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Events in C# .NET Explained


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;
        }
    }
}

References

1. C-sharpcorner
2. Tutorial Teacher
3.  MicrosoftDocumentation

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