c# create data matrix EVENTS in Visual C#.NET

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CHAPTER 16 EVENTS
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public class MyTimerClass { ... } class ClassA { public void TimerHandlerA(object obj, EventArgs e) { Console.WriteLine("Class A handler called"); } }
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// Event handler
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class ClassB { public static void TimerHandlerB(object obj, EventArgs e) { Console.WriteLine("Class B handler called"); } } class Program { static void Main( ) { ClassA ca = new ClassA(); MyTimerClass mc = new MyTimerClass(); mc.Elapsed += ca.TimerHandlerA; mc.Elapsed += ClassB.TimerHandlerB; Thread.Sleep(2000); } } This code produces the following output: Class Class Class Class A B A B handler handler handler handler called called called called
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// Static
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// Create the class object. // Create the timer object. // Add handler A -- instance. // Add handler B -- static.
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CHAPTER 16 EVENTS
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Removing Event Handlers
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You can remove an event handler from an event by using the -= operator, as shown here. mc.Elapsed -= ca.TimerHandlerA; // Remove handler A.
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For example, in the following code, you remove the event handler for ClassB after the first two times the event is raised, and then let the program run for another 2 seconds. ... mc.Elapsed += ca.TimerHandlerA; mc.Elapsed += ClassB.TimerHandlerB; Thread.Sleep(2250); mc.Elapsed -= ClassB.TimerHandlerB; Thread.Sleep(2250);
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// Add instance handler A. // Add static handler B. // Sleep more than 2 seconds. // Remove static handler B. // Sleep more than 2 seconds.
This code produces the following output. The first four lines are the result of both handlers being called twice, in the first two seconds. After you removed the handler for ClassB, only the handler for the instance of ClassA was called, during the last 2 seconds. Class Class Class Class Class Class A B A B A A handler handler handler handler handler handler called called called called called called
The event handler for ClassB has been removed.
CHAPTER 16 EVENTS
Adding Anonymous Method Event Handlers
You can also add anonymous method event handlers, just as with delegates. For example, the following code adds an anonymous method to the event and then sleeps for a little more than 2 seconds. The event is raised twice in that time. Anonymous method mc.Elapsed += delegate(object obj, EventArgs e) { Console.WriteLine("This is the anonymous method."); }; Semicolon required Thread.Sleep(2250); } This code produces the following output: This is the anonymous method. This is the anonymous method.
CHAPTER 16 EVENTS
Standard Event Usage
GUI programming is event driven, which means that while the program is running, it can be interrupted at any time by events such as button clicks, key presses, or system timers. When this happens, the program needs to handle the event and then continue on its course. Clearly, this asynchronous handling of program events is the perfect situation to use C# events. Windows GUI programming uses events so extensively that there is a standard .NET Framework pattern for using them, which you are strongly encouraged to follow. The foundation of the standard pattern for event usage is the EventHandler delegate type, which is declared in the System namespace. The declaration of the EventHandler delegate type is shown in the following code. The first parameter is meant to hold a reference to the object that raised the event. It is of type object and can, therefore, match any instance of any type. The second parameter is meant to hold state information of whatever type is appropriate for the application. The return type is void. public delegate void EventHandler(object sender, EventArgs e);
Using the EventArgs Class
The second parameter in the EventHandler delegate type is an object of class EventArgs, which is declared in the System namespace. You might be tempted to think that, since the second parameter is meant for passing data, an EventArgs class object would be able to store data of some sort. You would be wrong. The EventArgs class is designed to carry no data. It is used for event handlers that do not need to pass data and is generally ignored by them. If you want to pass data, you must declare a class derived from EventArgs, with the appropriate fields to hold the data you want to pass. Even though the EventArgs class does not actually pass data, it is an important part of the pattern of using the EventHandler delegate. Class object and class EventArgs are the base classes of whatever actual types are used as the parameters. This allows EventHandler to provide a signature that is the lowest common denominator for all events and event handlers, allowing them to have exactly two parameters, rather than having different signatures for each case.
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