Two questions on Singleton C#

Question

Consider the following code

 public sealed class Singleton
 {
     private static readonly Singleton instance = new Singleton();
     public static Singleton Instance { get { return instance; } }

     static Singleton() {}
     private Singleton() {}
 }

Question

1) Here what is the purpose of static constructor ? (I know static constructor will be called before the first instance of the class is created).But in the context of above code can't i use it without the static constructor?

2) I heard that one of the advantages of singleton is that it can be extended into factory. Since it is a sealed class ,how will you extend it into factory?can you give some example?

Answer 1

The static constructor ensures that the singleton really isn't constructed before it's used. If the static constructor isn't present, the CLR has a lot more leeway about when it runs the type initializer. See my article on beforefieldinit and my blog post about .NET 4 type initialization changes for more info.

As for turning a singleton into a factory - you'd really have to give more context. Your Instance property could choose whether to always return a reference to the same object or not, I suppose...

Answer 2

1. There is no need for a static constructor. SKEET!
2. I've never heard of that. You can't, obviously, if the class is sealed. But you could moderately easily rewrite it as a factory, as there is only one way to instantiate it--through a property. Change that into a method, and you have the starts of a factory. The method may return the same instance each time, or may return an instance from a pool, or do whatever it wants.

I wouldn't suggest you have a factory property, as properties shouldn't be doing too much work.