What are some alternatives to RegistryKey.OpenBaseKey in .NET 3.5?

For anyone who is interested in a C# solution for some of the previous versions of .NET in order to not have to refactor too much code, its not pretty but here it is, totally doable using reflection. I found this trick in the XSharper source code.

public static class RegistryExtensions
{

    public enum RegistryHiveType
    {
        X86,
        X64
    }

    static Dictionary<RegistryHive, UIntPtr> _hiveKeys = new Dictionary<RegistryHive, UIntPtr> {
        { RegistryHive.ClassesRoot, new UIntPtr(0x80000000u) },
        { RegistryHive.CurrentConfig, new UIntPtr(0x80000005u) },
        { RegistryHive.CurrentUser, new UIntPtr(0x80000001u) },
        { RegistryHive.DynData, new UIntPtr(0x80000006u) },
        { RegistryHive.LocalMachine, new UIntPtr(0x80000002u) },
        { RegistryHive.PerformanceData, new UIntPtr(0x80000004u) },
        { RegistryHive.Users, new UIntPtr(0x80000003u) }
    };

    static Dictionary<RegistryHiveType, RegistryAccessMask> _accessMasks = new Dictionary<RegistryHiveType, RegistryAccessMask> {
        { RegistryHiveType.X64, RegistryAccessMask.Wow6464 },
        { RegistryHiveType.X86, RegistryAccessMask.WoW6432 }
    };

    [Flags]
    public 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)]
    public static extern int RegOpenKeyEx(
      UIntPtr hKey,
      string subKey,
      uint ulOptions,
      uint samDesired,
      out IntPtr hkResult);

    public static RegistryKey OpenBaseKey(RegistryHive registryHive, RegistryHiveType registryType)
    {
        UIntPtr hiveKey = _hiveKeys[registryHive];
        if (Environment.OSVersion.Platform == PlatformID.Win32NT && Environment.OSVersion.Version.Major > 5)
        {
            RegistryAccessMask flags = RegistryAccessMask.QueryValue | RegistryAccessMask.EnumerateSubKeys | RegistryAccessMask.SetValue | RegistryAccessMask.CreateSubKey | _accessMasks[registryType];
            IntPtr keyHandlePointer = IntPtr.Zero;
            int result = RegOpenKeyEx(hiveKey, String.Empty, 0, (uint)flags, out 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
                if (safeRegistryHandleConstructor == null)
                    safeRegistryHandleConstructor = safeRegistryHandleType.GetConstructor(BindingFlags.Instance | BindingFlags.Public, 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);
                var net4Constructor = typeof(RegistryKey).GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic, null, new[] { typeof(IntPtr), typeof(bool), typeof(bool), typeof(bool), typeof(bool) }, null);
                object key;
                if (net4Constructor != null)
                    key = net4Constructor.Invoke(new object[] { keyHandlePointer, true, false, false, hiveKey == _hiveKeys[RegistryHive.PerformanceData] });
                else if (net3Constructor != null)
                    key = net3Constructor.Invoke(new object[] { keyHandle, true });
                else
                {
                    var keyFromHandleMethod = typeof(RegistryKey).GetMethod("FromHandle", BindingFlags.Static | BindingFlags.Public, null, new[] { safeRegistryHandleType }, null);
                    key = keyFromHandleMethod.Invoke(null, new object[] { keyHandle });
                }
                var field = typeof(RegistryKey).GetField("keyName", BindingFlags.Instance | BindingFlags.NonPublic);
                if (field != null)
                    field.SetValue(key, String.Empty);
                return (RegistryKey)key;
            }
            else if (result == 2) // The key does not exist.
                return null;
            throw new Win32Exception(result);
        }
        throw new PlatformNotSupportedException("The platform or operating system must be Windows XP or later.");
    }
}

Example usage:

var key64 = RegistryExtensions.OpenBaseKey(RegistryHive.LocalMachine, RegistryExtensions.RegistryHiveType.X64);
var key32 = RegistryExtensions.OpenBaseKey(RegistryHive.LocalMachine, RegistryExtensions.RegistryHiveType.X86);

For .NET versions earlier than version 4 there is no framework API that allows access to alternate registry views. In order to access alternate views you must call the native API RegOpenKeyEx passing one of the KEY_WOW64_32KEY or KEY_WOW64_64KEY flags as appropriate.

Common ways to do this are with C++/CLI mixed mode assemblies, or using P/Invoke. However, this is not very much fun at all. The registry APIs are some of the more awkward to use, because they support multiple data types for values.

Targeting needed registry:

RegistryKey localMachine = Registry.LocalMachine; //For example

and get the value that you need:

RegistryKey lKey = localMachine.OpenSubKey(@"SOFTWARE\...\", false);