When should you use the singleton pattern instead of a static class? [closed]

Question

Name the design considerations in deciding between use of a singleton versus a static class. In doing this, you're kind of forced to contrast the two, so whatever contrasts you can come up with are also useful in showing your thought process! Also, every interviewer likes to see illustrative examples. :)

Answer 1

• Singletons can implement interfaces and inherit from other classes.
• Singletons can be lazy loaded. Only when it is actually needed. That's very handy if the initialisation includes expensive resource loading or database connections.
• Singletons offer an actual object.
• Singletons can be extended into a factory. The object management behind the scenes is abstract so it's better maintainable and results in better code.

Answer 2

How about "avoid both"? Singletons and static classes:

• May introduce global state
• Get tightly coupled to multiple other classes
• Hide dependencies
• Can make unit testing classes in isolation difficult

Instead, look into Dependency Injection and Inversion of Control Container libraries. Several of the IoC libraries will handle lifetime management for you.

(As always, there are exceptions, such as static math classes and C# extension methods.)