Photo by Chetan Menaria on Unsplash

Singleton has a bad rep for being an anti-pattern. You might have been burned bad with it.

I learned from Michael Outlaw from Coding Blocks podcast episode 16 that Singleton pattern can come in handy in conjunction with Null object pattern.

Let's dive in.

Why implement a Null object as a singleton?

42 minutes into the Coding Blocks Episode 16, Michael Outlaw explains two reasons.

1. "If you have two versions of null objects, they should be identical, why waste memory to have same nothingness repeated".
2. For equality checks - Each null object can be compared by reference.

Depending on a situation you might not even need to do a equality check as a null object usually does nothing (as you can see in the example section below.)

TL;DR - You can stop reading here.

Example

Here is an example of how you can use a Singleton pattern with a Null object pattern.

Suppose that there is a simple factory (PaymentStrategyFactory) that returns a strategy object instance for processing a payment given a provider name.

https://gist.github.com/dance2die/aad6b604536afeb10f71771cc42e3853

PayPal & ApplePay strategies return "Successful" or "Failed" process status at random.

https://gist.github.com/dance2die/8d0f0f6f47f01a516f1657a7e500dd7b

https://gist.github.com/dance2die/496655cf6ec63b4dd74da93c4e6e180b

If a provider name is not supported, the PaymentStrategyFactory simply returns an object of type NullPaymentStrategy, which implements IPaymentStrategy.

Singleton instance is achieved with a private constructor and a static Instance property.

https://gist.github.com/dance2die/8ae2a62261d3784c713a44b6eed24b56

https://gist.github.com/dance2die/dee349d007060c5701652fadfcbc4009

Let's put them together and process payments.

https://gist.github.com/dance2die/5897ad07ed0411dfec9cc2c22b0d3fbc

Results of ProcessPayments().

https://gist.github.com/dance2die/1c76839c8c7f358018c2df0fa27ee2a0

Conclusion

Googling Singleton Anti Pattern results in many reasons why Singleton pattern is bad.

But when used judiciously, it can improve your code quality/memory/speed.

Source code is available on GitHub.