Understanding events and event handlers in C#

Question

I understand the purpose of events, especially within the context of creating user interfaces. I think this is the prototype for creating an event:

public void EventName(object sender, EventArgs e);

What do event handlers do, why are they needed, and how do I to create one?

Organizer

As noted by @Andy, the code snippet here describes the method registered to the event, not the event itself. – Orgulous

codeproject.com/Articles/4773/Events-and-Delegates-simplified – Hohenlinden

Possible duplicate of Difference between events and delegates and its respective applications – Grishilda

Answer 1

To understand event handlers, you need to understand delegates. In C#, you can think of a delegate as a pointer (or a reference) to a method. This is useful because the pointer can be passed around as a value.

The central concept of a delegate is its signature, or shape. That is (1) the return type and (2) the input arguments. For example, if we create a delegate void MyDelegate(object sender, EventArgs e), it can only point to methods which return void, and take an object and EventArgs. Kind of like a square hole and a square peg. So we say these methods have the same signature, or shape, as the delegate.

So knowing how to create a reference to a method, let's think about the purpose of events: we want to cause some code to be executed when something happens elsewhere in the system - or "handle the event". To do this, we create specific methods for the code we want to be executed. The glue between the event and the methods to be executed are the delegates. The event must internally store a "list" of pointers to the methods to call when the event is raised.* Of course, to be able to call a method, we need to know what arguments to pass to it! We use the delegate as the "contract" between the event and all the specific methods that will be called.

So the default EventHandler (and many like it) represents a specific shape of method (again, void/object-EventArgs). When you declare an event, you are saying which shape of method (EventHandler) that event will invoke, by specifying a delegate:

//This delegate can be used to point to methods
//which return void and take a string.
public delegate void MyEventHandler(string foo);

//This event can cause any method which conforms
//to MyEventHandler to be called.
public event MyEventHandler SomethingHappened;

//Here is some code I want to be executed
//when SomethingHappened fires.
void HandleSomethingHappened(string foo)
{
    //Do some stuff
}

//I am creating a delegate (pointer) to HandleSomethingHappened
//and adding it to SomethingHappened's list of "Event Handlers".
myObj.SomethingHappened += new MyEventHandler(HandleSomethingHappened);

//To raise the event within a method.
SomethingHappened("bar");

(*This is the key to events in .NET and peels away the "magic" - an event is really, under the covers, just a list of methods of the same "shape". The list is stored where the event lives. When the event is "raised", it's really just "go through this list of methods and call each one, using these values as the parameters". Assigning an event handler is just a prettier, easier way of adding your method to this list of methods to be called).

Kennel

And now can anyone explain why the event is called EventHandler?? Of all the confusing naming conventions, this is the worst... – Malpractice

@Joel in Go the event is not called EventHandler - EventHandler is the contract the event must have with anyone who communicates with it. It's like "string MyString" - the string is declaring the type. event MyEventHandler TheEvent is declaring that anyone who interacts with this event must conform to the MyEventHandler contract. The Handler convention is because the contract primarily describes how to handle the event. – Kennel

@LuftMensch from inside the class which owns the event, the syntax EventName(delegateArg1, delegateArg2); is valid. That invokes the event like a method, which kind of kicks off a chain, calling each handler assigned to that event with those arguments. – Kennel

@Rex M : thank you for the first coherent explanation for "MyEventHandler" that I have ever seen :) – Malpractice

@Joel in Go: I felt the same way, until I started reading it as: public event<MyEventHandler> eventName :) – Menell

Thank you for the phase: "The glue between the event and the methods to be executed are the delegates. ",this is really awesome. – Induction

Just want to mention: "HandleSomethingHappened" is often called "OnSomethingHappened" – Onus

important to note WHY the event keyword is required. It restricts access to types that do not own the event to += and -= only – Heterochromosome

Thanks for your well thought out post. It's worth mentioning the standard format: public delegate void MyEventHandler(object Sender, MyEventArgs e) where MyEventArgs derives from EventArgs. See msdn.microsoft.com/en-us/library/aa645739%28v=vs.71%29.aspx. This offers the benefit of following the generic system.eventhandler that's been built in since .Net 2.0 which allows you to use public event MyGenericEventHandler<MyPublisher, MyEventArgs> MyEvent. See codeproject.com/Articles/20550/…. – Shannonshanny

See msdn.microsoft.com/en-US/library/ms182178%28v=vs.80%29.aspx. It allows you to skip some of the extra work in declaring delegates, etc. – Shannonshanny

The most wonderful explanation of anything i have seen in stackoverflow. – Mccormick

Could the last line be simlpified to myObj.SomethingHappened += HandleSomethingHappened;? Why, why not, or under what circumstances? – Mockery

I printed this answer on paper. – Righteous

I just finished reading the microsoft documentation on delegate and events when i found your post and i can confirm that your explaination are better. Thanks a lot. – Moloch

Answer 2

C# knows two terms, delegate and event. Let's start with the first one.

