C# 中的委托類似於 C 或 C++ 中的函數指針。使用委托使程序員可以將方法引用封裝在委托對象內。然後可以將該委托對象傳遞給可調用所引用方法的代碼，而不必在編譯時知道將調用哪個方法。與 C 或 C++ 中的函數指針不同，委托是面向對象、類型安全的，並且是安全的。
委托聲明定義一種類型，它用一組特定的參數以及返回類型封裝方法。對於靜態方法，委托對象封裝要調用的方法。對於實例方法，委托對象同時封裝一個實例和該實例上的一個方法。如果您有一個委托對象和一組適當的參數，則可以用這些參數調用該委托。
委托的一個有趣且有用的屬性是，它不知道或不關心自己引用的對象的類。任何對象都可以；只是方法的參數類型和返回類型必須與委托的參數類型和返回類型相匹配。這使得委托完全適合“匿名”調用。
此教程包括兩個示例：
示例 1 展示如何聲明、實例化和調用委托。
示例 2 展示如何組合兩個委托。
此外，還討論以下主題：
委托和事件
委托與接口
示例 1
下面的示例闡釋聲明、實例化和使用委托。BookDB 類封裝一個書店數據庫，它維護一個書籍數據庫。它公開 ProcessPaperbackBooks 方法，該方法在數據庫中查找所有平裝書，並為每本書調用一個委托。所使用的 delegate 類型稱為 ProcessBookDelegate。Test 類使用該類輸出平裝書的書名和平均價格。
委托的使用促進了書店數據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後將對平裝書進行什麼處理。
// bookstore.cs
using System;
// 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 Test
{
// Print the title of the book.
static void PrintTitle(Book b)
{
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:
Console.WriteLine("Paperback Book Titles:");
// Create a new delegate object associated with the static
// method Test.PrintTitle:
bookDB.ProcessPaperbackBooks(new ProcessBookDelegate(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(new ProcessBookDelegate(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);
