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      举例讲解C#编程中委托的实例化使用
      发布时间:2016-12-21 07:57:56 关键词: C,委托,实例化
      内容简介:这篇文章主要介绍了C#编程中委托的实例化使用,包括委托的声明和多播委托的创建等内容,需要的朋友可以参考下

    合并委托
    本示例演示如何创建多播委托。 委托对象的一个有用属性是:可以使用 + 运算符将多个对象分配给一个委托实例。多播委托包含已分配委托的列表。在调用多播委托时,它会按顺序调用列表中的委托。只能合并相同类型的委托。
    - 运算符可用于从多播委托中移除组件委托。

    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");
      }
    }
    
    

    输出:

    Invoking delegate hiDel:
     Hello, A!
    Invoking delegate byeDel:
     Goodbye, B!
    Invoking delegate multiDel:
     Hello, C!
     Goodbye, C!
    Invoking delegate multiMinusHiDel:
     Goodbye, D!
    


    声明、实例化和使用委托
    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,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); };
    

    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);
        }
      }
    }
    
    

    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。

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