This post continues the analisys of the SOLID principles started some blog posts ago. This is the turn for the Open Closed Priciple (OCP).

The definition

An object/entity should be open for extension but closed for modification.

What we are basically talking about is to design our modules, classes and functions in a way that when a new functionality is needed, we should not modify our existing code but rather write new code that will be used by existing code.

The example

In this example we’re designing a class that represents the human resources department of a company and one of its main activities: hire people.

public class HumanResourceDepartment
{

    private IList<Developer> _hiredDevelopers;

    private IList<Manager> _hiredManagers;    

    public void Hire(Developer developer){ 

        developer.SignContract();
        _hiredDevelopers.Add(developer);
    }

    public void Hire(Manager manager){ 

        manager.SignContract();
        _hiredManagers.Add(manager);
    }
}

This class violates the OCP because if HR wants to hire a Secretary we have to add another Hire method and another IList where T is Secretary in addition to create the new Secretary class. So our initial purpose to write new code that will be used by existing code hasn’t been met.

Refactoring

interface IEmployee
{
    public void SignContract();
}

class Developer : IEmployee
{
    public void SignContract()
    {
       //...
    }
}

class Manager : IEmployee
{
    public void SignContract()
    {
       //...
    }
}

class Secretary : IEmployee
{
    public void SignContract()
    {
       //...
    }
}

public class HumanResourceDepartment
{

    private IList<IEmployee> _employees;

    public void Hire(IEmployee employee)
    {
        employee.SignContract();

        _employees.Add(employee);
    }
}

The HumanResorceDepartement class now respects the OCP because the new code (the Secretary class) can be used by old code (the Hire method). We did this by abstracting the concept of a manager/developer/secertary stating that they are all employees that sign a contract when are hired. So we created an interface called IEmployee that exposes the SignContract method.

The OCP is tightly coupled with the Single Responsability Principle. If we design a class with SRP in mind it is very likely that we also respect OCP or that little work is required to meet both and vice versa.

TL;DR

The OCP makes our code more reusable and less coupled. This way we can write new code that with little impact in our existing codebase. SRP and OCP are closely related parents and their application makes our code more clean and mantainable. What’s your experience with SOLID priciples?