Question

I was thinking about the classic issue of lazy singleton initialization – the whole matter of the inefficiency of:

if (instance == null)
{
    instance = new Foo();
}
return instance;

Anyone who knows what a Singleton is is familiar with the issue(you only need the if once). It’s trivial but irritating.

So, I thought of an alternate solution, at least for .NET(although it should work anywhere
that has some equivalent to function pointers:

public class Foo
{
    private delegate Foo FooReturner();

    private static Foo innerFoo;

    private static FooReturner fooReturnHandler = new FooReturner(InitialFooReturner);

    public static Foo Instance
    {
        get
        {
            return fooReturnHandler();
        }
    }
    private static Foo InitialFooReturner()
    {
        innerFoo = new Foo();
        fooReturnHandler = new FooReturner(NewFooReturner); 
        return innerFoo;
    }

    private static Foo NewFooReturner()
    {
        return innerFoo;
    }

}

In short – the Instance returns a delegate method. The delegate is initially set to a method
that initializes your instance, then changes the delegate to point at a simple Return method.

Now, I like to think I’m not terrible at my job, but I have no pretensions about being awesome. I have not seen an example of this code anywhere.

Ergo, I come to the conclusion that I am missing something. Something major. Either that the whole problem is too trivial to bother thinking that much about or this does something horrible that will destroy the universe. Or I fail at searching and therefore haven’t seen
the hundreds of developers using this method. Something, anyway.

I was hoping the good folks here at Stack Overflow could clue me in as to what(leaving aside the controversy on whether one should use a Singleton at all).

EDIT for clarification:

This is not performance code(although if the design actively degrades performance beyond the traditional model, that would be interesting to know).

It was written purely as proof-of-concept, and I am further aware that it is not thread-safe as it properly should be. Is there any reason why it could NOT be made thread-safe by it’s very nature?

Answer 1

This is the canonical, thread safe, lazy Singleton pattern in C#:

public sealed class Singleton
{
    Singleton(){}
    public static Singleton Instance
    {
        get
        {
            return Nested.instance;
        }
    }        
    class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested() {}    
        internal static readonly Singleton instance = new Singleton();
    }
}