C#

Usage examples: The Strategy pattern is very common in C# code. It’s often used in various frameworks to provide users a way to change the behavior of a class without extending it.

Identification: Strategy pattern can be recognized by a method that lets a nested object do the actual work, as well as a setter that allows replacing that object with a different one.

Conceptual Example

This example illustrates the structure of the Strategy design pattern. It focuses on answering these questions:

• What classes does it consist of?

• What roles do these classes play?

• In what way the elements of the pattern are related?

Program.cs: Conceptual example

using System;
using System.Collections.Generic;

namespace RefactoringGuru.DesignPatterns.Strategy.Conceptual
{
    // The Context defines the interface of interest to clients.
    class Context
    {
        // The Context maintains a reference to one of the Strategy objects. The
        // Context does not know the concrete class of a strategy. It should
        // work with all strategies via the Strategy interface.
        private IStrategy _strategy;

        public Context()
        { }

        // Usually, the Context accepts a strategy through the constructor, but
        // also provides a setter to change it at runtime.
        public Context(IStrategy strategy)
        {
            this._strategy = strategy;
        }

        // Usually, the Context allows replacing a Strategy object at runtime.
        public void SetStrategy(IStrategy strategy)
        {
            this._strategy = strategy;
        }

        // The Context delegates some work to the Strategy object instead of
        // implementing multiple versions of the algorithm on its own.
        public void DoSomeBusinessLogic()
        {
            Console.WriteLine("Context: Sorting data using the strategy (not sure how it'll do it)");
            var result = this._strategy.DoAlgorithm(new List<string> { "a", "b", "c", "d", "e" });

            string resultStr = string.Empty;
            foreach (var element in result as List<string>)
            {
                resultStr += element + ",";
            }

            Console.WriteLine(resultStr);
        }
    }

    // The Strategy interface declares operations common to all supported
    // versions of some algorithm.
    //
    // The Context uses this interface to call the algorithm defined by Concrete
    // Strategies.
    public interface IStrategy
    {
        object DoAlgorithm(object data);
    }

    // Concrete Strategies implement the algorithm while following the base
    // Strategy interface. The interface makes them interchangeable in the
    // Context.
    class ConcreteStrategyA : IStrategy
    {
        public object DoAlgorithm(object data)
        {
            var list = data as List<string>;
            list.Sort();

            return list;
        }
    }

    class ConcreteStrategyB : IStrategy
    {
        public object DoAlgorithm(object data)
        {
            var list = data as List<string>;
            list.Sort();
            list.Reverse();

            return list;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // The client code picks a concrete strategy and passes it to the
            // context. The client should be aware of the differences between
            // strategies in order to make the right choice.
            var context = new Context();

            Console.WriteLine("Client: Strategy is set to normal sorting.");
            context.SetStrategy(new ConcreteStrategyA());
            context.DoSomeBusinessLogic();
            
            Console.WriteLine();
            
            Console.WriteLine("Client: Strategy is set to reverse sorting.");
            context.SetStrategy(new ConcreteStrategyB());
            context.DoSomeBusinessLogic();
        }
    }
}

Output.txt: Execution result

Client: Strategy is set to normal sorting.
Context: Sorting data using the strategy (not sure how it'll do it)
a,b,c,d,e

Client: Strategy is set to reverse sorting.
Context: Sorting data using the strategy (not sure how it'll do it)
e,d,c,b,a