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Sixth version - using .NET 4's Lazy<T> type

If you're using .NET 4 (or higher), you can use the System.Lazy<T> type to make the laziness really simple. All you need to do is pass a delegate to the constructor which calls the Singleton constructor - which is done most easily with a lambda expression.

 

 



public sealed class Singleton
{
    private static readonly Lazy lazy =
        new Lazy(() => new Singleton());
    
    public static Singleton Instance { get { return lazy.Value; } }

    private Singleton()
    {
    }
} 

 

It's simple and performs well. It also allows you to check whether or not the instance has been created yet with the IsValueCreated property, if you need that.

 

 

http://csharpindepth.com/articles/general/singleton.aspx#lazy

 

Conclusion (modified slightly on January 7th 2006; updated Feb 12th 2011)

There are various different ways of implementing the singleton pattern in C#. A reader has written to me detailing a way he has encapsulated the synchronization aspect, which while I acknowledge may be useful in a few very particular situations (specifically where you want very high performance, and the ability to determine whether or not the singleton has been created, and full laziness regardless of other static members being called). I don't personally see that situation coming up often enough to merit going further with on this page, but please mail me if you're in that situation.

My personal preference is for solution 4: the only time I would normally go away from it is if I needed to be able to call other static methods without triggering initialization, or if I needed to know whether or not the singleton has already been instantiated. I don't remember the last time I was in that situation, assuming I even have. In that case, I'd probably go for solution 2, which is still nice and easy to get right.

Solution 5 is elegant, but trickier than 2 or 4, and as I said above,

 

the benefits it provides seem to only be rarely useful. Solution 6 is a simpler way to achieve laziness, if you're using .NET 4.

 It also has the advantage that it's obviously lazy.

 

I currently tend to still use solution 4, simply through habit - but if I were working with inexperienced developers I'd quite possibly go for solution 6 to start with as an easy and universally applicable pattern.

(I wouldn't use solution 1 because it's broken, and I wouldn't use solution 3 because it has no benefits over 5.)

 

 


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