Program.cs

using Microsoft.Deployment.WindowsInstaller;
using System;
using WixSharp;
using WixSharp.CommonTasks;

namespace Setup1
{
    class Program
    {
        static void Main()
        {
            var project = new ManagedProject("MyProject",
                new Dir(@"%ProgramFiles%\My Company\My Product",
                   new File("Program.cs")));

            project.GUID = new Guid("6fe30b47-2577-43ad-9095-1861ba25889b");

            project.AddAction(new ElevatedManagedAction(
                CustomActions.DoAction,
                Return.check,
                When.After,
                Step.InstallInitialize,
                null));

            project.BuildMsi();
        }
    }

    public class CustomActions
    {
        [CustomAction]
        public static ActionResult DoAction(Session session)
        {

            var baseKey = RegistryExtensions.OpenBaseKey(Microsoft.Win32.RegistryHive.LocalMachine, RegistryView.Registry64);
            var key = baseKey.OpenSubKey(@"SOFTWARE\My Company\My Product");

            key.GetValue(...);
            key.SetValue(...);


            return ActionResult.Success;
        }
    }
}

RegistryExtensions.cs

using Microsoft.Win32;
using Microsoft.Win32.SafeHandles;
using System;
using System.ComponentModel;
using System.Reflection;
using System.Runtime.InteropServices;

namespace WixSharp
{
    public enum RegistryView
    {
        Default = 0,
        Registry64 = RegistryExtensions.RegistryAccessMask.Wow6464,
        Registry32 = RegistryExtensions.RegistryAccessMask.WoW6432
    };

    public static class RegistryExtensions
    {

        [Flags]
        internal enum RegistryAccessMask
        {
            QueryValue = 0x0001,
            SetValue = 0x0002,
            CreateSubKey = 0x0004,
            EnumerateSubKeys = 0x0008,
            Notify = 0x0010,
            CreateLink = 0x0020,
            WoW6432 = 0x0200,
            Wow6464 = 0x0100,
            Write = 0x20006,
            Read = 0x20019,
            Execute = 0x20019,
            AllAccess = 0xF003F
        }

        [DllImport("advapi32.dll", CharSet = CharSet.Auto)]
        private static extern int RegOpenKeyEx(
          UIntPtr hKey,
          string subKey,
          uint ulOptions,
          uint samDesired,
          out IntPtr hkResult);

        public static RegistryKey OpenBaseKey(Microsoft.Win32.RegistryHive registryHive, RegistryView registryType)
        {
            var openBaseKey = typeof(RegistryKey).GetMethod("OpenBaseKey", BindingFlags.Static | BindingFlags.Public, null, new[] { typeof(Microsoft.Win32.RegistryHive), typeof(RegistryKey).Assembly.GetType("Microsoft.Win32.RegistryView") ?? typeof(int) }, null);
            if (openBaseKey != null)
                return (RegistryKey)openBaseKey.Invoke(null, new object[] { registryHive, registryType });

            var hiveKey = new UIntPtr((uint)registryHive);
            RegistryAccessMask flags = RegistryAccessMask.QueryValue | RegistryAccessMask.EnumerateSubKeys | RegistryAccessMask.SetValue | RegistryAccessMask.CreateSubKey | (RegistryAccessMask)registryType;
            var result = RegOpenKeyEx(hiveKey, string.Empty, 0, (uint)flags, out var keyHandlePointer);
            if (result == 0)
            {
                var safeRegistryHandleType = typeof(SafeHandleZeroOrMinusOneIsInvalid).Assembly.GetType("Microsoft.Win32.SafeHandles.SafeRegistryHandle");
                var safeRegistryHandleConstructor = safeRegistryHandleType.GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic, null, new[] { typeof(IntPtr), typeof(bool) }, null); // .NET < 4
                var keyHandle = safeRegistryHandleConstructor.Invoke(new object[] { keyHandlePointer, true });
                var net3Constructor = typeof(RegistryKey).GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic, null, new[] { safeRegistryHandleType, typeof(bool) }, null);

                return (RegistryKey)net3Constructor.Invoke(new object[] { keyHandle, true });
            }

            if (result == 2) return null; // The key does not exist.

            throw new Win32Exception(result);
        }

        public static RegistryKey CreateSubKey(this RegistryKey that, string subkey, bool writable)
        {
            return that.CreateSubKey(subkey, writable ? RegistryKeyPermissionCheck.ReadWriteSubTree : RegistryKeyPermissionCheck.ReadSubTree);
        }
    }
}