public class MultiComboBox : UserControl
{
    #region 屬性
    public ComboBox ComboBox { get; set; }
    public CheckedListBox CheckedListBox { get; set; }
    public ComboBox.ObjectCollection Items
    {
        get
        {
            return ComboBox?.Items;
        }
    }
    #endregion

    /// <summary>
    /// 多選下拉框
    /// </summary>
    public MultiComboBox()
    {
        // 面板
        this.VerticalScroll.Enabled = true;
        this.AutoSize = true;

        // 多選列表
        CheckedListBox = new CheckedListBox();
        CheckedListBox.CheckOnClick = true;
        CheckedListBox.BorderStyle = BorderStyle.Fixed3D;
        CheckedListBox.Visible = false;
        CheckedListBox.Margin = new Padding(0);
        CheckedListBox.MouseUp += (ss, se) =>
        {
            // 更新ComboBox顯示文本
            var lst = new List<string>();
            foreach (var v in CheckedListBox.CheckedItems)
            {
                lst.Add(v.ToString());
            }
            ComboBox.Text = string.Join(",", lst);
            ComboBox.Tag = lst;
        };
        CheckedListBox.MouseLeave += (ss, se) =>
        {
            // 隱藏下拉多選框
            CheckedListBox.Hide();
        };

        // 下拉框
        ComboBox = new ComboBox();
        ComboBox.Width = 150;
        ComboBox.DrawMode = DrawMode.OwnerDrawFixed;
        ComboBox.IntegralHeight = false;
        ComboBox.DroppedDown = false;
        ComboBox.DropDownHeight = 1;
        ComboBox.Margin = new Padding(0);
        ComboBox.Location = new Point(0, 0);
        ComboBox.DropDownStyle = ComboBoxStyle.DropDown;
        ComboBox.AutoCompleteSource = AutoCompleteSource.ListItems;
        ComboBox.MouseDown += (ss, se) =>
        {
            ComboBox.DroppedDown = false;
        };
        ComboBox.MouseLeave += (ss, se) =>
        {
            // 不在下拉區(qū)則隱藏下拉
            var curMousePos = this.PointToClient(Control.MousePosition);
            var downArea = CheckedListBox.Location;
            if (curMousePos.X < downArea.X || curMousePos.X > (downArea.X + CheckedListBox.Width)
            || curMousePos.Y < downArea.Y || curMousePos.Y > (downArea.Y + CheckedListBox.Height))
            {
                CheckedListBox.Hide();
            }
        };
        ComboBox.DropDown += (ss, se) =>
        {
            // 顯示下拉多選框
            CheckedListBox.Items.Clear();

            // 添加并設置選中項
            var lastChecked = ComboBox.Tag as List<string>;
            ComboBox.BeginUpdate();
            foreach (var v in this.Items)
            {
                var ck = false;
                if (lastChecked != null && lastChecked.IndexOf(v.ToString()) >= 0)
                {
                    ck = true;
                }
                CheckedListBox.Items.Add(v, ck);
            }
            // 顯示下拉框
            CheckedListBox.Width = ComboBox.Width;
            CheckedListBox.ItemHeight = ComboBox.ItemHeight;
            CheckedListBox.Size = new Size(ComboBox.DropDownWidth, this.Items.Count * 18);
            CheckedListBox.Location = new Point(ComboBox.Left, ComboBox.Height);
            this.Controls.Add(CheckedListBox);
            CheckedListBox.Visible = true;
            ComboBox.EndUpdate();
        };

        // 添加控件
        this.Controls.Add(ComboBox);
    }
}