Events in C# .NET
C# and .NET supports event driven programming
via delegates. Delegates and events provide notifications to client
applications when some state changes of an object. It is an encapsulation of
idea that "Something happened". Events and Delegates are tightly coupled
concept because event handling requires delegate implementation to dispatch
events.
The class that sends or raises an event is called a Publisher
and class that receives or handle the event is called "Subscriber".
In general terms, an
event is something special that is going to happen. For example, Microsoft
launches events for developers, to make them aware about the features of new or
existing products. Microsoft notifies the developers about the event by email
or other advertisement options. So in this case, Microsoft is a publisher who
launches (raises) an event and notifies the developers about it and developers are the subscribers of the event
and attend (handle) the event.
Events in C# follow a similar concept
with the above Microsoft analogy. An event has a publisher, subscriber,
notification and a handler. Generally, UI controls use events extensively. For
example, the button control in a Windows form has multiple events such as
click, mouseover, etc. A custom class can also have an event to notify other
subscriber classes about something that has happened or is going to happen.
Characteristics of an Event
1. Event Handlers in C# return void and take two
parameters.
2. The First parameter of Event - Source of Event
means publishing object.
3. The Second parameter of Event - Object derived
from EventArgs.
4. The publishers determines when an event is
raised and the subscriber determines what action is taken in response.
5. An Event can have so many subscribers.
6. Events are basically used for the single user
action like button click.
7. If an Event has multiple subscribers then
event handlers are invoked synchronously.
Let's see how you can define an event
and notify other classes that have event handlers. An event is nothing but an
encapsulated delegate. As we have learned in the previousarticle, a delegate is a reference type data type. You can declare the
delegate as shown below:
Declaration of delegate type for Event in C#
public delegate void DelegateType();
Now we can declare an event from the above
delegate instance
public event DelegateType SomeEvent;
Thus, a delegate DelegateType becomes
an event (SomeEvent) using the event keyword.
Invoking an Event
public void
EventMethod()
{
if (SomeEvent != null)
{
SomeEvent();
}
}
Hooking up Event
//Subscribing
to an Event
_changedEvent.SomeEvent +=
MyEventChanged;
Detach from an Event
//Unsubscribe
to an Event
_changedEvent.SomeEvent -=
MyEventChanged;
Real time example of Event
namespace LegalManagementSystem.Models
{
public class CsharpnaijaAnonymous
{
public static void Main()
{
Publisher publisher = new Publisher();
Subscriber subscriber = new Subscriber(publisher);
publisher.Add(new Product
{
ProductName = "Rice",
Price = 200
});
publisher.Add(new Product
{
ProductName = "Beans",
Price = 160
});
publisher.Add(new Product
{
ProductName = "Yam",
Price = 100
});
subscriber.UnSubscribeEvent();
Console.ReadKey();
}
}
public class Product
{
public int Price {
get; set; }
public string
ProductName { get; set; }
}
public delegate void EventHandler(object sender, EventArgs e);
public class Publisher : ArrayList
{
public event
EventHandler ProdcutAddedInfo;
protected virtual void OnChanged(EventArgs e)
{
if (ProdcutAddedInfo != null) ProdcutAddedInfo(this, e);
}
public override int Add(Object
product)
{
int added = base.Add(product);
OnChanged(EventArgs.Empty);
return added;
}
public override void Clear()
{
base.Clear();
OnChanged(EventArgs.Empty);
}
public override object this[int index]
{
set
{
base[index] = value;
OnChanged(EventArgs.Empty);
}
}
}
public class Subscriber
{
private Publisher publishers;
public Subscriber(Publisher publisher)
{
this.publishers = publisher;
publishers.ProdcutAddedInfo +=
publishers_ProdcutAddedInfo;
}
void publishers_ProdcutAddedInfo(object sender, EventArgs e)
{
if (sender == null)
{
Console.WriteLine("No New Product Added.");
return;
}
Console.WriteLine("A New Prodct Added.");
}
public void
UnSubscribeEvent()
{
publishers.ProdcutAddedInfo -=
publishers_ProdcutAddedInfo;
}
}
}
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