C#

using System;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.ComponentModel.Design;
using System.Drawing.Design;

namespace MultiColumnComboBox
{
    public partial class MultiColumnComboBox : ComboBox
    {
        
        
        public MultiColumnComboBox()
        {
            InitializeComponent();
        }

             
    }

    [Editor(typeof(ColumnCollectionEditor),UITypeEditor)]
        public ColumnCollection Columns
        {
            get { return Columns; }
            set { Columns = value; }
        }

    
    
    public class Column
    {
        #region private variables
        
        private bool _autoColumnWidth = false;
        
        #endregion

        #region public properties
        
        public bool AutoColumnWidth
        {
            get { return _autoColumnWidth; }
            set { _autoColumnWidth = value; }
        }
        
        #endregion
        
        
    }

    public class ColumnCollection : CollectionBase
    {
        public Column this[int index]
        {
            get { return (Column)List[index]; }
        }

        public void Add(Column column)
        {
            List.Add(column);
        }

        public void Remove(Column column)
        {
            List.Remove(column);
        }
    }

    public class ColumnCollectionEditor : CollectionEditor
    {
        public ColumnCollectionEditor(Type type)
            : base(type)
        {
        }

        protected override string GetDisplayText(object value)
        {
            Column item = new Column();
            item = (Column)value;
            return base.GetDisplayText(string.Format("{0}", item.AutoColumnWidth));
        }
    }
}

[Editor(typeof(ColumnCollectionEditor),UITypeEditor)]
        public ColumnCollection Columns
        {
            get { return Columns; }
            set { Columns = value; }
        }