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.NET Framework 3.5
ComboBox Class
This page is specific to
Microsoft Visual Studio 2008/.NET Framework 3.5

Other versions are also available for the following:
.NET Framework Class Library
ComboBox Class

Updated: August 2008

Represents a Windows combo box control.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

Visual Basic (Declaration)
<ClassInterfaceAttribute(ClassInterfaceType.AutoDispatch)> _
<DefaultBindingPropertyAttribute("Text")> _
<ComVisibleAttribute(True)> _
Public Class ComboBox _
    Inherits ListControl
Visual Basic (Usage)
Dim instance As ComboBox
C#
[ClassInterfaceAttribute(ClassInterfaceType.AutoDispatch)]
[DefaultBindingPropertyAttribute("Text")]
[ComVisibleAttribute(true)]
public class ComboBox : ListControl
Visual C++
[ClassInterfaceAttribute(ClassInterfaceType::AutoDispatch)]
[DefaultBindingPropertyAttribute(L"Text")]
[ComVisibleAttribute(true)]
public ref class ComboBox : public ListControl
J#
/** @attribute ClassInterfaceAttribute(ClassInterfaceType.AutoDispatch) */
/** @attribute DefaultBindingPropertyAttribute("Text") */
/** @attribute ComVisibleAttribute(true) */
public class ComboBox extends ListControl
JScript
public class ComboBox extends ListControl

A ComboBox displays a text box combined with a ListBox, which enables the user to select items from the list or enter a new value.

The DropDownStyle property specifies whether the list is always displayed or whether the list is displayed in a drop-down. The DropDownStyle property also specifies whether the text portion can be edited. See ComboBoxStyle for the available settings and their effects. There is no setting to always display the list and disallow entering a new value. To display a list to which no new values can be added, use a ListBox control.

To add objects to the list at run time, assign an array of object references with the AddRange method. The list then displays the default string value for each object. You can add individual objects with the Add method.

In addition to display and selection functionality, the ComboBox also provides features that enable you to efficiently add items to the ComboBox and to find text within the items of the list. With the BeginUpdate and EndUpdate methods, you can add a large number of items to the ComboBox without the control being repainted each time an item is added to the list. The FindString and FindStringExact methods enable you to search for an item in the list that contains a specific search string.

You can use these properties to manage the currently selected item in the list, the Text property to specify the string displayed in the editing field, the SelectedIndex property to get or set the current item, and the SelectedItem property to get or set a reference to the object.

Note:

If you have a ListBox, ComboBox, or CheckedListBox on a base Windows Forms page and want to modify the string collections of those controls in a derived form, the string collections of those controls in the base form must be empty. If the string collections are not empty, they become read-only when you derive another form.

The following code example is a complete application showing how you can use the Add method to add items to a ComboBox, the FindString method to find items in a ComboBox, and the BeginUpdate and EndUpdate methods to efficiently add a large number items to a ComboBox. The ability to store values that are different from displayed text is inherited from ListControl. For an example of how to use this feature, see the ListControl class.

Visual Basic
Imports System
Imports System.Windows.Forms

Namespace ComboBoxSampleNamespace

    Public Class ComboBoxSample
        Inherits System.Windows.Forms.Form

        Private addButton As System.Windows.Forms.Button
        Private textBox2 As System.Windows.Forms.TextBox
        Private addGrandButton As System.Windows.Forms.Button
        Private comboBox1 As System.Windows.Forms.ComboBox
        Private showSelectedButton As System.Windows.Forms.Button
        Private textBox1 As System.Windows.Forms.TextBox
        Private findButton As System.Windows.Forms.Button
        Private label1 As System.Windows.Forms.Label

        Public Sub New()
            MyBase.New()
            Me.InitializeComponent()
        End Sub

        <System.STAThreadAttribute()> Public Shared Sub Main()
            System.Windows.Forms.Application.Run(New ComboBoxSample())
        End Sub

