Customizable ComboBox Drop-Down

public class NumericPanel : UserControl
{
   ... YOUR STUFF ...
 
   public ComboBox OwnerDropDown { get; set; }
 
   public void testFunction()
   {
       if (OwnerDropDown != null)
       {
           String text = OwnerDropDown.Text;
       }
   }
}

...
 
NumericPanel numericPanel = new NumericPanel();
numericPanel.OwnerDropDown = myComboBox;
 
...

[Category("Custom Drop-Down")]
public bool ReadOnly
{
    get { return base.DropDownStyle == ComboBoxStyle.DropDown; }
    set { base.DropDownStyle = value ? ComboBoxStyle.DropDownList : ComboBoxStyle.DropDown; }
}

myTreeCombo.HideDropDown();
myTreeCombo.Parent.Focus();

private TreeView treeView1 = null;
 
private void Form1_Load(object sender, EventArgs e)
{
    treeView1 = new TreeView();
    treeView1.Nodes.Add("Node 1");
    treeView1.Nodes.Add("Node 2");
    treeView1.AfterSelect += new TreeViewEventHandler(treeView1_AfterSelect);
 
    customComboBox1.DropDownSizeMode = CustomComboBox.CustomComboBox.SizeMode.UseDropDownSize;
    customComboBox1.DropDownControl = treeView1;
}
 
void treeView1_AfterSelect(object sender, TreeViewEventArgs e)
{
    customComboBox1.HideDropDown();     // causes form to lose focus
    //customComboBox1.Parent.Focus(); 
}

private TreeView treeView1 = null;
private Timer timerAutoFocus = null;
 
private void Form1_Load(object sender, EventArgs e)
{
    treeView1 = new TreeView();
    treeView1.Nodes.Add("Node 1");
    treeView1.Nodes.Add("Node 2");
    treeView1.AfterSelect += new TreeViewEventHandler(treeView1_AfterSelect);
 
    customComboBox1.DropDownSizeMode = CustomComboBox.CustomComboBox.SizeMode.UseDropDownSize;
    customComboBox1.DropDownControl = treeView1;
}
 
private void timerAutoFocus_Tick(object sender, EventArgs e)
{
    if (!this.Focused)
    {
        this.Focus();
        timerAutoFocus.Enabled = false;
    }
}
 
void treeView1_AfterSelect(object sender, TreeViewEventArgs e)
{
    customComboBox1.HideDropDown();     // causes form to lose focus

    // Re-focus main form...
    if (timerAutoFocus == null)
    {
        timerAutoFocus = new Timer();
        timerAutoFocus.Interval = 10;
        timerAutoFocus.Tick += new EventHandler(timerAutoFocus_Tick);
        timerAutoFocus.Enabled = true;
    }
}

    public interface IPopupControlHost
    {
        #region Methods
 
        /// <summary>
        /// Displays drop-down area of combo box, if not already shown.
        /// </summary>
        void ShowDropDown();
 
        /// <summary>
        /// Hides drop-down area of combo box, if shown.
        /// </summary>
        void HideDropDown();
 
        #endregion
    }

        private void m_dropDown_Closed(object sender, ToolStripDropDownClosedEventArgs e)
        {
            if (AutoResetWhenClosed)
                DisposeHost();
            
            // Hide drop down within popup control.
            if (PopupControlHost != null)
                PopupControlHost.HideDropDown();
        }

    public class CustomComboBox : ComboBox, IPopupControlHost

        /// <summary>
        /// Gets or sets the popup control host, this is used to hide/show popup.
        /// </summary>
        public IPopupControlHost PopupControlHost { get; set; }

using System;
using System.ComponentModel;
using System.Drawing;
using System.Drawing.Design;
using System.Windows.Forms;
 
namespace CustomComboDemo
{
    public class ListComboBox : Control
    {
        #region Construction and destruction
 
        public ListComboBox()
        {
            List = new ListBox();
            
            Combo = new CustomComboBox.CustomComboBox(List);
            Combo.Dock = DockStyle.Fill;
            Controls.Add(Combo);
 
            List.SelectedIndexChanged += new EventHandler(List_SelectedIndexChanged);
            Combo.TextChanged += new EventHandler(Combo_TextChanged);
            Combo.KeyDown += new KeyEventHandler(ListComboBox_KeyDown);
        }
 
