State Design Pattern

// State pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.State.Structural
{
  
  // MainApp test application

  class MainApp
  {
    static void Main()
    {
      // Setup context in a state
      Context c = new Context(new ConcreteStateA());

      // Issue requests, which toggles state
      c.Request();
      c.Request();
      c.Request();
      c.Request();

      // Wait for user
      Console.Read();
    }
  }

  // "State"

  abstract class State
  {
    public abstract void Handle(Context context);
  }

  // "ConcreteStateA"

  class ConcreteStateA : State
  {
    public override void Handle(Context context)
    {
      context.State = new ConcreteStateB();
    }
  }

  // "ConcreteStateB"

  class ConcreteStateB : State
  {
    public override void Handle(Context context)
    {
      context.State = new ConcreteStateA();
    }
  }

  // "Context"

  class Context
  {
    private State state;

    // Constructor
    public Context(State state)
    {
      this.State = state;
    }

    // Property
    public State State
    {
      get{ return state; }
      set
      {
        state = value;
        Console.WriteLine("State: " +
          state.GetType().Name);
      }
    }

    public void Request()
    {
      state.Handle(this);
    }
  }
}

using System;

namespace DoFactory.GangOfFour.State.RealWorld
{
  
  // MainApp test application

  class MainApp
  {
    static void Main()
    {
      // Open a new account
      Account account = new Account("Jim Johnson");

      // Apply financial transactions
      account.Deposit(500.0);
      account.Deposit(300.0);
      account.Deposit(550.0);
      account.PayInterest();
      account.Withdraw(2000.00);
      account.Withdraw(1100.00);

      // Wait for user
      Console.Read();
    }
  }

  // "State"

  abstract class State
  {
    protected Account account;
    protected double balance;

    protected double interest;
    protected double lowerLimit;
    protected double upperLimit;

    // Properties
    public Account Account
    {
      get{ return account; }
      set{ account = value; }
    }

    public double Balance
    {
      get{ return balance; }
      set{ balance = value; }
    }

    public abstract void Deposit(double amount);
    public abstract void Withdraw(double amount);
    public abstract void PayInterest();
  }

  // "ConcreteState"

  // Account is overdrawn

  class RedState : State
  {
    double serviceFee;

    // Constructor
    public RedState(State state)
    {
      this.balance = state.Balance;
      this.account = state.Account;
      Initialize();
    }

    private void Initialize()
    {
      // Should come from a datasource
      interest = 0.0;
      lowerLimit = -100.0;
      upperLimit = 0.0;
      serviceFee = 15.00;
    }

    public override void Deposit(double amount)
    {
      balance += amount;
      StateChangeCheck();
    }

    public override void Withdraw(double amount)
    {
      amount = amount - serviceFee;
      Console.WriteLine("No funds available for withdrawal!");
    }

    public override void PayInterest()
    {
      // No interest is paid
    }

    private void StateChangeCheck()
    {
      if (balance > upperLimit)
      {
        account.State = new SilverState(this);
      }
    }
  }

  // "ConcreteState"

  // Silver is non-interest bearing state

  class SilverState : State
  {
    // Overloaded constructors

    public SilverState(State state) :
      this( state.Balance, state.Account)
    {  
    }

    public SilverState(double balance, Account account)
    {
      this.balance = balance;
      this.account = account;
      Initialize();
    }

    private void Initialize()
    {
      // Should come from a datasource
      interest = 0.0;
      lowerLimit = 0.0;
      upperLimit = 1000.0;
    }

    public override void Deposit(double amount)
    {
      balance += amount;
      StateChangeCheck();
    }

    public override void Withdraw(double amount)
    {
      balance -= amount;
      StateChangeCheck();
    }

    public override void PayInterest()
    {
      balance += interest * balance;
      StateChangeCheck();
    }

    private void StateChangeCheck()
    {
      if (balance < lowerLimit)
      {
        account.State = new RedState(this);
      }
      else if (balance > upperLimit)
      {
        account.State = new GoldState(this);
      }
    }
  }

  // "ConcreteState"

  // Interest bearing state

  class GoldState : State
  {
    // Overloaded constructors
    public GoldState(State state)
      : this(state.Balance,state.Account)
    {  
    }

    public GoldState(double balance, Account account)
    {
      this.balance = balance;
      this.account = account;
      Initialize();
    }

    private void Initialize()
    {
      // Should come from a database
      interest = 0.05;
      lowerLimit = 1000.0;
      upperLimit = 10000000.0;
    }

    public override void Deposit(double amount)
    {
      balance += amount;
      StateChangeCheck();
    }

    public override void Withdraw(double amount)
    {
      balance -= amount;
      StateChangeCheck();
    }

    public override void PayInterest()
    {
      balance += interest * balance;
      StateChangeCheck();
    }

    private void StateChangeCheck()
    {
      if (balance < 0.0)
      {
        account.State = new RedState(this);
      }
      else if (balance < lowerLimit)
      {
        account.State = new SilverState(this);
      }
    }
  }

  // "Context"

  class Account
  {
    private State state;
    private string owner;

    // Constructor
    public Account(string owner)
    {
      // New accounts are 'Silver' by default
      this.owner = owner;
      state = new SilverState(0.0, this);
    }

    // Properties
    public double Balance
    {
      get{ return state.Balance; }
    }

    public State State
    {
      get{ return state; }
      set{ state = value; }
    }

    public void Deposit(double amount)
    {
      state.Deposit(amount);
      Console.WriteLine("Deposited {0:C} --- ", amount);
      Console.WriteLine(" Balance = {0:C}", this.Balance);
      Console.WriteLine(" Status = {0}\n" ,
        this.State.GetType().Name);
      Console.WriteLine("");
    }

    public void Withdraw(double amount)
    {
      state.Withdraw(amount);
      Console.WriteLine("Withdrew {0:C} --- ", amount);
      Console.WriteLine(" Balance = {0:C}", this.Balance);
      Console.WriteLine(" Status = {0}\n" ,
        this.State.GetType().Name);
    }

    public void PayInterest()
    {
      state.PayInterest();
      Console.WriteLine("Interest Paid --- ");
      Console.WriteLine(" Balance = {0:C}", this.Balance);
      Console.WriteLine(" Status = {0}\n" ,
        this.State.GetType().Name);
    }
  }
}

// State pattern -- .NET optimized

Two editions: C# and VB.NET

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