Registry.LocalMachine.OpenSubKey() returns null

It can happen if you are on a 64-bit machine. Create a helper class first (requires .NET 4.0 or later):

public class RegistryHelpers
{

    public static RegistryKey GetRegistryKey()
    {
        return GetRegistryKey(null);
    }

    public static RegistryKey GetRegistryKey(string keyPath)
    {
        RegistryKey localMachineRegistry
            = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine,
                                      Environment.Is64BitOperatingSystem
                                          ? RegistryView.Registry64
                                          : RegistryView.Registry32);

        return string.IsNullOrEmpty(keyPath)
            ? localMachineRegistry
            : localMachineRegistry.OpenSubKey(keyPath);
    }

    public static object GetRegistryValue(string keyPath, string keyName)
    {
        RegistryKey registry = GetRegistryKey(keyPath);
        return registry.GetValue(keyName);
    }
}

Usage:

string keyPath = @"SOFTWARE\MyApp\Settings";
string keyName = "MyAppConnectionStringKey";

object connectionString = RegistryHelpers.GetRegistryValue(keyPath, keyName);

Console.WriteLine(connectionString);
Console.ReadLine();