// Declare the drop-down button and the items it will contain.
internal ToolStripDropDownButton dropDownButton1;
internal ToolStripDropDown dropDown;
internal ToolStripButton buttonRed;
internal ToolStripButton buttonBlue;
internal ToolStripButton buttonYellow;

private void InitializeDropDownButton()
{
    dropDownButton1 = new ToolStripDropDownButton();
    dropDown = new ToolStripDropDown();
    dropDownButton1.Text = "A";

    // Set the drop-down on the ToolStripDropDownButton.
    dropDownButton1.DropDown = dropDown;

    // Set the drop-down direction.
    dropDownButton1.DropDownDirection = ToolStripDropDownDirection.Left;

    // Do not show a drop-down arrow.
    dropDownButton1.ShowDropDownArrow = false;

    // Declare three buttons, set their foreground color and text, 
    // and add the buttons to the drop-down.
    buttonRed = new ToolStripButton();
    buttonRed.ForeColor = Color.Red;
    buttonRed.Text = "A";

    buttonBlue = new ToolStripButton();
    buttonBlue.ForeColor = Color.Blue;
    buttonBlue.Text = "A";

    buttonYellow = new ToolStripButton();
    buttonYellow.ForeColor = Color.Yellow;
    buttonYellow.Text = "A";
    
    buttonBlue.Click += new EventHandler(colorButtonsClick);
    buttonRed.Click += new EventHandler(colorButtonsClick);
    buttonYellow.Click += new EventHandler(colorButtonsClick);

    dropDown.Items.AddRange(new ToolStripItem[] 
        { buttonRed, buttonBlue, buttonYellow });
    toolStrip1.Items.Add(dropDownButton1);
}

// Handle the buttons' click event by setting the foreground color of the
// form to the foreground color of the button that is clicked.
private void colorButtonsClick(object sender, EventArgs e)
{
    ToolStripButton senderButton = (ToolStripButton)sender;
    this.ForeColor = senderButton.ForeColor;
}

//引入命名空间
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Security.Permissions;

public class Form1 : Form
{
    public Form1()
    {
        MyTreeViewCombo treeCombo = new MyTreeViewCombo();
        treeCombo.TreeView.Nodes.Add("one");
        treeCombo.TreeView.Nodes.Add("two");
        treeCombo.TreeView.Nodes.Add("three");
        this.Controls.Add(treeCombo);
    }
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
    }

    [SecurityPermissionAttribute(
        SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.UnmanagedCode)]
    public class MyTreeViewCombo : ComboBox
    {   
        ToolStripControlHost treeViewHost;
        ToolStripDropDown dropDown;
        public MyTreeViewCombo()
        {
            TreeView treeView = new TreeView();
            treeView.BorderStyle = BorderStyle.None;
            treeViewHost = new ToolStripControlHost(treeView);
            dropDown = new ToolStripDropDown();
            dropDown.Items.Add(treeViewHost);
        }

        public TreeView TreeView
        {
            get { return treeViewHost.Control as TreeView; }
        }

        private void ShowDropDown()
        {
            if (dropDown != null)
            {
                treeViewHost.Width = DropDownWidth;
                treeViewHost.Height = DropDownHeight;
                dropDown.Show(this, 0, this.Height);
            }
        }

        private const int WM_USER = 0x0400,
                          WM_REFLECT = WM_USER + 0x1C00,
                          WM_COMMAND = 0x0111,
                          CBN_DROPDOWN = 7;

        public static int HIWORD(int n)
        {
            return (n >> 16) & 0xffff;
        }

        protected override void WndProc(ref Message m)
        {
            if (m.Msg == (WM_REFLECT + WM_COMMAND))
            {
                if (HIWORD((int)m.WParam) == CBN_DROPDOWN)
                {
                    ShowDropDown();
                    return;
                }
            }
            base.WndProc(ref m);
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                if (dropDown != null)
                {
                    dropDown.Dispose();
                    dropDown = null;
                }
            }
            base.Dispose(disposing);
        }
    }
}