        Private Sub InitializeComponent()
            Me.addButton = New System.Windows.Forms.Button()
            Me.textBox2 = New System.Windows.Forms.TextBox()
            Me.addGrandButton = New System.Windows.Forms.Button()
            Me.comboBox1 = New System.Windows.Forms.ComboBox()
            Me.showSelectedButton = New System.Windows.Forms.Button()
            Me.textBox1 = New System.Windows.Forms.TextBox()
            Me.findButton = New System.Windows.Forms.Button()
            Me.label1 = New System.Windows.Forms.Label()
            Me.addButton.Location = New System.Drawing.Point(248, 32)
            Me.addButton.Size = New System.Drawing.Size(40, 24)
            Me.addButton.TabIndex = 1
            Me.addButton.Text = "Add"
            AddHandler Me.addButton.Click, AddressOf Me.addButton_Click
            Me.textBox2.Location = New System.Drawing.Point(8, 64)
            Me.textBox2.Size = New System.Drawing.Size(232, 20)
            Me.textBox2.TabIndex = 6
            Me.textBox2.Text = ""
            Me.addGrandButton.Location = New System.Drawing.Point(8, 96)
            Me.addGrandButton.Size = New System.Drawing.Size(280, 23)
            Me.addGrandButton.TabIndex = 2
            Me.addGrandButton.Text = "Add 1,000 Items"
            AddHandler Me.addGrandButton.Click, AddressOf Me.addGrandButton_Click
            Me.comboBox1.Anchor = ((System.Windows.Forms.AnchorStyles.Bottom Or System.Windows.Forms.AnchorStyles.Left) _
                        Or System.Windows.Forms.AnchorStyles.Right)
            Me.comboBox1.DropDownWidth = 280
            Me.comboBox1.Items.AddRange(New Object() {"Item 1", "Item 2", "Item 3", "Item 4", "Item 5"})
            Me.comboBox1.Location = New System.Drawing.Point(8, 248)
            Me.comboBox1.Size = New System.Drawing.Size(280, 21)
            Me.comboBox1.TabIndex = 7
            Me.showSelectedButton.Location = New System.Drawing.Point(8, 128)
            Me.showSelectedButton.Size = New System.Drawing.Size(280, 24)
            Me.showSelectedButton.TabIndex = 4
            Me.showSelectedButton.Text = "What Item is Selected?"
            AddHandler Me.showSelectedButton.Click, AddressOf Me.showSelectedButton_Click
            Me.textBox1.Location = New System.Drawing.Point(8, 32)
            Me.textBox1.Size = New System.Drawing.Size(232, 20)
            Me.textBox1.TabIndex = 5
            Me.textBox1.Text = ""
            Me.findButton.Location = New System.Drawing.Point(248, 64)
            Me.findButton.Size = New System.Drawing.Size(40, 24)
            Me.findButton.TabIndex = 3
            Me.findButton.Text = "Find"
            AddHandler Me.findButton.Click, AddressOf Me.findButton_Click
            Me.label1.Location = New System.Drawing.Point(8, 224)
            Me.label1.Size = New System.Drawing.Size(144, 23)
            Me.label1.TabIndex = 0
            Me.label1.Text = "Test ComboBox"
            Me.ClientSize = New System.Drawing.Size(292, 273)
            Me.Controls.AddRange(New System.Windows.Forms.Control() {Me.comboBox1, Me.textBox2, Me.textBox1, Me.showSelectedButton, Me.findButton, Me.addGrandButton, Me.addButton, Me.label1})
            Me.Text = "ComboBox Sample"
        End Sub

        Private Sub addButton_Click(ByVal sender As Object, ByVal e As System.EventArgs)
            comboBox1.Items.Add(textBox1.Text)
        End Sub

        Private Sub findButton_Click(ByVal sender As Object, ByVal e As System.EventArgs)
            Dim index As Integer
            index = comboBox1.FindString(textBox2.Text)
            comboBox1.SelectedIndex = index
        End Sub

        Private Sub addGrandButton_Click(ByVal sender As Object, ByVal e As System.EventArgs)
            comboBox1.BeginUpdate()
            Dim I As Integer
            For I = 0 To 1000
                comboBox1.Items.Add("Item 1" + i.ToString())
            Next
            comboBox1.EndUpdate()
        End Sub

        Private Sub showSelectedButton_Click(ByVal sender As Object, ByVal e As System.EventArgs)
            Dim selectedIndex As Integer
            selectedIndex = comboBox1.SelectedIndex
            Dim selectedItem As Object
            selectedItem = comboBox1.SelectedItem

