Design patterns have been a cornerstone of object-oriented software development for decades. Yet, with the evolution of .NET — from .NET Framework to .NET 10, C# 14, and the rise of cloud-native architectures — the relevance of each pattern has shifted dramatically. This deep dive explores which design patterns remain essential in modern .NET development, which have become anti-patterns, and how to apply them effectively for performance, maintainability, and scalability. Why Design Patterns Still Matter In 2026, the .NET ecosystem is richer than ever. From high‑performance services to AI‑driven applications, the underlying principles of good software design remain constant: separation of concerns, testability, and adaptability. Design patterns provide a shared vocabulary and proven solutions to recurring problems. However, blindly applying “textbook” patterns can lead to over‑engineered, rigid systems.…