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();
                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);
                EndUpdate();
            }
        }

        public void LoadFromRegistry()
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey);
            if (regKey == null) regKey = Application.UserAppDataRegistry.CreateSubKey(regSubKey);

            int count = regKey.GetValueInt("Count", 0);
            BeginUpdate();
            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(), "");
            }
            EndUpdate();
        }

        public void SaveToRegistry()
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey, true);
            if (regKey == null) regKey = 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++;
            }
        }
    }
}