Generic Singleton<T>

Question

Guys,

I have a question, is this the correct approach to make a Generic Singleton?

 public class Singleton<T> where T : class, new()
    {
        private static T instance = null;

        private Singleton() { }

        public static T Instancia
        {
            get 
            {
                if (instance == null)
                    instance = new T();
                return instance;
            }
        }
    }

Thanks in advanced.

EDIT:

Checking some PDFs I found a generic Singleton made this other way, is this other correct?

public class Singleton<T> where T : class, new()
{
    Singleton() { }

    class SingletonCreator
    {
        static SingletonCreator() { }
        // Private object instantiated with private constructor
        internal static readonly T instance = new T();
    }

    public static T UniqueInstance
    {
        get { return SingletonCreator.instance; }
    }
}

Thanks!!!

Answer 1

The problem with a generic singleton factory is that since it is generic you do not control the "singleton" type that is instantiated so you can never guarantee that the instance you create will be the only instance in the application.

If a user can provide a type to as a generic type argument then they can also create instances of that type. In other words, you cannot create a generic singleton factory - it undermines the pattern itself.

Answer 2

For a generic piece of code that will be reused, you should consider thread safety at the point where you create the singleton instance.

As it is, (instance == null) could evaluate to true on separate threads.