Question

我正在读一本关于C＃的书，它有关于复合赋值的说法（例如+ =， - =，* =，/ =，＆lt;＆lt; =，＆gt;＆gt; =）：

此规则的一个微妙的例外是事件，我们在第4章中描述：此处的+ =和 - =运算符被特殊处理并映射到事件的添加和删除访问器。

任何人都可以用简单的英语解释这意味着什么吗？我还没到第4章。

Answer 1

通常情况下，+=会将右侧的表达式/变量添加到左侧的表达式/变量，并将结果分配到左侧。

// if a = 4, after this statement, a would be 5
a += 1;

但是如果带有+=的表达式的左侧是一个事件，那么情况并非如此，但它将是右侧的事件处理程序，它被添加到列表中该事件的事件处理程序。

// whereas on the below statement someEventHandler is added to the collection of event handlers for the 'OnSomeEvent' event
self.OnSomeEvent += someEventHandler

Answer 2

它只是意味着+ =将事件方法附加到例如控制而不是添加（例如算术加法）

// += as in arithmetic addition
int i = 3;
i += 1;  // i now outputs 4

// += as in programmatically attaching an event to a control 
Button btn = new Button();
btn.Click += new EventHandler(btn_Click); 

private void btn_Click(object obj) { ... ; }

看到区别？

Answer 3

C＃中有一个委托的概念，它可以指向一个方法。您可以将事件视为特殊类型的代理。您可以add（或remove）对事件采用多种方法。它允许您在发生特定事件时执行指定的方法。

删除在Console和MessageBox上显示删除结果的文件时的一个简单示例。

// This is the delegate. Any instance with DeletedEventHandler type 
// can point a method which returns voids and accepts parameters (object,bool)
public delegate void DeletedEventHandler(object sender, bool deleted);

public class FileDeleter
{   
     private DeletedEventHandler onDelete;

     public event DeletedEventHandler OnDelete
     {
         // The add and remove accessors
         add { onDelete += value; }
         remove { onDelete -= value; }
     }

     public void Delete(string filePath)
     {
         try
         {
             File.Delete(filePath);
             RaiseOnDelete(true);
         }
         catch
         {
             RaiseOnDelete(false);
         }
     }

     private void RaiseOnDelete(bool deleted) 
     {
         if (onDelete != null)
             onDelete(this, deleted); // all methods added executes here
     }
}


public void TestMethod()
{
    FileDeleter deleter = new FileDeleter();
    // Note that both ShowDeleteInfoWindows and ShowDeleteInfoConsole are in form of void(object,bool) which is compatible with DeletedEventHandler 
    deleter.OnDelete += new DeletedEventHandler(DeletedOnConsole);
    deleter.OnDelete += new DeletedEventHandler(ShowDeleteInfoWindows);
    deleter.Delete(@"C:\test.txt");
}

private void ShowDeleteInfoConsole(object deleter, bool isDeleted)
{
    Console.WriteLine(isDeleted);
}

private void ShowDeleteInfoWindows(object deleter, bool isDeleted)
{
    MessageBox.Show(isDeleted.ToString());
}

C＃复合赋值（例如+ =， - =等）“异常”

3 个答案: