合并委托
本示例演示如何創建多播委托。 委托對象的一個有用屬性是：可以使用 + 運算符將多個對象分配給一個委托實例。多播委托包含已分配委托的列表。在調用多播委托時，它會按順序調用列表中的委托。只能合并相同類型的委托。
- 運算符可用于從多播委托中移除組件委托。
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using System;// Define a custom delegate that has a string parameter and returns void.delegate void CustomDel(string s);class TestClass{ // Define two methods that have the same signature as CustomDel. static void Hello(string s) { System.Console.WriteLine(" Hello, {0}!", s); } static void Goodbye(string s) { System.Console.WriteLine(" Goodbye, {0}!", s); } static void Main() { // Declare instances of the custom delegate. CustomDel hiDel, byeDel, multiDel, multiMinusHiDel; // In this example, you can omit the custom delegate if you // want to and use Action<string> instead. //Action<string> hiDel, byeDel, multiDel, multiMinusHiDel; // Create the delegate object hiDel that references the // method Hello. hiDel = Hello; // Create the delegate object byeDel that references the // method Goodbye. byeDel = Goodbye; // The two delegates, hiDel and byeDel, are combined to // form multiDel. multiDel = hiDel + byeDel; // Remove hiDel from the multicast delegate, leaving byeDel, // which calls only the method Goodbye. multiMinusHiDel = multiDel - hiDel; Console.WriteLine("Invoking delegate hiDel:"); hiDel("A"); Console.WriteLine("Invoking delegate byeDel:"); byeDel("B"); Console.WriteLine("Invoking delegate multiDel:"); multiDel("C"); Console.WriteLine("Invoking delegate multiMinusHiDel:"); multiMinusHiDel("D"); }} |
輸出：
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Invoking delegate hiDel: Hello, A!Invoking delegate byeDel: Goodbye, B!Invoking delegate multiDel: Hello, C! Goodbye, C!Invoking delegate multiMinusHiDel: Goodbye, D! |
聲明、實例化和使用委托
在 C# 1.0 及更高版本中，可以按以下示例所示聲明委托。
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// 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 提供了更簡單的方法來編寫上面的聲明，如以下示例所示。
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// 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，還可以使用匿名方法來聲明和初始化委托，如以下示例所示。
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// 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，還可以使用 Lambda 表達式來聲明和實例化委托，如以下示例所示。
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// Instantiate Del by using a lambda expression.Del del4 = name => { Console.WriteLine("Notification received for: {0}", name); }; |
下面的示例闡釋聲明、實例化和使用委托。 BookDB 類封裝一個書店數據庫，它維護一個書籍數據庫。它公開 ProcessPaperbackBooks 方法，該方法在數據庫中查找所有平裝書，并對每本平裝書調用一個委托。使用的 delegate 類型名為 ProcessBookDelegate。 Test 類使用該類打印平裝書的書名和平均價格。
委托的使用促進了書店數據庫和客戶代碼之間功能的良好分隔。客戶代碼不知道書籍的存儲方式和書店代碼查找平裝書的方式。書店代碼也不知道找到平裝書后將對平裝書執行什么處理。
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// 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); } }} |
輸出：
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Paperback Book Titles: The C Programming Language The Unicode Standard 2.0 Dogbert's Clues for the CluelessAverage Paperback Book Price: $23.97 |
可靠編程
聲明委托。
下面的語句聲明一個新的委托類型。
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public delegate void ProcessBookDelegate(Book book); |
每個委托類型都描述參數的數目和類型，以及它可以封裝的方法的返回值類型。每當需要一組新的參數類型或新的返回值類型時，都必須聲明一個新的委托類型。
實例化委托。
聲明了委托類型后，必須創建委托對象并使之與特定方法關聯。在上一個示例中，您通過按下面示例中的方式將 PrintTitle 方法傳遞到 ProcessPaperbackBooks 方法來實現這一點：
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bookDB.ProcessPaperbackBooks(PrintTitle); |
這將創建與靜態方法 Test.PrintTitle 關聯的新委托對象。類似地，對象 totaller 的非靜態方法 AddBookToTotal 是按下面示例中的方式傳遞的：
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bookDB.ProcessPaperbackBooks(totaller.AddBookToTotal); |
在兩個示例中，都向 ProcessPaperbackBooks 方法傳遞了一個新的委托對象。
委托創建后，它的關聯方法就不能更改；委托對象是不可變的。
調用委托。
創建委托對象后，通常將委托對象傳遞給將調用該委托的其他代碼。通過委托對象的名稱（后面跟著要傳遞給委托的參數，括在括號內）調用委托對象。下面是委托調用的示例：
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processBook(b); |
與本例一樣，可以通過使用 BeginInvoke 和 EndInvoke 方法同步或異步調用委托。
