Difference Between Singleton Class vs. Static Class - Recap

In C#, understanding the difference between a Singleton class and a Static class is essential for various scenarios in software development. Both have unique characteristics and uses:

Singleton Class

1. Purpose: A Singleton class ensures that a class has only one instance and provides a global point of access to it. Instantiation: It is instantiated once, and the same instance is reused.
2. Lazy Loading: Singleton supports lazy loading, meaning the instance is created only when it is needed.
3. Inheritance: A Singleton can inherit from other classes and implement interfaces.
4. State: It can maintain state across multiple calls and instances of the application.

Example Use:

Database connections or a file manager where a single instance manages the resources throughout the application.

Static Class

1. Purpose: A static class is a class that cannot be instantiated. Its members are accessible without creating an object of the class.
2. Instantiation: Cannot be instantiated. All members are static and accessed directly with the class name.
3. Lazy Loading: Does not support lazy loading in a conventional way. The class is loaded once by the CLR when it is accessed for the first time.
4. Inheritance: Cannot inherit from other classes (except for Object) and cannot implement interfaces.
5. State: Cannot maintain state as it doesn't support instance variables. All members are static.

Example Use:

Utility or helper functions that are generic and don't require object state, like mathematical functions.

Key Differences:

1. Instantiation Control: Singleton controls the instantiation, allowing only one instance. Static classes are never instantiated.
2. Memory Allocation: Singleton objects are stored in the heap, while static class objects are stored in the high frequency heap area.
3. Inheritance and Interfaces: Singleton can implement interfaces and inherit, while static classes cannot.
4. State Maintenance: Singleton can maintain state, but static classes cannot.

Choosing Between Them:

Use a Singleton when you need a single instance with stateful data across the application.

Use a static class for stateless utility or helper functions that don't require instantiation.

Both Singleton and Static classes promote specific design principles in C#, each serving different purposes based on the requirements of the application.