舉例講解C#編程中委托的實例化使用

2020-01-24 01:18:00

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合并委托
本示例演示如何創(chuàng)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。

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：

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。

 // 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。

// 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。

// 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。

// 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(shù)據(jù)庫，它維護一個書籍數(shù)據(jù)庫。它公開 ProcessPaperbackBooks 方法，該方法在數(shù)據(jù)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(shù)據(jù)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(zhí)行什么處理。

// 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);    }  }}

輸出：

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。

public delegate void ProcessBookDelegate(Book book);

每個委托類型都描述參數(shù)的數(shù)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(shù)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(chuàng)建委托對象并使之與特定方法關聯(lián)。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(xiàn)這一點：

bookDB.ProcessPaperbackBooks(PrintTitle);

這將創(chuàng)建與靜態(tài)方法 Test.PrintTitle 關聯(lián)的新委托對象。類似地，對象 totaller 的非靜態(tài)方法 AddBookToTotal 是按下面示例中的方式傳遞的：

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(chuàng)建后，它的關聯(lián)方法就不能更改；委托對象是不可變的。
調用委托。
創(chuàng)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(shù)，括在括號內）調用委托對象。下面是委托調用的示例：