C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.ComponentModel.Design;
using System.Drawing.Design;
using System.Collections.ObjectModel;

namespace MultiColumnComboBox
{
    
    public partial class MultiColumnComboBox : ComboBox
    {

        private List<column> values = new List<column>();
        
        public class Column
        {
            private bool _autoWidth = true;
            private int _columnWidth;
            private bool _visible = true;

            [Category("Parameters")]
            [DisplayName("Auto Width?")]
            public bool AutoWidth
            {
                get { return _autoWidth; }
                set { _autoWidth = value; }
            }
            
            [Category("Parameters")]
            [DisplayName("Column Width")]
            public int ColumnWidth
            {
                get { return _columnWidth; }
                set { _columnWidth = value; }
            }
            
            [Category("Appearance")]
            public bool Visible
            {
                get { return _visible; }
                set { _visible = value; }
            }

            public enum ColumnType
            {
                Image = 0,
                Data = 1,
                Checkbox = 2
            }

            
            [Category("Parameters")]
            [DisplayName("Column Data Type")]
            public ColumnType columnType
            {
                get { return columnType; }
                set { columnType = value; }
            }

            
        }

        public class ColumnEditor : CollectionEditor
        {
            public ColumnEditor()
                : base(typeof(List<column>))
            {}

            protected override object SetItems(object editValue, object[] value)
            {
                return base.SetItems(editValue, value);
            }

            protected override string GetDisplayText(object value)
            {
                Column item = new Column();
                item = (Column)value;
                                
                return base.GetDisplayText(string.Format("Column", item.AutoWidth, item.ColumnWidth));
            }
        }
        
        public MultiColumnComboBox()
        {
           InitializeComponent();
        }

        [Category("Columns")]
        [DisplayName("Columns")]
        [Browsable(true)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
        [RefreshProperties(RefreshProperties.All)]
        [Editor(typeof(ColumnEditor), typeof(UITypeEditor))]
        public List<column> Values
        {
            get { return values; }
        }
    }
}
</column></column></column></column>