source: Common/ApplicationSettings.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Drawing;
using System.IO;
using System.Windows.Forms;
using Microsoft.Win32;

namespace Common
{
    public static class ApplicationSettings
    {
        public static string variable;

        public static void SetDefault()
        {
            variable = "";
        }

        public static void LoadFromRegistry(string regSubKey = "")
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey);
            if (regKey == null) return;

            variable = regKey.GetValueString("Variable", variable);
        }

        public static void SaveToRegistry(string regSubKey = "")
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey, true);
            if (regKey == null) regKey = Application.UserAppDataRegistry.CreateSubKey(regSubKey);

            regKey.SetValueString("Variable", variable);
        }

        public static void CopyRegistry(RegistryKey src, RegistryKey dst)
        {
            // Copy the values
            foreach (var name in src.GetValueNames())
            {
                dst.SetValue(name, src.GetValue(name), src.GetValueKind(name));
            }

            // Copy the subkeys
            foreach (var name in src.GetSubKeyNames())
            {
                using (var srcSubKey = src.OpenSubKey(name, false))
                {
                    var dstSubKey = dst.CreateSubKey(name);
                    CopyRegistry(srcSubKey, dstSubKey);
                }
            }
        }

        private static bool IsValidVersion(string version)
        {
            return Regex.Match(version, "^(\\d+\\.){3}(\\d+)$").Success;
        }

        public static void UpgradeRegistrySettings()
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey("", true);
            if (regKey == null) regKey = Application.UserAppDataRegistry.CreateSubKey("");
            if (regKey.GetValue("Upgraded") == null)
            {
                string parentKey = Application.UserAppDataRegistry.Name.Substring(0, Application.UserAppDataRegistry.Name.LastIndexOf('\\'));
                parentKey = parentKey.Substring(parentKey.IndexOf('\\') + 1);
                RegistryKey parentRegKey = Registry.CurrentUser.OpenSubKey(parentKey, false);
                if (parentRegKey != null)
                {
                    List<string> subKeys = new List<string>(parentRegKey.GetSubKeyNames());
                    var versions = new List<Version>();
                    foreach (var key in subKeys)
                    {
                        if (IsValidVersion(key) && ("HKEY_CURRENT_USER\\" + parentKey + '\\' + key + '\\' != regKey.ToString()))
                            versions.Add(new Version(key));
                    }
                    versions.Sort();
                    versions.Reverse();

                    if (versions.Count > 0)
                    {
                        RegistryKey srcKey = Registry.CurrentUser.OpenSubKey(parentKey + '\\' + versions[0].ToString());
                        CopyRegistry(srcKey, regKey);
                    }
                }
                regKey.SetValue("Upgraded", true);
            }
        }
    }
}