Strategy

Updated: 03 September 2023

The strategy pattern is about using composition instead of inheritence

The strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. The algorithm can vary independent of clients using it

Algorithm/implementation is decoupled from its clients

This works by creating an object which wraps the behaviour we want to implement in a family of classes, allowing clients to choose which behaviour they would like to implement by referencing the interface that these behaviours implement.

Example

Definition of Classes

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namespace DesignPatterns.Strategy
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{
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    public class User {
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        // strategy to be used for logging in a user
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        // this is what we want to change depending on an implementation
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        private ILoginStrategy _loginStrategy { get; set; }
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        public string Username { get; set; }
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        public string Password { get; set; }
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        // we set the behaviour when instantiating
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        public User(ILoginStrategy loginStrategy)
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        {
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            this._loginStrategy = loginStrategy;
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        }
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        // we can possibly change the strategy later
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        public void SetLoginStrategy(ILoginStrategy loginStrategy)
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        {
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            this._loginStrategy = loginStrategy;
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        }
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        // login function that can be called by a consumer of this class
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        public bool Login() => _loginStrategy.Execute(Username, Password);
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    }
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    // interface to define login strategies
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    public interface ILoginStrategy
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    {
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        public bool Execute(string username, string password);
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    }
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    // login strategy for logging in from a local service
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    public class LocalLoginStrategy: ILoginStrategy
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    {
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        public bool Execute(string username, string password) => true;
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    }
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    // login strategy for logging in from a remote service
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    public class RemoteLoginStrategy : ILoginStrategy
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    {
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        public bool Execute(string username, string password) => false;
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    }
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}

Usage

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var user = new User(new LocalLoginStrategy()) {
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    Username = "user",
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    Password = "Password"
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};
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// calling this uses the LocalLoginStrategy
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user.Login();
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user.SetLoginStrategy(new RemoteLoginStrategy());
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// calling this uses the RemoteLoginStrategy
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user.Login();

References