            MessageBox.Show("Selected Item Text: " & selectedItem.ToString() & Microsoft.VisualBasic.Constants.vbCrLf & _
                                "Index: " & selectedIndex.ToString())
        End Sub
    End Class
End Namespace
C#
using System;
using System.Windows.Forms;

namespace Win32Form1Namespace {


    public class Win32Form1 : System.Windows.Forms.Form {
        private System.Windows.Forms.Button addButton;
        private System.Windows.Forms.TextBox textBox2;
        private System.Windows.Forms.Button addGrandButton;
        private System.Windows.Forms.ComboBox comboBox1;
        private System.Windows.Forms.Button showSelectedButton;
        private System.Windows.Forms.TextBox textBox1;
        private System.Windows.Forms.Button findButton;
        private System.Windows.Forms.Label label1;

        public Win32Form1() {
            this.InitializeComponent();
        }

        [System.STAThreadAttribute()]
        public static void Main() {
            System.Windows.Forms.Application.Run(new Win32Form1());
        }

        private void InitializeComponent() {
            this.addButton = new System.Windows.Forms.Button();
            this.textBox2 = new System.Windows.Forms.TextBox();
            this.addGrandButton = new System.Windows.Forms.Button();
            this.comboBox1 = new System.Windows.Forms.ComboBox();
            this.showSelectedButton = new System.Windows.Forms.Button();
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.findButton = new System.Windows.Forms.Button();
            this.label1 = new System.Windows.Forms.Label();
            this.addButton.Location = new System.Drawing.Point(248, 32);
            this.addButton.Size = new System.Drawing.Size(40, 24);
            this.addButton.TabIndex = 1;
            this.addButton.Text = "Add";
            this.addButton.Click += new System.EventHandler(this.addButton_Click);
            this.textBox2.Location = new System.Drawing.Point(8, 64);
            this.textBox2.Size = new System.Drawing.Size(232, 20);
            this.textBox2.TabIndex = 6;
            this.textBox2.Text = "";
            this.addGrandButton.Location = new System.Drawing.Point(8, 96);
            this.addGrandButton.Size = new System.Drawing.Size(280, 23);
            this.addGrandButton.TabIndex = 2;
            this.addGrandButton.Text = "Add 1,000 Items";
            this.addGrandButton.Click += new System.EventHandler(this.addGrandButton_Click);
            this.comboBox1.Anchor = ((System.Windows.Forms.AnchorStyles.Bottom | System.Windows.Forms.AnchorStyles.Left) 
                        | System.Windows.Forms.AnchorStyles.Right);
            this.comboBox1.DropDownWidth = 280;
            this.comboBox1.Items.AddRange(new object[] {"Item 1",
                        "Item 2",
                        "Item 3",
                        "Item 4",
                        "Item 5"});
            this.comboBox1.Location = new System.Drawing.Point(8, 248);
            this.comboBox1.Size = new System.Drawing.Size(280, 21);
            this.comboBox1.TabIndex = 7;
            this.showSelectedButton.Location = new System.Drawing.Point(8, 128);
            this.showSelectedButton.Size = new System.Drawing.Size(280, 24);
            this.showSelectedButton.TabIndex = 4;
            this.showSelectedButton.Text = "What Item is Selected?";
            this.showSelectedButton.Click += new System.EventHandler(this.showSelectedButton_Click);
            this.textBox1.Location = new System.Drawing.Point(8, 32);
            this.textBox1.Size = new System.Drawing.Size(232, 20);
            this.textBox1.TabIndex = 5;
            this.textBox1.Text = "";
            this.findButton.Location = new System.Drawing.Point(248, 64);
            this.findButton.Size = new System.Drawing.Size(40, 24);
            this.findButton.TabIndex = 3;
            this.findButton.Text = "Find";
            this.findButton.Click += new System.EventHandler(this.findButton_Click);
            this.label1.Location = new System.Drawing.Point(8, 224);
            this.label1.Size = new System.Drawing.Size(144, 23);
            this.label1.TabIndex = 0;
            this.label1.Text = "Test ComboBox";
            this.ClientSize = new System.Drawing.Size(292, 273);
            this.Controls.AddRange(new System.Windows.Forms.Control[] {this.comboBox1,
                        this.textBox2,
                        this.textBox1,
                        this.showSelectedButton,
                        this.findButton,
                        this.addGrandButton,
                        this.addButton,
                        this.label1});
            this.Text = "ComboBox Sample";
        }

