How to Obtain a RegistryKey Object for the 64-Bit Windows Registry from a 32-Bit Process without .NET 4 or Above

It is totally doable to fetch a RegistryKey object for the 64-bit and 32-bit Windows registries with .NET versions prior to .NET 4. Using reflection this can be achieved as follows, by calling some unexposed constructors that are a part of these frameworks (even previous framework editions contain them, but just don’t necessarily expose them):

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")]
	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, null, 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 2000 or later.");
	}
}

Example usage:

RegistryKey localMachineX64 = RegistryExtensions.OpenBaseKey(RegistryHive.LocalMachine, RegistryExtensions.RegistryHiveType.X64);
RegistryKey localMachineX86 = RegistryExtensions.OpenBaseKey(RegistryHive.LocalMachine, RegistryExtensions.RegistryHiveType.X86);

Once you have a RegistryKey object reference, it should work just fine with the existing RegistryKey methods.