design-patterns

Apply SOLID in C# by separating responsibilities into focused classes, using abstract base classes for extension, designing substitutable hierarchies, creating small focused interfaces, and depending on abstractions injected via constructors. The Skill provides before/after examples for each principle.

Use Factory Method when a class delegates object creation to subclasses, Abstract Factory when creating families of related objects like platform-specific UI elements, and Builder when constructing complex objects step by step with many optional parameters.

Use Decorator when you need to add responsibilities to objects dynamically without subclassing, such as layering logging or SMS delivery onto a notification service. Decorators wrap the same interface and compose at runtime.

Classic Singleton using Lazy<T> works independently, but in modern .NET you typically register services with AddSingleton in IServiceCollection instead. The Skill shows thread-safe Lazy-based Singleton implementations for cases outside the DI container.

Manual Observer implementations require explicit registration and unregistration to avoid memory leaks, and notification order is not guaranteed. For many .NET scenarios, events, delegates, or IObservable<T> with Reactive Extensions are preferable alternatives.