Dan McKinley

I googled around, and couldn't find a generic singleton implementation that was 1) correct and 2) met all of my needs. This is a clever approach, but unfortunately it is limited to objects that are created by calls to constructors.

So I went ahead and wrote the singleton generic that I fully expect to be included in the next BCL. (Not really. It's completely straightforward. However, the Steelers won the super bowl yesterday, so I'm not really in a modest mood.)

I started out defining a class factory type, which is responsible for creating the an instance in a purposefully vague way. I also wrote the default version which just calls new().

/// <summary>
/// Interface for objects that create instances of
/// another type.
/// </summary>
public interface IClassFactory<T> where T : class
{
        T CreateInstance();
}

/// <summary>
/// A default <see cref="IClassFactory"/> implementation,
/// which uses a parameterless constructor to create the
/// instance.
/// </summary>
public class DefaultClassFactory<T> : IClassFactory<T>
        where T : class, new()
{
        public T CreateInstance()
        {
                return new T();
        }
}

From there, I went for the slam dunk in writing both a singleton class with a class factory and a default version that doesn't require one.

/// <summary>
/// A base (or helper) singleton class. Defines the
/// singleton instance.
/// </summary>
/// <typeparam name="T">
/// The type of the singleton object.
/// </typeparam>
/// <typeparam name="class_factory">
/// The type of the class factory to use to create an
/// instance of type <typeparamref name="T"/>.
/// </typeparam>
public class Singleton<T, class_factory>
    where T : class
    where class_factory : IClassFactory<T>, new()
{
    private static object _sync = new object();
    private static T _default;

    /// <summary>
    /// Gets the singleton instance.
    /// </summary>
    public static T Default
    {
        get
        {
            EnsureDefault();
            return _default;
        }
    }

    /// <summary>
    /// Ensures that the singleton has been created.
    /// </summary>
    private static void EnsureDefault()
    {
        if (_default == null)
        {
            lock (_sync)
            {
                if (_default == null)
                {
                     CreateDefault();
                }
             }
        }
    }

    /// <summary>
    /// Uses the class factory to create the instance.
    /// </summary>
    private static void CreateDefault()
    {
        class_factory cf = new class_factory();
        T value = cf.CreateInstance();

        // This ensures that writes in the creation of
        // the default instance won't be shuffled beyond
        // the write to _default. Only matters on multiproc
        // machines where the hardware allows this. Does
        // nothing on an x86.
        Thread.MemoryBarrier();
        _default = value;
    }
}

/// <summary>
/// A basic singleton type that can be used with objects
/// that are created with a parameterless constructor.
/// </summary>
public class Singleton<T> :
    Singleton<T, DefaultClassFactory<T>>
    where T : class, new()
{
}

(For more on why I added that call to Thread.MemoryBarrier(), see this writeup.)

Here's a really simple example that does not take advantage of the customizability:

public class Foo : Singleton<Foo>
{
}

If you can't use Singleton as a base class, you have to write a little more code.

public class Foo : Bar
{
    public static Foo Default
    {
        get { return Singleton<Foo>.Default; }
    }
}