windows C#-声明、实例化和使用委托
可以使用以下任一方法声明委托：
1. 使用匹配签名声明委托类型并声明方法：
// Declare a delegate.
delegate void NotifyCallback(string str);
// Declare a method with the same signature as the delegate.
static void Notify(string name)
{
Console.WriteLine($"Notification received for: {name}");
}
// Create an instance of the delegate.
NotifyCallback del1 = new NotifyCallback(Notify);2. 将方法组分配给委托类型：
// C# 2.0 provides a simpler way to declare an instance of NotifyCallback.
NotifyCallback del2 = Notify;3. 声明匿名方法：
// Instantiate NotifyCallback by using an anonymous method.
NotifyCallback del3 = delegate(string name)
{ Console.WriteLine($"Notification received for: {name}"); };4. 使用 lambda 表达式：
// Instantiate NotifyCallback by using a lambda expression.
NotifyCallback del4 = name => { Console.WriteLine($"Notification received for: {name}"); };下面的示例演示如何声明、实例化和使用委托。 BookDB 类封装用来维护书籍数据库的书店数据库。 它公开一个方法 ProcessPaperbackBooks，用于在数据库中查找所有平装书并为每本书调用委托。 使用的 delegate 类型名为 ProcessBookCallback。 Test 类使用此类打印平装书的书名和平均价格。
使用委托提升书店数据库和客户端代码之间的良好分隔功能。 客户端代码程序不知道如何存储书籍或书店代码如何查找平装书。 书店代码不知道它在找到平装书之后对其执行什么处理。
示例
// A set of classes for handling a bookstore:
namespace Bookstore
{
using System.Collections;
// Describes a book in the book list:
public struct Book
{
public string Title; // Title of the book.
public string Author; // Author of the book.
public decimal Price; // Price of the book.
public bool Paperback; // Is it paperback?
public Book(string title, string author, decimal price, bool paperBack)
{
Title = title;
Author = author;
Price = price;
Paperback = paperBack;
}
}
// Declare a delegate type for processing a book:
public delegate void ProcessBookCallback(Book book);
// Maintains a book database.
public class BookDB
{
// List of all books in the database:
ArrayList list = new ArrayList();
// Add a book to the database:
public void AddBook(string title, string author, decimal price, bool paperBack)
{
list.Add(new Book(title, author, price, paperBack));
}
// Call a passed-in delegate on each paperback book to process it:
public void ProcessPaperbackBooks(ProcessBookCallback processBook)
{
foreach (Book b in list)
{
if (b.Paperback)
// Calling the delegate:
processBook(b);
}
}
}
}
// Using the Bookstore classes:
namespace BookTestClient
{
using Bookstore;
// Class to total and average prices of books:
class PriceTotaller
{
int countBooks = 0;
decimal priceBooks = 0.0m;
internal void AddBookToTotal(Book book)
{
countBooks += 1;
priceBooks += book.Price;
}
internal decimal AveragePrice()
{
return priceBooks / countBooks;
}
}
// Class to test the book database:
class Test
{
// Print the title of the book.
static void PrintTitle(Book b)
{
Console.WriteLine($" {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:
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);
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
*/可靠编程
1. 声明委托。以下语句声明新的委托类型。
public delegate void ProcessBookCallback(Book book);每个委托类型描述自变量的数量和类型，以及它可以封装的方法的返回值类型。 每当需要一组新的自变量类型或返回值类型，则必须声明一个新的委托类型。
2.实例化委托。
声明委托类型后，则必须创建委托对象并将其与特定的方法相关联。 在上例中，你通过将 PrintTitle 方法传递给 ProcessPaperbackBooks 方法执行此操作，如下面的示例所示：
bookDB.ProcessPaperbackBooks(PrintTitle);这将创建一个新的与静态方法 Test.PrintTitle 关联的委托对象。 同样，如下面的示例所示，传递对象 totaller 中的非静态方法 AddBookToTotal：
bookDB.ProcessPaperbackBooks(totaller.AddBookToTotal);在这两种情况下，都将新的委托对象传递给 ProcessPaperbackBooks 方法。
创建委托后，它与之关联的方法就永远不会更改；委托对象是不可变的。
3.调用委托。
创建委托对象后，通常会将委托对象传递给将调用该委托的其他代码。 委托对象是通过使用委托对象的名称调用的，后跟用圆括号括起来的将传递给委托的自变量。 下面是一个委托调用示例：
processBook(b);委托可以同步调用(如在本例中)或通过使用 BeginInvoke 和 EndInvoke 方法异步调用。