Stuff Happens C# Events in C#

Drawing ANSI/AIM Code 39 in C# Stuff Happens C# Events

Making the method protected and virtual allows subclasses to override it That s optional After you have that method, which always takes the same form (making it easy to write quickly), you call the method when you need to raise the event:

void SomeMethod() { // Do stuff here and then: NewEditionEventArgs e = new NewEditionEventArgs(DateTimeToday, Peace Breaks Out! ); OnNewEdition(e); }

The subscribing object specifies the name of a handler method when it subscribes it s the argument to the constructor (boldfaced):

button1Click += new EventHandler<EventArgs>(button1_Click);

This line sort of says, Send my paper to this address, please Here s a handler for the NewEdition event:

myPublisherNewEdition += new EventHandler<NewEditionEventArgs>(NewEdHandler); void NewEdHandler(object sender, NewEditionEventArgs e) { // Do something in response to the event }

For example, a BankAccount class can raise a custom TransactionAlert event when anything occurs in a BankAccount object, such as a deposit, withdrawal, or transfer or even an error A Logger observer object can subscribe and log these events to a file or a database as an audit trail

Events: Use events when you may have multiple subscribers or when communicating with client software that uses your classes Delegates: Use delegates or anonymous methods when you need a callback or need to customize an operation Lambdas: There is an article about lambdas on this book s companion Web site A lambda expression is, in essence, just a short way to specify the method you re passing to a delegate You can use lambdas instead of anonymous methods

When you create a button handler in Visual Studio (by double-clicking the button on your form), Visual Studio generates the subscription code in the Form1Designercs file You shouldn t edit the subscription, but you can delete it and replace it with the same code written in your half of the partial form class Thereafter, the form designer knows nothing about it In your subscriber s handler method, you do whatever is supposed to happen when your class learns of this kind of event To help you write that code, you can cast the sender parameter to the type you know it to be:

Button theButton = (Button)sender;

and then call methods and properties of that object as needed Because you have a reference to the sending object, you can ask the subscriber questions and carry out operations on it if you need to like the person who delivers your newspaper knocking on your door to collect the monthly subscription fees And, you can extract information from the e parameter by getting at its properties in the same way:

ConsoleWriteLine(eHatSize);

Dealing with separately compiled assemblies Writing a class library More access-control keywords: protected, internal, protected

You can use a method to divide a long string of code into separate, maintainable units Use the class structure to group both data and methods in meaningful ways to further reduce the complexity of the program Programs are complex already, and we simple humans become confused easily, so we need all the help we can get C# provides another level of grouping: You can group similar classes into a separate library Beyond writing your own libraries, you can use anybody s libraries in your programs These programs contain multiple modules known as assemblies I describe libraries and assemblies in this chapter Meanwhile, the access-control story in 5 of this minibook leaves a few untidy loose ends the protected, internal, and protected internal keywords and is slightly complicated further by the use of namespaces, another way to group similar classes and allow the use of duplicate names in two parts of a program I cover namespaces in this chapter as well The program examples mentioned in this chapter are part of the chapter download You can also download them separately on the Example Code page of the Web site at csharp102info or csharpfordummiesnet