1. C# Event Processing Model1

2. C# Event Processing Macro View
Generally speaking, two logical components are required to implement the event processing model:
1) An event producer (or publisher)
2) An event consumer (or subscriber)
Each logical component has assigned responsibilities
Consider the following diagram

3. C# Event Processing Macro View
When an Event occurs notification is sent to all the subscribers on the list for that particular event…
Object B processes the
event notification in its
event handler code
Object A
(Event Publisher)
Object B
(Event Subscriber)
Object B subscribes to event
(or events) generated by Object A.
Subscriber List
(Object B)
Event Handler Code
Object A maintains a list of subscribers for each publishable event

4. C# Event Processing Macro View
This diagram hides a lot of details
How is the subscriber list maintained?
How is the event generated?
How is the notification sent to each subscriber?
What is an event – really?
How can you add custom event processing to your applications?

5. Required Components
To implement custom event processing in your programs you need to understand how to create the following component types:
Delegates
Event Generating Objects (publishers)
Events
Event Notification Methods
Event Handling Objects (subscribers)
Event Handler Methods

6. Required Components
You will also need to know how to pass information related to the event between the event generating object and the subscriber object
The EventArgs class can be used as-is or subclassed
The EventArgs class captures generic information about an object and the event that occurred

7. Role Of Each Component Delegate Example
Delegate types represent references to methods with a particular parameter list and return type
Example
void EventHandler(Object sender, EventArgs e)
Represents a method that has two parameters, the first one being of type Object and the second being of type EventArgs. Its return type is void
Any method, so long as its signature matches that expected by the delegate, can be handled by the delegate

8. Role Of Each Component Delegate A delegate is a reference type object.
A delegate extends either the System.Delegate or MulticastDelegate class
Depends on whether one (Delegate) or more (MulticaseDelegate) subscribers are involved
You do not extend Delegate or MulticastDelegate
The C# compiler does it for you

9. Role Of Each Component Delegate
The delegate object contains the subscriber list
It is actually implemented as a linked list where each node of the list contains a pointer to a subscriber’s event handler method
Delegates are types – like classes
Except – you declare them with the delegate keyword and specify the types of methods they can reference

10. Role Of Each Component Publisher
A publisher is any class that can fire an event and send event notifications to interested subscribers
A publisher class contains the following critical elements:
An event field
This is what subscribers subscribe to…
An event notification method
This activates the subscriber notification process when the event occurs
And some means of generating the event or recognizing the event in question has occurred
This usually happens in a method as well

11. Role Of Each Component Event An event is a field in a class
Events are declared with the event keyword
Events must be a delegate type
Delegates, remember, are objects that contain a list of pointers to subscriber methods that delegate can process
An event field will be null until the first subscriber subscribes to that event

12. Role Of Each Component Event Notification Method
In addition to an event field a publisher will have a method whose job it is to start the subscriber notification process when the event in question occurs
An event notification method is just a normal method
It usually has a parameter of EventArgs or a user-defined subtype of EventArgs.
But it can have any number and type of parameters you require

13. Role Of Each Component Subscriber
A subscriber is a class that registers its interest in a publisher’s events
A subscriber class contains one or more event handler methods

14. Role Of Each Component Event Handler Method
An event handler methods is an ordinary method that is registered with a publisher’s event
The event handler method’s signature must match the signature required by the publisher’s event delegate

15. ?

16. References