' Declare the drop-down button and the items it will contain.
Friend WithEvents dropDownButton1 As ToolStripDropDownButton
Friend WithEvents dropDown As ToolStripDropDown
Friend WithEvents buttonRed As ToolStripButton
Friend WithEvents buttonBlue As ToolStripButton
Friend WithEvents buttonYellow As ToolStripButton

Private Sub 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"

    dropDown.Items.AddRange(New ToolStripItem() {buttonRed, buttonBlue, buttonYellow})
    toolStrip1.Items.Add(dropDownButton1)
End Sub

' Handle the buttons' click event by setting the foreground color of the
' form to the foreground color of the button that is clicked.
Public Sub colorButtonsClick(ByVal sender As [Object], ByVal e As EventArgs) _
    Handles buttonRed.Click, buttonBlue.Click, buttonYellow.Click
    Dim senderButton As ToolStripButton = CType(sender, ToolStripButton)
    Me.ForeColor = senderButton.ForeColor

End Sub

        // 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;
        }

Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Data
Imports System.Drawing
Imports System.Text
Imports System.Windows.Forms
Imports System.Security.Permissions

Public Class Form1
   Inherits Form

   Public Sub New()
      Dim treeCombo As New MyTreeViewCombo()
        treeCombo.MyTreeView.Nodes.Add("one")
        treeCombo.MyTreeView.Nodes.Add("two")
        treeCombo.MyTreeView.Nodes.Add("three")
      Me.Controls.Add(treeCombo)
   End Sub

   <STAThread()>  _
   Shared Sub Main()
      Application.EnableVisualStyles()
      Application.SetCompatibleTextRenderingDefault(False)
      Application.Run(New Form1())
   End Sub

    <SecurityPermissionAttribute( _
         SecurityAction.LinkDemand, Flags:=SecurityPermissionFlag.UnmanagedCode)> _
   Public Class MyTreeViewCombo
        Inherits ComboBox

        Private treeViewHost As ToolStripControlHost
        Private Shadows dropDown As ToolStripDropDown

        Public Sub New()

            Dim treeView As New TreeView()
            treeView.BorderStyle = BorderStyle.None
            treeViewHost = New ToolStripControlHost(treeView)
            dropDown = New ToolStripDropDown()
            dropDown.Items.Add(treeViewHost)
        End Sub

        Public ReadOnly Property MyTreeView() As TreeView
            Get
                Return treeViewHost.Control '
            End Get
        End Property

        Private Sub ShowDropDown()
            If Not (dropDown Is Nothing) Then
                treeViewHost.Width = DropDownWidth
                treeViewHost.Height = DropDownHeight
                dropDown.Show(Me, 0, Me.Height)
            End If
        End Sub

        Private Const WM_USER As Integer = &H400
        Private Const WM_REFLECT As Integer = WM_USER + &H1C00
        Private Const WM_COMMAND As Integer = &H111
        Private Const CBN_DROPDOWN As Integer = 7

        Public Shared Function HIWORD(ByVal n As Integer) As Integer
            Return (n >> 16) And &HFFFF
        End Function

        Protected Overrides Sub WndProc(ByRef m As Message)
            If m.Msg = WM_REFLECT + WM_COMMAND Then
                If HIWORD(CType(m.WParam, Integer)) = CBN_DROPDOWN Then
                    ShowDropDown()
                    Return
                End If
            End If
            MyBase.WndProc(m)
        End Sub

        Protected Overrides Sub Dispose(ByVal disposing As Boolean)
            If disposing Then
                If Not (dropDown Is Nothing) Then
                    dropDown.Dispose()
                    dropDown = Nothing
                End If
            End If
            MyBase.Dispose(disposing)
        End Sub
    End Class
End Class

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);
        }
    }
}