Project Discount Strategy System

Context: Build a console application that applies different discount strategies to an order based on customer type or total amount. This project uses the Strategy pattern.

Complete Code

using System;

public interface IDiscountStrategy
{
    decimal CalculateDiscount(decimal total);
}

public class NoDiscount : IDiscountStrategy
{
    public decimal CalculateDiscount(decimal total) => 0;
}

public class PercentageDiscount : IDiscountStrategy
{
    private readonly decimal _percentage;
    public PercentageDiscount(decimal percentage) => _percentage = percentage;
    public decimal CalculateDiscount(decimal total) => total * _percentage / 100;
}

public class FixedAmountDiscount : IDiscountStrategy
{
    private readonly decimal _amount;
    public FixedAmountDiscount(decimal amount) => _amount = amount;
    public decimal CalculateDiscount(decimal total) => Math.Min(_amount, total);
}

public class SeasonalDiscount : IDiscountStrategy
{
    public decimal CalculateDiscount(decimal total) => total * 0.15m;
}

public class Order
{
    public decimal Total { get; set; }
    private IDiscountStrategy _discountStrategy;

    public void SetDiscountStrategy(IDiscountStrategy strategy)
    {
        _discountStrategy = strategy;
    }

    public decimal GetFinalTotal()
    {
        if (_discountStrategy == null) return Total;
        return Total - _discountStrategy.CalculateDiscount(Total);
    }
}

public class Program
{
    static void Main()
    {
        Console.WriteLine("=== Discount Strategy System ===");
        Console.Write("Enter order total: ");
        decimal total = decimal.Parse(Console.ReadLine());

        var order = new Order { Total = total };

        Console.WriteLine("Select discount strategy:");
        Console.WriteLine("1. No discount");
        Console.WriteLine("2. Percentage discount (10%)");
        Console.WriteLine("3. Fixed amount discount ($20)");
        Console.WriteLine("4. Seasonal discount (15%)");
        Console.Write("Choice: ");
        string choice = Console.ReadLine();

        switch (choice)
        {
            case "1": order.SetDiscountStrategy(new NoDiscount()); break;
            case "2": order.SetDiscountStrategy(new PercentageDiscount(10)); break;
            case "3": order.SetDiscountStrategy(new FixedAmountDiscount(20)); break;
            case "4": order.SetDiscountStrategy(new SeasonalDiscount()); break;
            default: order.SetDiscountStrategy(new NoDiscount()); break;
        }

        Console.WriteLine($"Original total: {total:C}");
        Console.WriteLine($"Final total after discount: {order.GetFinalTotal():C}");
    }
}

Example Run

=== Discount Strategy System ===
Enter order total: 100
Select discount strategy:
1. No discount
2. Percentage discount (10%)
3. Fixed amount discount ($20)
4. Seasonal discount (15%)
Choice: 2
Original total: $100.00
Final total after discount: $90.00

Real-world usage example

E‑commerce cart: The discount strategy pattern allows dynamic switching between promotions – Black Friday (percentage), loyalty program (fixed amount), or seasonal sales. The Order class remains unchanged when new discount types are added. This is similar to how IComparer<T> allows plugging different sorting strategies.

Related sections

• 5.3.1 Strategy
• 5.4.3 Dependency Injection