#785 – The Singleton Pattern

Singleton is a design pattern that you can use when you have a situation where you want to limit a particular class to have at most one instance.

A singleton behaves similarly to a static class in C#, but has some advantages, in that it behaves as a traditional object.  (E.g. Can implement an interface, or be passed to a method).

Here’s a common (thread-safe) pattern for implementing a singleton in C#.

    public sealed class DogFactory
        // Instance created when first referenced
        private static readonly DogFactory instance = new DogFactory();

        // Prevent early instantiation due to beforefieldinit flag
        static DogFactory() { }

        // Prevent instantiation
        private DogFactory() { }

        public static DogFactory Instance
            get { return instance; }

        // Actual methods go here, e.g.:
        public Dog CreateDog(string name)
            return new Dog(name);

To use the singleton, you use the Instance property:

            Dog d = DogFactory.Instance.CreateDog("Bob");