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Of the nine class member types, two of them are data member types. The first, which has already been discussed, is fields. The second is constants. You can think of a constant as a type of variable for which the value cannot be changed. A constant must be initialized at its declaration. A constant cannot be changed after its declaration. There are two kinds of constants: Member constants, which are like fields except constant Local constants, which are like local variables except constant The core declaration for a constant is shown following. Note that the syntax is the same as that of a field or variable declaration, except for the following: The addition of the keyword const in front The mandatory initializer

Once again, this is such a common thing to need that the framework provides us with a generic type, EventHandler<T>, to save us the boilerplate code. So we can replace the ProcessEventHandler with an EventHandler<ProcessEventArgs>. Let s update our event declarations (see Example 5-25).

public event EventHandler<ProcessEventArgs> Processing; public event EventHandler<ProcessEventArgs> Processed;

and then our helper methods which raise the event that will need to take a ProcessE ventArgs (see Example 5-26).

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

Note The keyword const is not a modifier, but part of the core declaration. It must be placed immediately

And finally, our calls to those methods will need to create an appropriate ProcessEven tArgs object, as shown in Example 5-27.

public void Process(Document doc) { ProcessEventArgs e = new ProcessEventArgs(doc); OnProcessing(e);

The object assigned to the team variable has three properties: name, members, and count. Note that '' represents the empty string, and [] is an empty array. Even the value of the count property is a literal a function literal:

// 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(e); return; } } // Then perform the action foreach (ActionCheckPair process in processes) { process.Action(doc); if (LogTextProvider != null) { Console.WriteLine(LogTextProvider(doc)); } } OnProcessed(e);

}

Note Unlike C and C++, in C# there are no global constants. Every constant must be declared within

Notice how we happen to reuse the same event data for each event we raise. That s safe to do because our event argument instance cannot be modified its only property has a private setter. If it were possible for event handlers to change the event argument object, it would be risky to use the same one for both events. We could offer our colleagues on the production team another facility using these events. We already saw how they need to perform a quick check before each individual process to determine whether they should abort processing. We can take advantage of our Processing event to give them the option of canceling the whole process before it even gets off the ground. The framework defines a class called CancelEventArgs which adds a Boolean property called Cancel to the basic EventArgs. Subscribers can set the property to True, and the publisher is expected to abort the operation. Let s add a new EventArgs class for that (see Example 5-28).

class ProcessCancelEventArgs : CancelEventArgs { public ProcessCancelEventArgs(Document document) { Document = document; } public Document Document

{ }

get; private set;

Constant members, like fields, are declared in the class declaration, as follows: class MyClass { const int IntVal = 100; } Type Initializer const double PI = 3.1416;

}

We ll update the declaration of our Processing event, and its corresponding helper, as shown in Example 5-29 (but we ll leave the Processed event as it is if the document has already been processed, it s too late to cancel it).

public event EventHandler<ProcessCancelEventArgs> Processing; private void OnProcessing(ProcessCancelEventArgs e) { if (Processing != null) { Processing(this, e); } }

function() { return members.length }

Finally, we need to update the Process method to create the right kind of event argument object, and to honor requests for cancellation (see Example 5-30).

The value used to initialize a constant must be computable at compile time. class MyClass { const int IntVal1 = 100; const int IntVal2 = 2 * IntVal1; }

public void Process(Document doc) { ProcessEventArgs e = new ProcessEventArgs(doc); ProcessCancelEventArgs ce = new ProcessCancelEventArgs(doc); OnProcessing(ce); if (ce.Cancel) { Console.WriteLine("Process canceled."); if (LogTextProvider != null) { Console.WriteLine(LogTextProvider(doc)); } return; } // ... }

Now we ll make use of this in one of our production tools, as shown in Example 5-31.

class ProductionDeptTool1 { public void Subscribe(DocumentProcessor processor) { processor.Processing += processor_Processing; processor.Processed += processor_Processed; }

public void Unsubscribe(DocumentProcessor processor) { processor.Processing -= processor_Processing; processor.Processed -= processor_Processed; } void processor_Processing(object sender, ProcessCancelEventArgs e) { Console.WriteLine("Tool1 has seen processing, and not canceled."); } void processor_Processed(object sender, EventArgs e) { Console.WriteLine("Tool1 has seen that processing is complete."); }

You cannot assign to a constant after its declaration. class MyClass { const int IntVal; IntVal = 100; }

}

class ProductionDeptTool2 { public void Subscribe(DocumentProcessor processor) { processor.Processing += (sender, e) => { Console.WriteLine("Tool2 has seen processing and canceled it"); if(e.Document.Text.Contains("document")) { e.Cancel = true; } }; processor.Processed += (sender, e) => Console.WriteLine("Tool2 has seen that processing is complete."); } }

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