When and when not to use Extension functions

Use extension methods when:

1. You want to add small, self-contained pieces of functionality to an existing type. Extension methods are ideal for adding simple utility methods that don't require deep integration with the type's structure.

2. You don't have access to the original source code of the type. If you're using third-party libraries or working with legacy code that you can't modify, extension methods allow you to extend the functionality without having to modify the original code.

3. You need to add functionality that is specific to a certain context or layer of your application. Extension methods can be used to add layer-specific functionality to domain entities or data transfer objects without cluttering their core behaviour.

Avoid using extension methods when:

1. The functionality you want to add is complex or tightly coupled to the type's internal structure. Extension methods are not suitable for complex operations that require intimate knowledge of the type's implementation.

2. The functionality you want to add needs to be overridden in derived classes. Extension methods cannot be overridden, so they are not appropriate for adding functionality that should be tailored to specific derived types.

3. The functionality you want to add is already available in a more appropriate form, such as a static class or interface method. Extension methods should be used for genuinely new functionality, not for duplicating existing methods or adding complexity to the type's design.