Delegate

A delegate is a reference to a method. Just like you can create a reference to an instance:

MyClass instance = myFactory.GetInstance();

You can use a delegate to create an reference to a method:

Action myMethod = myFactory.GetInstance;

Now that you have this reference to a method, you can call the method via the reference:

MyClass instance = myMethod();

But why would you? You can also just call myFactory.GetInstance() directly. In this case you can. However, there are many cases to think about where you don't want the rest of the application to have knowledge of myFactory or to call myFactory.GetInstance() directly.

An obvious one is if you want to be able to replace myFactory.GetInstance() into myOfflineFakeFactory.GetInstance() from one central place (aka factory method pattern).

Factory method pattern

So, if you have a TheOtherClass class and it needs to use the myFactory.GetInstance(), this is how the code will look like without delegates (you'll need to let TheOtherClass know about the type of your myFactory):

TheOtherClass toc;
//...
toc.SetFactory(myFactory);


class TheOtherClass
{
   public void SetFactory(MyFactory factory)
   {
      // set here
   }

}

If you'd use delegates, you don't have to expose the type of my factory:

TheOtherClass toc;
//...
Action factoryMethod = myFactory.GetInstance;
toc.SetFactoryMethod(factoryMethod);


class TheOtherClass
{
   public void SetFactoryMethod(Action factoryMethod)
   {
      // set here
   }

}

Thus, you can give a delegate to some other class to use, without exposing your type to them. The only thing you're exposing is the signature of your method (how many parameters you have and such).

"Signature of my method", where did I hear that before? O yes, interfaces!!! interfaces describe the signature of a whole class. Think of delegates as describing the signature of only one method!

Another large difference between an interface and a delegate is that when you're writing your class, you don't have to say to C# "this method implements that type of delegate". With interfaces, you do need to say "this class implements that type of an interface".

Further, a delegate reference can (with some restrictions, see below) reference multiple methods (called MulticastDelegate). This means that when you call the delegate, multiple explicitly-attached methods will be executed. An object reference can always only reference to one object.

The restrictions for a MulticastDelegate are that the (method/delegate) signature should not have any return value (void) and the keywords out and ref is not used in the signature. Obviously, you can't call two methods that return a number and expect them to return the same number. Once the signature complies, the delegate is automatically a MulticastDelegate.

Event

Events are just properties (like the get;set; properties to instance fields) which expose subscription to the delegate from other objects. These properties, however, don't support get;set;. Instead, they support add; remove;

So you can have:

    Action myField;

    public event Action MyProperty
    {
        add { myField += value; }
        remove { myField -= value; }
    }

Usage in UI (WinForms,WPF,UWP So on)

So, now we know that a delegate is a reference to a method and that we can have an event to let the world know that they can give us their methods to be referenced from our delegate, and we are a UI button, then: we can ask anyone who is interested in whether I was clicked, to register their method with us (via the event we exposed). We can use all those methods that were given to us and reference them by our delegate. And then, we'll wait and wait.... until a user comes and clicks on that button, then we'll have enough reason to invoke the delegate. And because the delegate references all those methods given to us, all those methods will be invoked. We don't know what those methods do, nor we know which class implements those methods. All we do care about is that someone was interested in us being clicked, and gave us a reference to a method that complied with our desired signature.

Java

Languages like Java don't have delegates. They use interfaces instead. The way they do that is to ask anyone who is interested in 'us being clicked', to implement a certain interface (with a certain method we can call), then give us the whole instance that implements the interface. We keep a list of all objects implementing this interface and can call their 'certain method we can call' whenever we get clicked.

Gleesome

cheers for the explanation but how does an event different from an instance of a delegate which takes on subscribers? they both look like exactly the same thing? – Gibbie

@BKSpurgeon that's because they are "delegates that take on subscribers" - event is mere syntax sugar, nothing more. – Alli

"The restrictions for a MulticastDelegate are that the (method/delegate) signature should not have any return value (void) ", I dont think this is correct. If they have return values, it will return the last one. – Weiman

"Thus, you can give a delegate to some other class to use, without exposing your type to them. The only thing you're exposing is the signature of your method ..." -- that to me is the critical point. Thank you! – Midlothian

Answer 3

That is actually the declaration for an event handler - a method that will get called when an event is fired. To create an event, you'd write something like this:

public class Foo
{
    public event EventHandler MyEvent;
}

And then you can subscribe to the event like this:

Foo foo = new Foo();
foo.MyEvent += new EventHandler(this.OnMyEvent);

With OnMyEvent() defined like this:

private void OnMyEvent(object sender, EventArgs e)
{
    MessageBox.Show("MyEvent fired!");
}

Whenever Foo fires off MyEvent, then your OnMyEvent handler will be called.

You don't always have to use an instance of EventArgs as the second parameter. If you want to include additional information, you can use a class derived from EventArgs (EventArgs is the base by convention). For example, if you look at some of the events defined on Control in WinForms, or FrameworkElement in WPF, you can see examples of events that pass additional information to the event handlers.

