委托的声明如下所示：
public delegate void Del(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：
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。