如何重構此代碼以使其符合開放式關閉原則？

Question

The question is actually derived from this link.

假設我有這樣一個問題：

---

書商店購買和出售兩種類型的書： （1）非技術{標題，作者，價格} （ 2）技術{標題，作者，價格，CD}

此外，客戶購買技術書籍時獲得CD。 CD對象被定義爲CD {標題，價格}。

非技術書的價格將只是書的價格。技術書的價格將是書和CD的價格的總和。

創建一個C＃程序，以顯示以下信息：

Total number of book Bought & Price: XXX & XXX.XX 
Total number of book Sold & Price: XXX & XXX.XX 
Total Technical Book Sold & Price: XXX & XXX.XX 
Total Non-technical Book sold & Price: XXX & XXX.XX 

---

abstract class Publication 
{ 
    public virtual string Title { get; set; } 
    public virtual double Price { get; set; } 
} 

class CD : Publication 
{ 
} 

abstract class Book : Publication 
{ 
    public virtual string Author { get; set; } 
} 

class TechnicalBook : Book 
{ 
    public CD Cd { get; set; } 
    public override double Price 
    { 
     get 
     { 
      return (base.Price + Cd.Price); 
     } 
    } 
} 

class NonTechnicalbook : Book 
{ 
} 

class Shop 
{ 
    private IDictionary<string, Book> boughtDictionary; 
    private IDictionary<string, Book> soldDictionary; 

    public Shop() 
    { 
     boughtDictionary = new Dictionary<string, Book>(); 
     soldDictionary = new Dictionary<string, Book>(); 
    } 

    public virtual void Buy(Book item) 
    { 
     boughtDictionary.Add(item.Title, item); 
    } 

    public virtual void Sell(string title) 
    { 
     Book book = boughtDictionary[title]; 
     boughtDictionary.Remove(book.Title); 
     soldDictionary.Add(book.Title, book); 
    } 

    public virtual int GetBoughtBookCount() 
    { 
     return boughtDictionary.Count; 
    } 

    public virtual double GetBoughtBookPrice() 
    { 
     double price = 0.0; 

     foreach (string title in boughtDictionary.Keys) 
     { 
      price = price + boughtDictionary[title].Price; 
     } 
    } 

    public virtual int GetSoldBookCount() 
    { 
     return boughtDictionary.Count; 
    } 

    public virtual double GetSoldBookPrice() 
    { 
     double price = 0.0; 

     foreach (string title in soldDictionary.Keys) 
     { 
      price = price + soldDictionary[title].Price; 
     } 
    } 

    public virtual double GetTotalBookCount() 
    { 
     return this.GetBoughtBookCount() + this.GetSoldBookCount(); 
    } 

    public virtual double GetTotalBookPrice() 
    { 
     return this.GetBoughtBookPrice() + this.GetSoldBookPrice(); 
    } 

    public virtual void Show() 
    { 
     Console.WriteLine("Total number of books Bought & Price: ", this.GetTotalBookCount() + " & " + this.GetTotalBookPrice()); 
     Console.WriteLine("Total number of books Sold & Price: ", this.GetSoldBookCount() + " & " + this.GetSoldBookPrice()); 
    } 
} 

如何顯示技術和非技術書籍的價格，同時保持開閉原則？

派生商店級沒有任何意義。

如果我如下代碼：

if(book is TechnicalBook) { 
    // ... 
} else if(book is NonTechnicalBook) { 
    // ... 
} 

我不認爲它使OCP。

那該怎麼辦？

Answer 1

你可以有一個方法

getTotal(Option op) 

其中Option是delagte這需要一本書，它是否符合你的標準，你的情況，如果書是TechnicalBook或NonTechnicalBook返回true。

the getTotal方法會遍歷所有書籍只彙總那些Option委託返回true的書籍。 並返回總和。

希望這是你的意思。