        private void addButton_Click(object sender, System.EventArgs e) {
           comboBox1.Items.Add(textBox1.Text);
        }

        private void addGrandButton_Click(object sender, System.EventArgs e) {
            comboBox1.BeginUpdate();
            for (int i = 0; i < 1000; i++) {
                comboBox1.Items.Add("Item 1" + i.ToString());
            }
            comboBox1.EndUpdate();
        }

        private void findButton_Click(object sender, System.EventArgs e) {
            int index = comboBox1.FindString(textBox2.Text);
            comboBox1.SelectedIndex = index;
        }

        private void showSelectedButton_Click(object sender, System.EventArgs e) {
            int selectedIndex = comboBox1.SelectedIndex;
            Object selectedItem = comboBox1.SelectedItem;

            MessageBox.Show("Selected Item Text: " + selectedItem.ToString() + "\n" +
                            "Index: " + selectedIndex.ToString());
        }
    }
}
Visual C++
#using <System.dll>
#using <System.Drawing.dll>
#using <System.Windows.Forms.dll>

using namespace System;
using namespace System::Windows::Forms;

namespace Win32Form1Namespace
{
   public ref class Win32Form1: public System::Windows::Forms::Form
   {
   private:
      System::Windows::Forms::Button^ addButton;
      System::Windows::Forms::TextBox^ textBox2;
      System::Windows::Forms::Button^ addGrandButton;
      System::Windows::Forms::ComboBox^ comboBox1;
      System::Windows::Forms::Button^ showSelectedButton;
      System::Windows::Forms::TextBox^ textBox1;
      System::Windows::Forms::Button^ findButton;
      System::Windows::Forms::Label ^ label1;

   public:
      Win32Form1()
      {
         this->InitializeComponent();
      }

   private:
      void InitializeComponent()
      {
         this->addButton = gcnew System::Windows::Forms::Button;
         this->textBox2 = gcnew System::Windows::Forms::TextBox;
         this->addGrandButton = gcnew System::Windows::Forms::Button;
         this->comboBox1 = gcnew System::Windows::Forms::ComboBox;
         this->showSelectedButton = gcnew System::Windows::Forms::Button;
         this->textBox1 = gcnew System::Windows::Forms::TextBox;
         this->findButton = gcnew System::Windows::Forms::Button;
         this->label1 = gcnew System::Windows::Forms::Label;
         this->addButton->Location = System::Drawing::Point( 248, 32 );
         this->addButton->Size = System::Drawing::Size( 40, 24 );
         this->addButton->TabIndex = 1;
         this->addButton->Text = "Add";
         this->addButton->Click += gcnew System::EventHandler(
            this, &Win32Form1::addButton_Click );
         this->textBox2->Location = System::Drawing::Point( 8, 64 );
         this->textBox2->Size = System::Drawing::Size( 232, 20 );
         this->textBox2->TabIndex = 6;
         this->textBox2->Text = "";
         this->addGrandButton->Location = System::Drawing::Point( 8, 96 );
         this->addGrandButton->Size = System::Drawing::Size( 280, 23 );
         this->addGrandButton->TabIndex = 2;
         this->addGrandButton->Text = "Add 1, 000 Items";
         this->addGrandButton->Click += gcnew System::EventHandler(
            this, &Win32Form1::addGrandButton_Click );
         this->comboBox1->Anchor = (System::Windows::Forms::AnchorStyles)(
            (System::Windows::Forms::AnchorStyles::Bottom | System::Windows::Forms::AnchorStyles::Left) |
             System::Windows::Forms::AnchorStyles::Right);
         this->comboBox1->DropDownWidth = 280;
         array<Object^>^ objectArray = {"Item 1",
            "Item 2",
            "Item 3",
            "Item 4",
            "Item 5"};
         this->comboBox1->Items->AddRange( objectArray );
         this->comboBox1->Location = System::Drawing::Point( 8, 248 );
         this->comboBox1->Size = System::Drawing::Size( 280, 21 );
         this->comboBox1->TabIndex = 7;
         this->showSelectedButton->Location = System::Drawing::Point( 8, 128 );
         this->showSelectedButton->Size = System::Drawing::Size( 280, 24 );
         this->showSelectedButton->TabIndex = 4;
         this->showSelectedButton->Text = "What Item is Selected?";
         this->showSelectedButton->Click += gcnew System::EventHandler( 
            this, &Win32Form1::showSelectedButton_Click );
         this->textBox1->Location = System::Drawing::Point( 8, 32 );
         this->textBox1->Size = System::Drawing::Size( 232, 20 );
         this->textBox1->TabIndex = 5;
         this->textBox1->Text = "";
         this->findButton->Location = System::Drawing::Point( 248, 64 );
         this->findButton->Size = System::Drawing::Size( 40, 24 );
         this->findButton->TabIndex = 3;
         this->findButton->Text = "Find";
         this->findButton->Click += gcnew System::EventHandler( 
            this, &Win32Form1::findButton_Click );
         this->label1->Location = System::Drawing::Point( 8, 224 );
         this->label1->Size = System::Drawing::Size( 144, 23 );
         this->label1->TabIndex = 0;
         this->label1->Text = "Test ComboBox";
         this->ClientSize = System::Drawing::Size( 292, 273 );
         array<System::Windows::Forms::Control^>^ controlsArray = {this->comboBox1,
            this->textBox2,
            this->textBox1,
            this->showSelectedButton,
            this->findButton,
            this->addGrandButton,
            this->addButton,
            this->label1};
         this->Controls->AddRange( controlsArray );
         this->Text = "ComboBox Sample";
      }

