Program.cs

using System;

namespace Strategy;

// Strategy interface
public interface IPaymentStrategy
{
    void Pay(decimal amount);
}

// Concrete Strategies
public class CreditCardPayment(string cardNumber) : IPaymentStrategy
{
    // Using C# 14 field keyword for property
    public string CardNumber
    {
        get;
        init => field = value ?? throw new ArgumentNullException(nameof(value));
    } = cardNumber;

    public void Pay(decimal amount)
    {
        Console.WriteLine($"Paid ${amount} using Credit Card ending in {CardNumber.Substring(CardNumber.Length - 4)}");
    }
}

public class PayPalPayment(string email) : IPaymentStrategy
{
    // Using C# 14 field keyword
    public string Email
    {
        get;
        init => field = value ?? throw new ArgumentNullException(nameof(value));
    } = email;

    public void Pay(decimal amount)
    {
        Console.WriteLine($"Paid ${amount} using PayPal account {Email}");
    }
}

public class BitcoinPayment(string walletAddress) : IPaymentStrategy
{
    // Using C# 14 field keyword
    public string WalletAddress
    {
        get;
        init => field = value ?? throw new ArgumentNullException(nameof(value));
    } = walletAddress;

    public void Pay(decimal amount)
    {
        Console.WriteLine($"Paid ${amount} using Bitcoin wallet {WalletAddress}");
    }
}

// Context
public class ShoppingCart
{
    private IPaymentStrategy paymentStrategy;

    public void SetPaymentStrategy(IPaymentStrategy strategy)
    {
        this.paymentStrategy = strategy;
    }

    public void Checkout(decimal amount)
    {
        // Using C# 14 null-conditional assignment
        if (paymentStrategy == null)
        {
            Console.WriteLine("Please select a payment method");
            return;
        }

        paymentStrategy.Pay(amount);
    }
}

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("=== Strategy Pattern Demo ===");
        Console.WriteLine();

        var cart = new ShoppingCart();
        decimal totalAmount = 250.00m;

        // Pay with Credit Card
        Console.WriteLine("Paying with Credit Card:");
        cart.SetPaymentStrategy(new CreditCardPayment("1234-5678-9012-3456"));
        cart.Checkout(totalAmount);

        // Pay with PayPal
        Console.WriteLine();
        Console.WriteLine("Paying with PayPal:");
        cart.SetPaymentStrategy(new PayPalPayment("user@example.com"));
        cart.Checkout(totalAmount);

        // Pay with Bitcoin
        Console.WriteLine();
        Console.WriteLine("Paying with Bitcoin:");
        cart.SetPaymentStrategy(new BitcoinPayment("1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa"));
        cart.Checkout(totalAmount);

        Console.WriteLine();
        Console.WriteLine("Press any key to exit...");
        Console.ReadKey();
    }
}