🎯 Strategy Pattern — Salary Calculation Example (C#)

📖 Overview

This repository demonstrates the Strategy Design Pattern through a real-world payroll example.

Instead of writing long if/else or switch statements for every employee type,
the Strategy Pattern lets you define a family of salary calculation algorithms,
encapsulate each one in its own class, and make them interchangeable at runtime.

This keeps your code clean, extensible, and easy to maintain.

---

😵 Problem (Before)

In a typical payroll system, salary calculation often starts like this:

if (emp.Type == EmployeeType.FullTime)
    return emp.BaseSalary + emp.Bonus;
else if (emp.Type == EmployeeType.Hourly)
    return emp.HoursWorked * emp.HourlyRate;
else if (emp.Type == EmployeeType.Freelancer)
    return emp.GrossAmount - (emp.GrossAmount * 0.15m);
else if (emp.Type == EmployeeType.Intern)
    return 1000;

This approach quickly becomes rigid and messy:

• Adding a new employee type means editing this method.
• Unit testing is painful.
• Code violates the Open/Closed Principle.

---

✅ Solution (After)

With the Strategy Pattern, each calculation logic is moved into its own class that implements a common interface:

public interface ISalaryCalculationStrategy
{
    decimal Calculate(Employee emp);
}

Now each employee type has its own strategy:

Employee Type	Strategy Class	Description
Full-time	FulltimeStrategy	Base salary + bonus
Hourly	HourlyStrategy	Hourly rate × worked hours
Task-based	TaskBasedStrategy	Pay per completed task
Freelancer	FreelancerStrategy	Gross amount minus tax & commission
Intern	InternStrategy	Fixed allowance

The selection of the correct strategy is done by a Factory class,
so the main logic (PayrollService) doesn’t need to know any implementation details.

---

🧩 Class Responsibilities

• ISalaryCalculationStrategy → Defines the contract for all salary strategies
• FulltimeStrategy, HourlyStrategy, FreelancerStrategy, TaskBasedStrategy, InternStrategy → Implement different salary rules
• SalaryStrategyFactory → Chooses which strategy to use based on EmployeeType
• PayrollService → Uses the chosen strategy to calculate the final salary

---

▶ Example Usage

var emp = new Employee
{
    Name = "Ahmed",
    Type = EmployeeType.Freelancer,
    GrossAmount = 8000
};

var payroll = new PayrollService();
var salary = payroll.CalculateSalary(emp);

Console.WriteLine($"Employee: {emp.Name}, Salary: {salary:C}");

🧾 Output Example

Employee: Ahmed, Salary: 6,800.00

---

⚙️ Benefits

✅ Removes complex if/else and switch logic
✅ Follows the Open/Closed Principle (easy to add new types)
✅ Each strategy is independent and testable
✅ Improves readability and maintainability
✅ Makes behavior interchangeable at runtime

---

📦 Project Structure

/StrategyPattern
├── Strategies/
│   ├── ISalaryCalculationStrategy.cs
│   ├── FulltimeStrategy.cs
│   ├── HourlyStrategy.cs
│   ├── TaskBasedStrategy.cs
│   ├── FreelancerStrategy.cs
│   ├── InternStrategy.cs
│   ├── SalaryStrategyFactory.cs
│   └── PayrollService.cs
└── Program.cs

---

💬 Design Pattern Summary

The Strategy Pattern defines a family of algorithms,
encapsulates each one, and makes them interchangeable.
This allows the algorithm to vary independently from the clients that use it.

---