      void addButton_Click( Object^ sender, System::EventArgs^ e )
      {
         comboBox1->Items->Add( textBox1->Text );
      }

      void addGrandButton_Click( Object^ sender, System::EventArgs^ e )
      {
         comboBox1->BeginUpdate();
         for ( int i = 0; i < 1000; i++ )
         {
            comboBox1->Items->Add( "Item 1 " + i.ToString() );

         }
         comboBox1->EndUpdate();
      }

      void findButton_Click( Object^ sender, System::EventArgs^ e )
      {
         int index = comboBox1->FindString( textBox2->Text );
         comboBox1->SelectedIndex = index;
      }

      void showSelectedButton_Click( Object^ sender, System::EventArgs^ e )
      {
         int selectedIndex = comboBox1->SelectedIndex;
         Object^ selectedItem = comboBox1->SelectedItem;
         MessageBox::Show( "Selected Item Text: " + selectedItem->ToString() + "\n" +
            "Index: " + selectedIndex.ToString() );
      }
   };
}


[System::STAThreadAttribute]
int main()
{
   System::Windows::Forms::Application::Run( gcnew Win32Form1Namespace::Win32Form1 );
}
J#
package Win32Form1Namespace;

import System.*;
import System.Windows.Forms.*;

public class Win32Form1 extends System.Windows.Forms.Form
{
    private System.Windows.Forms.Button addButton;
    private System.Windows.Forms.TextBox textBox2;
    private System.Windows.Forms.Button addGrandButton;
    private System.Windows.Forms.ComboBox comboBox1;
    private System.Windows.Forms.Button showSelectedButton;
    private System.Windows.Forms.TextBox textBox1;
    private System.Windows.Forms.Button findButton;
    private System.Windows.Forms.Label label1;

    public Win32Form1()
    {
        this.InitializeComponent();
    } //Win32Form1

    /** @attribute System.STAThreadAttribute()
     */
    public static void main(String[] args)
    {
        System.Windows.Forms.Application.Run(new Win32Form1());
    } //main

