source: Common/ComboBoxEx.cs

using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;
using Microsoft.Win32;

namespace Common
{
    public partial class ComboBoxEx : ComboBox
    {
        public string RegSubKey;
        public int MaxHistoryCount;

        public ComboBoxEx()
        {
            InitializeComponent();
            RegSubKey = Name;
            MaxHistoryCount = 20;
        }

        public void UpdateHistory()
        {
            if (Text.Trim() != "")
            {
                BeginUpdate();
                try
                {
                    string previousText = Text;
                    int index = Items.IndexOf(Text);
                    if (index >= 0) Items.RemoveAt(index);
                    Text = previousText;
                    Items.Insert(0, previousText);

                    while (Items.Count > MaxHistoryCount)
                        Items.RemoveAt(Items.Count - 1);
                }
                finally
                {
                    EndUpdate();
                }
            }
        }

        public void LoadFromRegistry()
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(RegSubKey) ?? 
                                 Application.UserAppDataRegistry.CreateSubKey(RegSubKey);

            int count = regKey.GetValueInt("Count", 0);
            BeginUpdate();
            try
            {
                while (Items.Count > count) Items.RemoveAt(Items.Count - 1);
                while (Items.Count < count) Items.Add("");
                for (int i = 0; i < count; i++)
                {
                    Items[i] = (string)regKey.GetValue(i.ToString(), "");
                }
            }
            finally
            {
                EndUpdate();
            }
        }

        public void SaveToRegistry()
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(RegSubKey, true) ?? 
                                 Application.UserAppDataRegistry.CreateSubKey(RegSubKey);

            int i = 0;
            regKey.SetValue("Count", Items.Count);
            foreach (var item in Items)
            {
                regKey.SetValue(i.ToString(), item);
                i++;
            }

            while (true)
            {
                List<string> names = regKey.GetValueNames().ToList();
                if (names.IndexOf(i.ToString()) != -1)
                {
                    regKey.DeleteValue(i.ToString());
                }
                else break;
                i++;
            }
        }
    }
}