C# 委托

// bookstore.cs
using System;

// 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)
      {
         Title = title;
         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 Test
   {
      // Print the title of the book.
      static void PrintTitle(Book b)
      {
         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:
         Console.WriteLine("Paperback Book Titles:");
         // Create a new delegate object associated with the static 
         // method Test.PrintTitle:
         bookDB.ProcessPaperbackBooks(new ProcessBookDelegate(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(new ProcessBookDelegate(totaller.AddBookToTotal));
         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);
      }
   }
}