C#学习：委托

• 委托声明和实例化

     （1）通过与委托签名相同的方法的方法名声明实例化委托

        //声明委托
        public delegate void Del(string message);

        //与委托签名相同的方法
        //签名相同即参数列表相同，返回值类型相同
        public static void DelegateMethod(string message)
        {
            Console.WriteLine(message);
        }

        //通过与委托签名相同的方法实例化委托
        Del handler = DelegateMethod;

        //调用委托
        handler("hello world");

     （2）通过匿名方法声明实例化委托

            public delegate void Del(string message);
            Del del=delegate(string name){
                Console.WriteLine("Hello:" + name);
            };
            del("delegate");

     （3）通过 Lambda 表达式声明和实例化委托

            public delegate void Del(string message);
            Del del = name =>{Console.WriteLine("Hello:{0}",
                    name);};
            del("delegate");

•  委托使用

    （1）调用委托。调用方传递给委托的参数被传递给方法，方法如果有返回值由委托返回给调用方。

    class Program
    {
        public delegate int Add(int a, int b);

        public static int AddMethod(int a, int b)
        {
            int c = a + b;
            Console.WriteLine("{0}+{1}={2}", a, b, c);
            return c;
        }
        static void Main(string[] args)
        {
            Add add = AddMethod;
            int c = AddMethod(1, 1);
            Console.WriteLine("result:{0}", c);

            Console.Read();
        }
    }
输出：1+1=2
     result：2

   （2）异步回调委托。委托实例作为方法的参数使用，在该方法内使用委托无须关注委托对应方法的具体实现，该功能类似接口。

    class Program
    {
        public delegate int Add(int a, int b);

        public static int AddMethod(int a, int b)
        {
            int c = a + b;
            Console.WriteLine("{0}+{1}={2}", a, b, c);
            return c;
        }
        static void Main(string[] args)
        {
            Add add = AddMethod;
            int d = AddWithCallBack(1, 1, add);
            Console.WriteLine("result:{0}", d);

            Console.Read();
        }
        public static int AddWithCallBack(int a, int b, Add callback)
        {
            int c = callback(a, b);
            return c;
        }
    }
输出：1+1=2
     result:2

   （3）多路广播委托。调用委托时，可以调用多个方法，如果向委托方法列表中添加方法，可以通过加法运算符或加法赋值运算符（“+”或“+=”）添加委托实例。也可以通过减法运算符或减法赋值运算符（“-”或“-=”）移除方法。在多路广播委托中，委托将按顺序调用方法，方法引起的更改对下一个方法是可见的，在一个方法内发生异常，将不再调用后边的方法。

    class Program
    {
        public delegate void Del(string message);

        public static void DelegateMethod(string message)
        {
            Console.WriteLine(message);
        }

        static void Main(string[] args)
        {
            MethodClass obj = new MethodClass();
            Del del1 = obj.Method1;
            Del del2 = obj.Method2;
            Del del3 = DelegateMethod;
            Del allMethodsDelegate = del1 + del2;
            allMethodsDelegate += del3;
            allMethodsDelegate("Delegate Test1");

            allMethodsDelegate -= del1;
            allMethodsDelegate("Delegate Test2");

            Console.Read();
        }
     }
    public class MethodClass
    {
        public void Method1(string message) 
        {
            Console.WriteLine("Method1:" + message);
        }
        public void Method2(string message)
        {
            Console.WriteLine("Method2:" + message);
        }
    }
输出：Method1：Delegate Test1
     Method2：Delegate Test1
     Delegate Test1
     Method2：Delegate Test2
     Delegate Test2

•  MSDN委托使用实例

// 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
*/