    private void InitializeComponent()
    {
        this.addButton = new System.Windows.Forms.Button();
        this.textBox2 = new System.Windows.Forms.TextBox();
        this.addGrandButton = new System.Windows.Forms.Button();
        this.comboBox1 = new System.Windows.Forms.ComboBox();
        this.showSelectedButton = new System.Windows.Forms.Button();
        this.textBox1 = new System.Windows.Forms.TextBox();
        this.findButton = new System.Windows.Forms.Button();
        this.label1 = new System.Windows.Forms.Label();
        this.addButton.set_Location(new System.Drawing.Point(248, 32));
        this.addButton.set_Size(new System.Drawing.Size(40, 24));
        this.addButton.set_TabIndex(1);
        this.addButton.set_Text("Add");
        this.addButton.add_Click(new System.EventHandler(this.addButton_Click));
        this.textBox2.set_Location(new System.Drawing.Point(8, 64));
        this.textBox2.set_Size(new System.Drawing.Size(232, 20));
        this.textBox2.set_TabIndex(6);
        this.textBox2.set_Text("");
        this.addGrandButton.set_Location(new System.Drawing.Point(8, 96));
        this.addGrandButton.set_Size(new System.Drawing.Size(280, 23));
        this.addGrandButton.set_TabIndex(2);
        this.addGrandButton.set_Text("Add 1,000 Items");
        this.addGrandButton.add_Click(new System.EventHandler(
            this.addGrandButton_Click));
        this.comboBox1.set_Anchor(System.Windows.Forms.AnchorStyles.Bottom
            | System.Windows.Forms.AnchorStyles.Left
            | System.Windows.Forms.AnchorStyles.Right);
        this.comboBox1.set_DropDownWidth(280);
        this.comboBox1.get_Items().AddRange(new Object[] 
            { "Item 1", "Item 2", "Item 3", "Item 4", "Item 5" });
        this.comboBox1.set_Location(new System.Drawing.Point(8, 248));
        this.comboBox1.set_Size(new System.Drawing.Size(280, 21));
        this.comboBox1.set_TabIndex(7);
        this.showSelectedButton.set_Location(new System.Drawing.Point(8, 128));
        this.showSelectedButton.set_Size(new System.Drawing.Size(280, 24));
        this.showSelectedButton.set_TabIndex(4);
        this.showSelectedButton.set_Text("What Item is Selected?");
        this.showSelectedButton.add_Click(new System.EventHandler(
            this.showSelectedButton_Click));
        this.textBox1.set_Location(new System.Drawing.Point(8, 32));
        this.textBox1.set_Size(new System.Drawing.Size(232, 20));
        this.textBox1.set_TabIndex(5);
        this.textBox1.set_Text("");
        this.findButton.set_Location(new System.Drawing.Point(248, 64));
        this.findButton.set_Size(new System.Drawing.Size(40, 24));
        this.findButton.set_TabIndex(3);
        this.findButton.set_Text("Find");
        this.findButton.add_Click(new System.EventHandler(
            this.findButton_Click));
        this.label1.set_Location(new System.Drawing.Point(8, 224));
        this.label1.set_Size(new System.Drawing.Size(144, 23));
        this.label1.set_TabIndex(0);
        this.label1.set_Text("Test ComboBox");
        this.set_ClientSize(new System.Drawing.Size(292, 273));
        this.get_Controls().AddRange(new System.Windows.Forms.Control[]
            { this.comboBox1, this.textBox2, this.textBox1,
            this.showSelectedButton, this.findButton, this.addGrandButton,
            this.addButton, this.label1 });
        this.set_Text("ComboBox Sample");
    } //InitializeComponent

    private void addButton_Click(Object sender, System.EventArgs e)
    {
        comboBox1.get_Items().Add(textBox1.get_Text());
    } //addButton_Click

    private void addGrandButton_Click(Object sender, System.EventArgs e)
    {
        comboBox1.BeginUpdate();
        for (int i = 0; i < 1000; i++) {
            comboBox1.get_Items().Add("Item 1" + ((Int32)i).ToString());
        }
        comboBox1.EndUpdate();
    } //addGrandButton_Click

    private void findButton_Click(Object sender, System.EventArgs e)
    {
        int index = comboBox1.FindString(textBox2.get_Text());
        comboBox1.set_SelectedIndex(index);
    } //findButton_Click

    private void showSelectedButton_Click(Object sender, System.EventArgs e)
    {
        int selectedIndex = comboBox1.get_SelectedIndex();
        Object selectedItem = comboBox1.get_SelectedItem();
        MessageBox.Show("Selected Item Text: " + selectedItem.ToString() 
            + "\n" + "Index: " + ((Int32)selectedIndex).ToString());
    } //showSelectedButton_Click

The following code example uses ToolStripControlHost and ComboBox to show a ToolStripDropDown as a TreeView.

Visual Basic
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
C#
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);
        }
    }
}
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

Date

History

Reason

August 2008

Updated description text in Remarks section.

Content bug fix.

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