        #endregion
 
        #region Events
 
        void ListComboBox_KeyDown(object sender, KeyEventArgs e)
        {
            switch (e.KeyCode)
            {
                case Keys.Up:
                    if (List.SelectedIndex > 0)
                        --List.SelectedIndex;
                    break;
                case Keys.Down:
                    if (List.SelectedIndex + 1 < List.Items.Count)
                        ++List.SelectedIndex;
                    break;
                case Keys.Home:
                    List.SelectedIndex = List.Items.Count > 0 ? 0 : -1;
                    break;
                case Keys.End:
                    List.SelectedIndex = List.Items.Count - 1;
                    break;
            }
        }
 
        void List_SelectedIndexChanged(object sender, EventArgs e)
        {
            // Adjust content of combo box text.
            Combo.Text = (List.SelectedItem != null) ? List.SelectedItem.ToString() : "";
            Combo.HideDropDown();
            Combo.Select();
        }
 
        void Combo_TextChanged(object sender, EventArgs e)
        {
            // Adjust selection within list control.
            List.SelectedIndex = List.FindStringExact(Combo.Text);
        }
 
        #endregion
 
        #region Combo Methods
 
        public void ShowDropDown()
        {
            Combo.ShowDropDown();
        }
 
        public void HideDropDown()
        {
            Combo.HideDropDown();
        }
 
        #endregion
 
        #region Combo Properties
 
        [Browsable(false)]
        public bool IsDroppedDown
        {
            get { return Combo.IsDroppedDown; }
        }
 
        [Category("Custom Drop-Down"), Description("Indicates if drop-down is resizable.")]
        public bool AllowResizeDropDown
        {
            get { return Combo.AllowResizeDropDown; }
            set { Combo.AllowResizeDropDown = value; }
        }
 
        [Category("Custom Drop-Down"), Description("Indicates current sizing mode."), DefaultValue(CustomComboBox.CustomComboBox.SizeMode.UseComboSize)]
        public CustomComboBox.CustomComboBox.SizeMode DropDownSizeMode
        {
            get { return Combo.DropDownSizeMode; }
            set { Combo.DropDownSizeMode = value; }
        }
 
        [Category("Custom Drop-Down")]
        public Size DropSize
        {
            get { return Combo.DropSize; }
            set { Combo.DropSize = value; }
        }
 
        [Category("Custom Drop-Down"), Browsable(false)]
        public Size ControlSize
        {
            get { return Combo.ControlSize; }
            set { Combo.ControlSize = value; }
        }
 
        #endregion
 
        #region List Events
 
        public event EventHandler DataSourceChanged
        {
            add { List.DataSourceChanged += value; }
            remove { List.DataSourceChanged -= value; }
        }
        public event EventHandler DisplayMemberChanged
        {
            add { List.DisplayMemberChanged += value; }
            remove { List.DisplayMemberChanged -= value; }
        }
        public event ListControlConvertEventHandler Format
        {
            add { List.Format += value; }
            remove { List.Format -= value; }
        }
 
        [EditorBrowsable(EditorBrowsableState.Advanced)]
        [Browsable(false)]
        public event EventHandler FormatInfoChanged
        {
            add { List.FormatInfoChanged += value; }
            remove { List.FormatInfoChanged -= value; }
        }
        public event EventHandler FormatStringChanged
        {
            add { List.FormatStringChanged += value; }
            remove { List.FormatStringChanged -= value; }
        }
        public event EventHandler FormattingEnabledChanged
        {
            add { List.FormattingEnabledChanged += value; }
            remove { List.FormattingEnabledChanged -= value; }
        }
        public event EventHandler SelectedValueChanged
        {
            add { List.SelectedValueChanged += value; }
            remove { List.SelectedValueChanged -= value; }
        }
        public event EventHandler ValueMemberChanged
        {
            add { List.ValueMemberChanged += value; }
            remove { List.ValueMemberChanged -= value; }
        }
 
        #endregion
 
        #region List Methods
 
        public string GetItemText(object item)
        {
            return List.GetItemText(item);
        }
 
        public void ClearSelected()
        {
            List.ClearSelected();
        }
 
        public int FindString(string s)
        {
            return List.FindString(s);
        }
 
        public int FindString(string s, int startIndex)
        {
            return List.FindString(s, startIndex);
        }
 
        public int FindStringExact(string s)
        {
            return List.FindStringExact(s);
        }
 
        public int FindStringExact(string s, int startIndex)
        {
            return FindStringExact(s, startIndex);
        }
 
        public int GetItemHeight(int index)
        {
            return List.GetItemHeight(index);
        }
 
