All 11 Design Patterns in C# questions
More in Design Patterns in C#
How do you implement a thread-safe Singleton pattern in modern C# using Lazy<T>? How does the Factory Method pattern work in C#, and what problem does it solve? 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 would you implement the Strategy pattern in C# using either interfaces or delegates?

Intermediate

Answer

Define a common interface (or a delegate type) representing an interchangeable algorithm, implement multiple concrete strategies, and inject/pass the chosen one into the context class — modern C# often prefers a simple Func<> delegate over a full interface hierarchy for lightweight strategies.
// Interface-based
public interface IDiscountStrategy { decimal Apply(decimal price); }
public class PercentageDiscount : IDiscountStrategy {
  public decimal Apply(decimal price) => price * 0.9m;
}

// Delegate-based (lighter weight)
Func<decimal, decimal> percentageDiscount = price => price * 0.9m;
Real-world example Supporting multiple interchangeable pricing, sorting, or validation strategies that can be swapped at runtime.

Common follow-ups: When would you prefer the interface-based approach over the simpler delegate-based one for the Strategy pattern?

Delegates Events & Lambdas
All 11 Design Patterns in C# questions