Delegates

// 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);
}
}
}