举例讲解C#编程中委托的实例化使用

下面的示例阐释声明、实例化和使用委托。 BookDB 类封装一个书店数据库，它维护一个书籍数据库。它公开 ProcessPaperbackBooks 方法，该方法在数据库中查找所有平装书，并对每本平装书调用一个委托。使用的 delegate 类型名为 ProcessBookDelegate。 Test 类使用该类打印平装书的书名和平均价格。
委托的使用促进了书店数据库和客户代码之间功能的良好分隔。客户代码不知道书籍的存储方式和书店代码查找平装书的方式。书店代码也不知道找到平装书后将对平装书执行什么处理。

// A set of classes for handling a bookstore:
namespace Bookstore
{
  using System.Collections;

  // Describes a book in the book list:
  public struct Book
  {
    public string Title;    // Title of the book.
    public string Author;    // Author of the book.
    public decimal Price;    // Price of the book.
    public bool Paperback;   // Is it paperback?

    public Book(string title, string author, decimal price, bool paperBack)
    {
      Author = author;
      Price = price;
      Paperback = paperBack;
    }
  }

  // Declare a delegate type for processing a book:
  public delegate void ProcessBookDelegate(Book book);

  // Maintains a book database.
  public class BookDB
  {
    // List of all books in the database:
    ArrayList list = new ArrayList();

    // Add a book to the database:
    public void AddBook(string title, string author, decimal price, bool paperBack)
    {
      list.Add(new Book(title, author, price, paperBack));
    }

    // Call a passed-in delegate on each paperback book to process it: 
    public void ProcessPaperbackBooks(ProcessBookDelegate processBook)
    {
      foreach (Book b in list)
      {
        if (b.Paperback)
          // Calling the delegate:
          processBook(b);
      }
    }
  }
}


// Using the Bookstore classes:
namespace BookTestClient
{
  using Bookstore;

  // Class to total and average prices of books:
  class PriceTotaller
  {
    int countBooks = 0;
    decimal priceBooks = 0.0m;

    internal void AddBookToTotal(Book book)
    {
      countBooks += 1;
      priceBooks += book.Price;
    }

    internal decimal AveragePrice()
    {
      return priceBooks / countBooks;
    }
  }

  // Class to test the book database:
  class TestBookDB
  {
    // Print the title of the book.
    static void PrintTitle(Book b)
    {
      System.Console.WriteLine("  {0}", b.Title);
    }

    // Execution starts here.
    static void Main()
    {
      BookDB bookDB = new BookDB();

      // Initialize the database with some books:
      AddBooks(bookDB);

      // Print all the titles of paperbacks:
      System.Console.WriteLine("Paperback Book Titles:");

      // Create a new delegate object associated with the static 
      // method Test.PrintTitle:
      bookDB.ProcessPaperbackBooks(PrintTitle);

      // Get the average price of a paperback by using
      // a PriceTotaller object:
      PriceTotaller totaller = new PriceTotaller();

      // Create a new delegate object associated with the nonstatic 
      // method AddBookToTotal on the object totaller:
      bookDB.ProcessPaperbackBooks(totaller.AddBookToTotal);

      System.Console.WriteLine("Average Paperback Book Price: ${0:#.##}",
          totaller.AveragePrice());
    }

    // Initialize the book database with some test books:
    static void AddBooks(BookDB bookDB)
    {
      bookDB.AddBook("The C Programming Language", "Brian W. Kernighan and Dennis M. Ritchie", 19.95m, true);
      bookDB.AddBook("The Unicode Standard 2.0", "The Unicode Consortium", 39.95m, true);
      bookDB.AddBook("The MS-DOS Encyclopedia", "Ray Duncan", 129.95m, false);
      bookDB.AddBook("Dogbert's Clues for the Clueless", "Scott Adams", 12.00m, true);
    }
  }
}

								 
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