Recently I needed to read a registry key in C#. This is obviously pretty simple using the Registry class in the “Microsoft.Win32” namespace:

public string ReadHypervisorName()
{
	using (var key = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\Virtual Machine\Guest\Parameters", false))
	{
		return key.GetValue("PhysicalHostNameFullyQualified") as string;
	}
}

As you can tell, I’m trying to read out the servername of a virtual machine’s hypervisor (using MS Hyper-V virtualization). However this didn’t work!

OpenSubKey simply returned null.

I could get as far as opening SOFTWARE\Microsoft, and that was it. “Virtual Machine” apparently didn’t exist…

Very puzzling indeed, as I could see it in my registry editor on the server.

I tried checking the ACLs for the registry key, which was fine – my user had full access to it.

Then something struck me: “Wow6432Node”. This is a registry key which exists beneath SOFTWARE and is used for 32-bit software under 64-bit Windows.

I quickly checked my project configuration, and alas Visual Studio had configured it for x86 by default!

So when I asked the Registry class to open a subkey, it was in fact adding Wow6432Node to it, so it actually tried finding “SOFTWARE\Wow6432Node\Microsoft\Virtual Machine\Guest\Parameters” which obviously doesn’t exist.

I changed my build configuration to “Any CPU” (why Visual Studio defaults to x86 is beyond me…) and the code worked like it should 🙂