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OK, our production team is very happy with all of that, but they have another requirement. Apparently, they have one team working on some diagnostic components that

are going to track the time taken to execute some of their processes, and another team developing some real-time display of all the processes as they run through the system. They want to know when a process is about to be executed and when it has completed so that these teams can execute some of their own code. Our first thought might be to implement a couple of additional callbacks: one called as processing starts, and the other as it ends; but that won t quite meet their needs they have two separate teams who both want, independently, to hook into it. We need a pattern for notifying several clients that something has occurred. The .NET Framework steps up with events.

An event is raised (or sent) by a publisher (or sender) when something of interest occurs (such as an action taking place, or a property changing). Clients can subscribe to the event by providing a suitable delegate, rather like the callbacks we used previously. The method wrapped by the delegate is called the event handler. The neat thing is that more than one client can subscribe to the event. Here s an example of a couple of events that we can add to the DocumentProcessor to help our production team:

Figure 6-4. Static field with no class instances The code in Figure 6-4 produces the following output:

class DocumentProcessor { public event EventHandler Processing; public event EventHandler Processed; } // ...

Notice that we use the keyword event to indicate that what follows is an event declaration. We then specify the delegate type for the event (EventHandler) and the name of the event (using PascalCasing). So, this is just like a declaration for a public field of type EventHandler, but annotated with the event keyword. What does this EventHandler delegate look like The framework defines it like this:

But the preferred approach is to use an array literal, which is a comma-separated list of values enclosed in square braces:

delegate void EventHandler(object sender, EventArgs e);

Notice that our delegate takes two parameters. The first is a reference to the publisher of the event so that subscribers can tell who raised it. The second is some data associated with the event. The EventArgs class is defined in the framework, and is a placeholder for events that don t need any extra information. We ll see how to customize this later.

Besides static fields, there are also static function members. Static function members, like static fields, are independent of any class instance. Even if there are no instances of a class, you can call a static method. Static function members cannot access instance members. They can, however, access other static members. For example, the following class contains a static field and a static method. Notice that the body of the static method accesses the static field. class X { static public int A; // Static field static public void PrintValA() // Static method { Console.WriteLine("Value of A: {0}", A); } } Accessing the static field The following code uses class X, defined in the preceding code. Figure 6-5 illustrates the code. class Program { static void Main() { X.A = 10; X.PrintValA(); } } Class name

Almost all events follow this two-argument pattern. Technically, they re not required to you can use any delegate type for an event. But in practice, this pattern is almost universal.

So, how do we raise an event Well, it really is just like a delegate, so we can use the delegate calling syntax as shown in the OnProcessing and OnProcessed methods in Example 5-21.

public void Process(Document doc) { OnProcessing(EventArgs.Empty); // First time, do the quick check foreach (ActionCheckPair process in processes) { if (process.QuickCheck != null && !process.QuickCheck(doc)) { Console.WriteLine("The process will not succeed."); if (LogTextProvider != null) { Console.WriteLine(LogTextProvider(doc)); } OnProcessed(EventArgs.Empty); return; } } // Then perform the action foreach (ActionCheckPair process in processes) { process.Action(doc); if (LogTextProvider != null) { Console.WriteLine(LogTextProvider(doc)); } } OnProcessed(EventArgs.Empty);

}

This code produces the following output:

private void OnProcessing(EventArgs e) { if (Processing != null) { Processing(this, e); } } private void OnProcessed(EventArgs e) { if (Processed != null) { Processed(this, e); } }

Notice how we pulled out the code to check for null and execute the delegate into functions called OnXXX. This isn t strictly necessary, but it is a very common practice.

var somePrimes = [ 1, 2, 3, 5, 7 ];

If we are designing our class as a base, we often mark this kind of method as a protected virtual so that derived classes can override the eventraising function instead of subscribing to the event. This can be more efficient than going through the event, and it allows us (optionally) to decline to raise the event by not calling on the base implementation. Be careful to document whether derived classes are allowed not to call the base, though!

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