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

You can do something like this

public sealed class Singleton
{
    IEnumerable Values { get; set; }

    private Singleton()
    {
        Console.WriteLine("-- Private Singleton constructor");
        Values = new[] { "quick", "brown", "fox" };
    }

    public static Singleton Instance
    {
        get
        {
            Console.WriteLine("- Singleton Instance");
            return Nested.instance;
        }
    }

    public static void Initialize()
    {
        Console.WriteLine("- Singleton Initialize");
        Nested.Initialize();
    }

    internal class Nested
    {
        private static object syncRoot = new object();
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
            Console.WriteLine("-- Static Nested constructor");
        }

        internal static readonly Singleton instance = new Singleton();

        internal static void Initialize()
        {
            lock (syncRoot)
            {
                Console.WriteLine("-- Locked");
                Console.WriteLine("--- Nested Initialize");
                Console.WriteLine("-- Unlocked");
            }
        }
    }
}

Usage

class Program
{
    static void Main(string[] args)
    {
        var i = Singleton.Instance;
        i = Singleton.Instance;

        Console.WriteLine("-----");

        Singleton.Initialize();
        Singleton.Initialize();
        Singleton.Initialize();

        Console.Read();
    }
}

Which outputs

- Singleton Instance
-- Private Singleton constructor
-- Static Nested constructor
- Singleton Instance
-----
- Singleton Initialize
-- Locked
--- Nested Initialize
-- Unlocked
- Singleton Initialize
-- Locked
--- Nested Initialize
-- Unlocked
- Singleton Initialize
-- Locked
--- Nested Initialize
-- Unlocked