        #endregion
 
        #region List Properties
 
        [AttributeProvider(typeof(IListSource))]
        [DefaultValue("")]
        [RefreshProperties(RefreshProperties.Repaint)]
        public object DataSource
        {
            get { return List.DataSource; }
            set { List.DataSource = value; }
        }
 
        [TypeConverter("System.Windows.Forms.Design.DataMemberFieldConverter, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")]
        [DefaultValue("")]
        [Editor("System.Windows.Forms.Design.DataMemberFieldEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
        public string DisplayMember
        {
            get { return List.DisplayMember; }
            set { List.DisplayMember = value; }
        }
 
        [EditorBrowsable(EditorBrowsableState.Advanced)]
        [Browsable(false)]
        [DefaultValue("")]
        public IFormatProvider FormatInfo
        {
            get { return List.FormatInfo; }
            set { List.FormatInfo = value; }
        }
 
        [MergableProperty(false)]
        [Editor("System.Windows.Forms.Design.FormatStringEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
        [DefaultValue("")]
        public string FormatString
        {
            get { return List.FormatString; }
            set { List.FormatString = value; }
        }
 
        [DefaultValue(false)]
        public bool FormattingEnabled
        {
            get { return List.FormattingEnabled; }
            set { List.FormattingEnabled = value; }
        }
 
        public int SelectedIndex
        {
            get { return List.SelectedIndex; }
            set { List.SelectedIndex = value; }
        }
 
        [Browsable(false)]
        [DefaultValue("")]
        [Bindable(true)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public object SelectedValue
        {
            get { return List.SelectedValue; }
            set { List.SelectedValue = value; }
        }
 
        [DefaultValue("")]
        [Editor("System.Windows.Forms.Design.DataMemberFieldEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
        public string ValueMember
        {
            get { return List.ValueMember; }
            set { List.ValueMember = value; }
        }
 
        [RefreshProperties(RefreshProperties.Repaint)]
        [DefaultValue(true)]
        [Localizable(true)]
        public bool IntegralHeight
        {
            get { return List.IntegralHeight; }
            set { List.IntegralHeight = value; }
        }
 
        [Localizable(true)]
        [RefreshProperties(RefreshProperties.Repaint)]
        [DefaultValue(13)]
        public int ItemHeight
        {
            get { return List.ItemHeight; }
            set { List.ItemHeight = value; }
        }
 
        [Editor("System.Windows.Forms.Design.ListControlStringCollectionEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
        [Localizable(true)]
        [MergableProperty(false)]
        public ListBox.ObjectCollection Items
        {
            get { return List.Items; }
        }
 

        [DefaultValue(false)]
        public bool MultiColumn
        {
            get { return List.MultiColumn; }
            set { List.MultiColumn = value; }
        }
 
        [Localizable(true)]
        [DefaultValue(false)]
        public bool ScrollAlwaysVisible
        {
            get { return List.ScrollAlwaysVisible; }
            set { List.ScrollAlwaysVisible = value; }
        }
 
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        [Browsable(false)]
        public ListBox.SelectedIndexCollection SelectedIndices
        {
            get { return List.SelectedIndices; }
        }
 
        [Browsable(false)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        [Bindable(true)]
        public object SelectedItem
        {
            get { return List.SelectedItem; }
            set { List.SelectedItem = value; }
        }
 
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        [Browsable(false)]
        public ListBox.SelectedObjectCollection SelectedItems
        {
            get { return List.SelectedItems; }
        }
 
        public virtual SelectionMode SelectionMode
        {
            get { return List.SelectionMode; }
            set { List.SelectionMode = value; }
        }
 
        [DefaultValue(false)]
        public bool Sorted
        {
            get { return List.Sorted; }
            set { List.Sorted = value; }
        }
 
        [Browsable(false)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public int TopIndex
        {
            get { return List.TopIndex; }
            set { List.TopIndex = value; }
        }
 
        [Browsable(false)]
        [DefaultValue(false)]
        public bool UseCustomTabOffsets
        {
            get { return List.UseCustomTabOffsets; }
            set { List.UseCustomTabOffsets = value; }
        }
 
        [DefaultValue(true)]
        public bool UseTabStops
        {
            get { return List.UseTabStops; }
            set { List.UseTabStops = value; }
        }
 
        #endregion
 
        #region Properties
 
        protected CustomComboBox.CustomComboBox Combo { get; private set; }
        protected ListBox List { get; private set; }
 
        #endregion
    }
}