The singleton design pattern

The singleton design pattern enables easy access to a single instance of a class.

Benefits

• Makes it very easy to get a single instance of an object globally

Drawbacks

• They are effectively just globals, this is not object-oriented design
• Can cause concurrency issues if not the singleton is not thread-safe
• Can make code hard to follow
• Makes unit testing more difficult

From the looks of the benefits and drawbacks you would wonder why you would use a singleton at all, a lot of people even consider it an anti-pattern. My answer to that question is that you should only use this pattern very sparingly and preferably not at all in large projects. You can however save yourself quite a bit of time using them in small projects.

UML diagram

Note the private constructor.

Code examples

View on GitHub

Traditional singleton

public class Singleton
{
    private static Singleton _instance = new Singleton();

    private Singleton() { }

    public static Singleton Instance
    {
        get { return _instance; }
    }
}

Singleton with lazy loading

public class LazyLoadingSingleton
{
    private static LazyLoadingSingleton _instance;

    private LazyLoadingSingleton() { }

    public static LazyLoadingSingleton Instance
    {
        get
        {
            if (_instance == null)
                _instance = new LazyLoadingSingleton();

            return _instance;
        }
    }
}

Usage examples

Some common uses of the Singleton design pattern are:

• Caches
• Handling user preferences
• Factory or builder classes