't share that since it "belongs" to a client.

This code has been tested in FrameWork 4.5 and 3.0. It works with the custom sub-classed ListBox 'SelectionMode set to either 'MultiSimple or 'MultiExtended modes.