Csharp

From ZENWorks Wiki

C# Delegate Example 1

Delegates vs. Interfaces

Delegates and interfaces are similar in that they enable the separation of specification and implementation. Multiple independent authors can produce implementations that are compatible with an interface specification. Similarly, a delegate specifies the signature of a method, and authors can write methods that are compatible with the delegate specification. When should you use interfaces, and when should you use delegates?

Delegates are useful when:

   * A single method is being called.
   * A class may want to have multiple implementations of the method specification.
   * It is desirable to allow using a static method to implement the specification.
   * An event-like design pattern is desired (for more information, see the Events Tutorial).
   * The caller has no need to know or obtain the object that the method is defined on.
   * The provider of the implementation wants to "hand out" the implementation of the specification to only a few select components.
   * Easy composition is desired.

Interfaces are useful when:

   * The specification defines a set of related methods that will be called.
   * A class typically implements the specification only once.
   * The caller of the interface wants to cast to or from the interface type to obtain other interfaces or classes.

• http://msdn2.microsoft.com/en-us/library/aa288459(VS.71).aspx

// 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);
     }
  }

}