C# Singleton pattern with triggerable initialization

Question

I need a singleton that:

• is lazy loaded
• is thread safe
• loads some values at construction
• those values can be queried at any time

Answer 1

public class Singleton where T : class, new()
{
    private static T instance;
    public static T Instance
    {
        get
        {
            if (instance == null)
            {
                throw new Exception("singleton needs to be initialised before use");
            }
            return instance;
        }
    }
    public static void Initialise(Action initialisationAction)
    {
        lock(typeof(Singleton))
        {
            if (instance != null)
            {
                return;
            }
            instance = new T();
            initialisationAction(instance);
        }
    }
}