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How does the Factory Method pattern work in C#, and what problem does it solve? How would you implement the Strategy pattern in C# using either interfaces or delegates? How does the Repository pattern abstract data access, and what interface typically defines its contract? How would you implement the Decorator pattern in C# to add behavior to an object without modifying its original class? How would you implement the Observer pattern in C# using events, versus using the IObservable<T>/IObserver<T> interfaces from System? How would you implement the Unit of Work pattern alongside the Repository pattern to coordinate multiple repository changes within a single transaction? How would you implement the CQRS (Command Query Responsibility Segregation) pattern's basic structure in a C# application using MediatR-style handlers? How would you implement the Builder pattern using method chaining (a fluent API) to construct a complex object step by step? How would you implement the Chain of Responsibility pattern in C# to process a request through a series of independent handlers? What are design patterns, and how are the Singleton, Adapter, and Strategy patterns typically implemented in C#?
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How do you implement a thread-safe Singleton pattern in modern C# using Lazy<T>?

Beginner

Answer

Lazy<T> defers creation of the instance until it's first accessed AND guarantees thread-safe initialization by default, making it the cleanest modern way to implement a Singleton without manually writing double-checked locking.
public class ConfigManager {
  private static readonly Lazy<ConfigManager> _instance = new(() => new ConfigManager());
  public static ConfigManager Instance => _instance.Value;
  private ConfigManager() { }
}
Real-world example Ensuring a single, shared application configuration or logging instance across the entire application.

Common follow-ups: Why is a plain 'static readonly' field alone not always sufficient to guarantee thread-safe lazy initialization?

Multithreading & Task Parallel Library
All 11 Design Patterns in C# questions