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# 如何:声明、实例化和使用委托(C# 编程指南) 在 C# 1.0 及更高版本中,可以按以下示例所示声明委托。 ``` // Declare a delegate. delegate void Del(string str); // Declare a method with the same signature as the delegate. static void Notify(string name) { Console.WriteLine("Notification received for: {0}", name); } ``` ``` // Create an instance of the delegate. Del del1 = new Del(Notify); ``` C# 2.0 提供了更简单的方法来编写上面的声明,如以下示例所示。 ``` // C# 2.0 provides a simpler way to declare an instance of Del. Del del2 = Notify; ``` 在 C# 2.0 及更高版本中,还可以使用匿名方法来声明和初始化[委托](https://msdn.microsoft.com/zh-cn/library/900fyy8e.aspx),如以下示例所示。 ``` // Instantiate Del by using an anonymous method. Del del3 = delegate(string name) { Console.WriteLine("Notification received for: {0}", name); }; ``` 在 C# 3.0 及更高版本中,还可以使用 Lambda 表达式来声明和实例化委托,如以下示例所示。 ``` // Instantiate Del by using a lambda expression. Del del4 = name => { Console.WriteLine("Notification received for: {0}", name); }; ``` 有关更多信息,请参见 [Lambda 表达式(C# 编程指南)](https://msdn.microsoft.com/zh-cn/library/bb397687.aspx)。 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。使用的 **delegate** 类型名为 ProcessBookDelegate。 Test 类使用该类打印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执行什么处理。 ``` // 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); } } } /* Output: 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。 下面的语句声明一个新的委托类型。 ``` public delegate void ProcessBookDelegate(Book book); ``` 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。在上一个示例中,您通过按下面示例中的方式将 PrintTitle 方法传递到 ProcessPaperbackBooks 方法来实现这一点: ``` bookDB.ProcessPaperbackBooks(PrintTitle); ``` 这将创建与[静态](https://msdn.microsoft.com/zh-cn/library/98f28cdx.aspx)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 是按下面示例中的方式传递的: ``` 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。 委托创建后,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。 ## 请参阅 [C# 编程指南](https://msdn.microsoft.com/zh-cn/library/67ef8sbd.aspx) [事件(C# 编程指南)](https://msdn.microsoft.com/zh-cn/library/awbftdfh.aspx) [委托(C# 编程指南)](https://msdn.microsoft.com/zh-cn/library/ms173171.aspx)