Strategy

Context: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

public interface IStrategy
{
    void Execute();
}

public class ConcreteStrategyA : IStrategy
{
    public void Execute() => Console.WriteLine("Strategy A");
}

public class Context
{
    private IStrategy _strategy;
    public void SetStrategy(IStrategy strategy) => _strategy = strategy;
    public void DoWork() => _strategy.Execute();
}

Real-world usage example

Payment processing: An e‑commerce site selects different strategies: CreditCardStrategy, PayPalStrategy, CryptoStrategy based on user choice. The CheckoutContext uses the selected strategy to calculate final amount.

Example: Sorting in .NET: List<T>.Sort(IComparer<T>) allows plugging different comparer strategies (by name, by price, by date). Also, System.Text.Encoding strategies for different character encodings.

Related sections

• 5.3 Behavioural patterns
• 5.3.2 Command