State Pattern

Type	Behavioral
Purpose	Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

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Description

The State pattern enables the context to behave differently depending on the state that it's in. This is achieved by representing each state as a separate class that implements a common State interface.

Implementing the State pattern can ensure that:

• Large monolithic conditional statements can be avoided.
• State transitions can be made more explicit and easier to understand.
• New states can often be added easily with minimal changes to existing code.

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Overview

Context	Maintains a reference to the current State object and delegates state-dependent behaviour to it.
State	Defines the interface used by the Context to delegate state-dependent behaviour.
ConcreteState	Implements the State interface and provides behaviour associated with a particular state of the Context.

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Implementation

// State

public interface ITrafficLightState
{
    void Change(TrafficLight light);
}

// Concrete States

public class GreenLightState : ITrafficLightState
{
    public void Change(TrafficLight light)
    {
        Console.WriteLine("Green light - Go!");
        light.State = new YellowLightState();
    }
}

public class YellowLightState : ITrafficLightState
{
    public void Change(TrafficLight light)
    {
        Console.WriteLine("Yellow light - Be careful!");
        light.State = new RedLightState();
    }
}

public class RedLightState : ITrafficLightState
{
    public void Change(TrafficLight light)
    {
        Console.WriteLine("Red light - Stop!");
        light.State = new GreenLightState();
    }
}

// Context

public class TrafficLight(ITrafficLightState initialState)
{
    public ITrafficLightState State { get; set; } = initialState;

    // handler
    
    public void Change() => State.Change(this);
}

// Demonstration

public static class StateDemo
{
    public static void Execute()
    {
        var light = new TrafficLight(new RedLightState());
        
        for (var i = 0; i < 5; i++)
        {
            // request 
            
            light.Change();
        }
    }
}

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Summary

For more information, please see here.