委托的声明如下所示：
public delegate void Del<T>(T item);
public void Notify(int i) { }
Del<int> d1 = new Del<int>(Notify);
在 C# 2.0 中，还可以使用下面的简化语法来声明委托：
Del<int> d2 = Notify;
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。所使用的 delegate 类型称为 ProcessBookDelegate。Test 类使用该类输出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进行什么处理。
示例
// 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 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);
}
}
}
输出
Paperback Book Titles:
The C Programming Language
The Unicode Standard 2.0
Dogbert's Clues for the Clueless
Average Paperback Book Price: $23.97
可靠编程
-
声明委托。
下列语句：
public delegate void ProcessBookDelegate(Book book);
声明一个新的委托类型。每个委托类型都描述参数的数目和类型，以及它可以封装的方法的返回值类型。每当需要一组新的参数类型或新的返回值类型时，都必须声明一个新的委托类型。
-
实例化委托。
声明了委托类型后，必须创建委托对象并使之与特定方法关联。在上面的示例中，这是通过将 PrintTitle 方法传递给 ProcessPaperbackBooks 方法来完成的，如下所示：
bookDB.ProcessPaperbackBooks(PrintTitle);
这将创建与静态方法 Test.PrintTitle 关联的新委托对象。类似地，对象 totaller 的非静态方法 AddBookToTotal 是按如下方式传递的：
bookDB.ProcessPaperbackBooks(totaller.AddBookToTotal);
在两个示例中，都向 ProcessPaperbackBooks 方法传递了一个新的委托对象。
委托一旦创建，它的关联方法就不能更改；委托对象是不可变的。
-
调用委托。
创建委托对象后，通常将委托对象传递给将调用该委托的其他代码。通过委托对象的名称（后面跟着要传递给委托的参数，括在括号内）调用委托对象。下面是委托调用的示例：
processBook(b);
与本例一样，可以通过使用 BeginInvoke 和 EndInvoke 方法同步或异步调用委托。