今天來簡介程式開發裡 SOLID 原則的單一職責、開放/封閉原則，並用 C# 實作。

S: Single Responsibility Principle (單一職責)

一個類別負責一件工作。

// 負責計算
public class Order
{
    public int OrderId { get; set; }
    public string ProductName { get; set; }
    public int Quantity { get; set; }

    public double CalculateTotalPrice(double unitPrice)
    {
        return unitPrice * Quantity;
    }
}

// 負責印出資料
public class OrderPrinter
{
    public void PrintOrderDetails(Order order)
    {
        Console.WriteLine($"Order ID: {order.OrderId}");
        Console.WriteLine($"Product Name: {order.ProductName}");
        Console.WriteLine($"Quantity: {order.Quantity}");
    }
}

O: Open/Close Principle (開放/封閉原則)

可以開放擴充新功能，且盡可能減少對現有程式碼的修改。

// 一開始先設計好可擴充的類別
public abstract class Discount
{
    public abstract double ApplyDiscount(double price);
}

// 沒有折扣
public class NoDiscount : Discount
{
    public override double ApplyDiscount(double price)
    {
        return price; // 傳回原價
    }
}

// 有折扣
public class PercentageDiscount : Discount
{
    private double percentage;

    public PercentageDiscount(double percentage)
    {
        this.percentage = percentage;
    }

    public override double ApplyDiscount(double price)
    {
        return price - (price * percentage / 100);
    }
}

// 擴充後可以不修改現有類別
public class OrderWithDiscount
{
    public double CalculatePrice(double price, Discount discount)
    {
        return discount.ApplyDiscount(price);
    }
}

結論

用以下的程式呼叫上述類別：

public class Program
{
	public static void Main()
	{
		// 單一職責原則
        Order order = new Order { OrderId = 1, ProductName = "Laptop", Quantity = 2 };
        OrderPrinter printer = new OrderPrinter();
        printer.PrintOrderDetails(order);
		
		// 開放/封閉原則
        Discount discount = new PercentageDiscount(10);
        OrderWithDiscount orderWithDiscount = new OrderWithDiscount();
        Console.WriteLine($"Price after discount: {orderWithDiscount.CalculatePrice(1000, discount)}");
        Console.WriteLine($"Price after discount: {orderWithDiscount.CalculatePrice(1000, noDiscount)}");
	}
}

當程式變得需要時常修改時，上述原則讓程式更容易擴充，且減少雜亂的程度。

參考資料

• 個人 Github Pages
• ChatGPT 產生部分程式碼