举例讲解C#编程中委托的实例化使用

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。