Design Patterns – Singleton

Singleton is a creational design pattern.
Idea: We want to have some classes that should have EXACTLY ONE INSTANCE.

Singleton Pattern ensures a class has only one Instance.


  • When creating the instance is expensive –> a Singleton can improve performance.
  • There must be one and only one instance of a class.
  • The class should not require parameters as a part its construction.
Classic Singleton

The image shows us a Singleton class in its classic version. This version has:

  • Private static instance of Singleton class
  • Private Constructor
  • Public static Method which returns the UNIQUE singleton Instance.

Version 1 – Classic Singleton

This is the original version of the pattern. In other words, the Default Implementation.
This version is NOT THREAD-SAFE. For this reason should not be used in multi-threaded environments (E.g: Web Servers – ASP.NET)

Version 2 – Thead-safe Singleton using double-check locking

This version is thead-safe but doesn’t perform as well as Lazy Implementation.
It’s easy to get wrong with this alternative –> A pattern should be more cleaning.

Version 3 – Recommended – Thead-safe Singleton with Lazy<>

  • It’s simple and performs well
  • By Default, Lazy objects are thread-safe
  • We can use this alternative in web projects and multi-threaded environments.

Finally, I will call all Singletons!

static void Main(string[] args)