Why do Events need Delegates? Why do we even Need Events?

Question

I've been confused over the past weeks now about events. I understand how delegates work, not how it works in detail but enough to know that delegate datatype is a single cast delegate. delegate void is a multicast delegate - a list of references to methods.

I know a delegate type compiles to a class, but unfortunately I am still not sure how the method is referenced. For example

delegate void TestDelegate();
TestDelegate testDelegate = new TestDelegate(myObject.SomeMethod) ;

Question 1: I think myObject is the target, and SomeMethod is the method to reference, but I'm only passing one input. So is myObject.SomeMethod compiled to a string and is the string split by the period? Ridiculous I know.

Question 2: When you add to a multicast delegate

multicastdelegate+=newmethodtobereference
multicastdelegate() ;

Every method in the invocation list is called or notified? If that's true, why the hell do I need events or the event keyword? Is it simply to tell the developers that Hey, this is acting as an event? Because I'm seriously confused, I just want to move on at this stage lol. This is a sample code I wrote to test it today whether I need event keyword or not.

using System;
namespace LambdasETs
{
    public delegate void IsEvenNumberEventHandler(int numberThatIsEven);

    public class IsEvenNumberFound
    {
        public  IsEvenNumberEventHandler IsEvenNumberEvent;
        private int number;

        public void InputNumber(int n)
        {
            if(number %2 ==0)
            {
                if (IsEvenNumberEvent != null)
                {
                    IsEvenNumberEvent(n);
                }
            }
        }


        public static void Main()
        {
            IsEvenNumberFound isEvenNumberFound = new IsEvenNumberFound();

            isEvenNumberFound.IsEvenNumberEvent += IsEvenNumberAction;

             isEvenNumberFound.InputNumber(10);

            Console.ReadLine();

        }

        public static void IsEvenNumberAction(int number)
        {
            Console.WriteLine("{0} is an even number!", number);
        }
    }


}

Adding the event keyword to the field public IsEvenNumberEventHandler IsEvenNumberEvent; has no difference. Please can some explain so that a noob can understand thanks.

Answer 1

An event is an accessor for a delegate, just like a property is an accessor for a field. With much the same goals, it prevents other code from messing with the delegate object. Like setting it null when a bunch of code you don't know about have subscribed a callback. An event restricts access to only adding and removing event handlers with the += and -= operators, external code cannot access the private delegate object at all.

And to customize the subscription with the add and remove accessors . You don't often do so because you are typically happy with the default accessors generated by the compiler. Including a hidden backing field that stores a delegate. But it is not uncommon in the framework code for example. Like all the event handlers for the many events that System.Windows.Forms.Control supports, they are all stored in a single EventHandlerList. Or the WPF equivalent, EventHandlersStore.

Answer 2

but enough to know that delegate datatype is a single cast delegate. delegate void is a multicast delegate - a list of references to methods.

Not true. All "normal" delegates are multicast, even if they have a non void return type.

Question 1: I think myObject is the target, and SomeMethod is the method to reference, but I'm only passing one input. So is myObject.SomeMethod compiled to a string and is the string split by the period? Ridiculous I know.

No, myObject.SomeMethod is a method group. This way of delegate instance creation involves a bit of compiler magic.

multicastdelegate+=newmethodtobereference

If multicastdelegate is a normal delegate variable, this is equivalent to multicastdelegate = multicastdelegate + newmethodtobereference ie it creates a new delegate that calls several methods, and assigns it to multicastdelegate .

---

Now to your main question: What's the purpose of events?

Events have delegate types. They behave similarly to properties. Their purpose is encapsulation, in particular they only allow consumers to subscribe( += ) and unsubscribe( -= ) but not to read the value of the event.

Properties are a combination of two methods: get and set .

Events are a combination of two public methods subscribe and unsubscribe , and in the case of a field-like event also something similar to a private getter.