OpenSubKey() returns null for a registry key that I can see in regedit.exe

A 32-bit application on a 64-bit OS will be looking at the HKLM\Software\Wow6432Node node by default. To read the 64-bit version of the key, you'll need to specify the RegistryView:

using (var hklm = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry64))
using (var key = hklm.OpenSubKey(@"SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall"))
{
   // key now points to the 64-bit key
}

The API to do this was added in .NET 4.0; if you're still using 3.5, you'll need to use P/Invoke to access the 64-bit keys: http://www.rhyous.com/2011/01/24/how-read-the-64-bit-registry-from-a-32-bit-application-or-vice-versa/

In Visual Studio 2017 go to

Project > Properties > Build > Uncheck **Prefer 32-bit** and Platform target as **Any CPU**.