Software Architecture Cheat Blog 3: Open Closed Principle

Define Principle

Open Closed Principle (OCP) states that software entities (classes, modules, functions, etc.) should be open for extension but closed for modifications.

Violation Example

Lets consider a simple banking module that is responsible for calculating interest of accounts respect to account type. Say app owner bank offers to their client three types of accounts : savings accounts, basic checking accounts, money market deposit account. Each of the account type has different interest rate, and interest is calculated on different other factors such as amount deposited, average balance per month and so on. Thus based on this business rules we designed this class Accounts which calculates interest based on account types.

public enum AccountTYpe
    {
        Savings,
        Checking,
        MoneyMarket 
    }

    public class Accounts
    {
        private AccountTYpe accountType { get; set; }

        public Decimal CalculateInterest()
        {
            Decimal interest = 0;
            if (accountType == AccountTYpe.Savings)
            {
                //consider business rules and calculate interest of savings account
                interest = 2.50m;
            }
            else if (accountType == AccountTYpe.Checking)
            {
                //consider business rules and calculate interest of checking account
                interest = 3.50m;
            }
            else
            {
                //consider business rules and calculate interest of money market account
                interest = 5.00m;
            }

            return interest;
        }
    }

Above solution work perfectly for our client bank and app went to live and every one is happy. Few months later bank realize other banks are making money from certificates of deposit account , so we should do that, lets change our system to accommodate that. Now our revised system looks like this,

public enum AccountTYpe
    {
        Savings,
        Checking,
        MoneyMarket,
        CertificatesOfDeposit
    }

    public class Accounts
    {
        private AccountTYpe accountType { get; set; }

        public Decimal CalculateInterest()
        {
            Decimal interest = 0;
            if (accountType == AccountTYpe.Savings)
            {
                //consider business rules and calculate interest of savings account
                interest = 2.50m;
            }
            else if (accountType == AccountTYpe.Checking)
            {
                //consider business rules and calculate interest of checking account
                interest = 3.50m;
            }
            else if (accountType == AccountTYpe.CertificatesOfDeposit)
            {
                //consider business rules and calculate interest of certificates of deposit account
                interest = 6.35m;
            }
            else
            {
                //consider business rules and calculate interest of money market account
                interest = 5.00m;
            }

            return interest;
        }
    }

Considering the above scenario its not hard to guess, it will go through same process each time bank wants to launch another account type or made change on existing account types business rules. This is where open closed principle violates.

Resolution

To solve this We can create an common Interface which will be implemented by all classes represent account types. Benefit is later change can be accommodate better without modifying other corresponding class.

public interface IAccount
    {
        Decimal CalculateInterest();
    }

    public class SavingsAccounts:IAccount
    {
        public decimal CalculateInterest()
        {
            return 2.50m;
        }
    }

    public class CheckingAccounts : IAccount
    {
        public decimal CalculateInterest()
        {
            return 3.50m;
        }
    }

    public class MoneyMarketAccounts : IAccount
    {
        public decimal CalculateInterest()
        {
            return 5.00m;
        }
    }

Through approach we can add as many account type is needed, all we need is add another class for that account type. Also modification of business logic resides in respective account type class. Hence IAccount interface implement the idea of open for extension but closed for modifications.

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