Geocentric

Thank you for answering the question and not going into Delegates and Events. – Ethic

I would recommend against using the OnXXX naming pattern for your event handler. (Stupidly, OnXXX is taken to mean 'handle XXX' in MFC, and 'raise XXX' in .net, and so now its meaning is unclear and confusing - see this post for details ). Preferred names would be RaiseXXX to raise events, and HandleXXX or Sender_XXX to for event handlers. – Teodor

Can you show a working example with a simple WinForms application? – Unchurch

Answer 4

Here is a code example which may help:

using System;
using System.Collections.Generic;
using System.Text;

namespace Event_Example
{
  // First we have to define a delegate that acts as a signature for the
  // function that is ultimately called when the event is triggered.
  // You will notice that the second parameter is of MyEventArgs type.
  // This object will contain information about the triggered event.

  public delegate void MyEventHandler(object source, MyEventArgs e);

  // This is a class which describes the event to the class that receives it.
  // An EventArgs class must always derive from System.EventArgs.

  public class MyEventArgs : EventArgs
  {
    private string EventInfo;

    public MyEventArgs(string Text) {
      EventInfo = Text;
    }

    public string GetInfo() {
      return EventInfo;
    }
  }

  // This next class is the one which contains an event and triggers it
  // once an action is performed. For example, lets trigger this event
  // once a variable is incremented over a particular value. Notice the
  // event uses the MyEventHandler delegate to create a signature
  // for the called function.

  public class MyClass
  {
    public event MyEventHandler OnMaximum;

    private int i;
    private int Maximum = 10;

    public int MyValue
    {
      get { return i; }
      set
      {
        if(value <= Maximum) {
          i = value;
        }
        else 
        {
          // To make sure we only trigger the event if a handler is present
          // we check the event to make sure it's not null.
          if(OnMaximum != null) {
            OnMaximum(this, new MyEventArgs("You've entered " +
              value.ToString() +
              ", but the maximum is " +
              Maximum.ToString()));
          }
        }
      }
    }
  }

  class Program
  {
    // This is the actual method that will be assigned to the event handler
    // within the above class. This is where we perform an action once the
    // event has been triggered.

    static void MaximumReached(object source, MyEventArgs e) {
      Console.WriteLine(e.GetInfo());
    }

    static void Main(string[] args) {
      // Now lets test the event contained in the above class.
      MyClass MyObject = new MyClass();
      MyObject.OnMaximum += new MyEventHandler(MaximumReached);
      for(int x = 0; x <= 15; x++) {
        MyObject.MyValue = x;
      }
      Console.ReadLine();
    }
  }
}

Venegas

Delegate invocation can in C#6 be simplified to: OnMaximum?.Invoke(this,new MyEventArgs("you've entered...")); – Dunigan

Answer 5

Just to add to the existing great answers here - building on the code in the accepted one, which uses a delegate void MyEventHandler(string foo)...

Because the compiler knows the delegate type of the SomethingHappened event, this:

myObj.SomethingHappened += HandleSomethingHappened;

Is totally equivalent to:

myObj.SomethingHappened += new MyEventHandler(HandleSomethingHappened);

And handlers can also be unregistered with -= like this:

// -= removes the handler from the event's list of "listeners":
myObj.SomethingHappened -= HandleSomethingHappened;

For completeness' sake, raising the event can be done like this, only in the class that owns the event:

//Firing the event is done by simply providing the arguments to the event:
var handler = SomethingHappened; // thread-local copy of the event
if (handler != null) // the event is null if there are no listeners!
{
    handler("Hi there!");
}

The thread-local copy of the handler is needed to make sure the invocation is thread-safe - otherwise a thread could go and unregister the last handler for the event immediately after we checked if it was null, and we would have a "fun" NullReferenceException there.

---

C# 6 introduced a nice short hand for this pattern. It uses the null propagation operator.

SomethingHappened?.Invoke("Hi there!");

Alli

Answer 6

My understanding of the events is;

Delegate:

A variable to hold reference to method / methods to be executed. This makes it possible to pass around methods like a variable.

Steps for creating and calling the event:

1. The event is an instance of a delegate

2. Since an event is an instance of a delegate, then we have to first define the delegate.

3. Assign the method / methods to be executed when the event is fired (Calling the delegate)

4. Fire the event (Call the delegate)

Example:

using System;

namespace test{
    class MyTestApp{
        //The Event Handler declaration
        public delegate void EventHandler();

        //The Event declaration
        public event EventHandler MyHandler;

        //The method to call
        public void Hello(){
            Console.WriteLine("Hello World of events!");
        }

        public static void Main(){
            MyTestApp TestApp = new MyTestApp();

            //Assign the method to be called when the event is fired
            TestApp.MyHandler = new EventHandler(TestApp.Hello);

            //Firing the event
            if (TestApp.MyHandler != null){
                TestApp.MyHandler();
            }
        }

    }   

}

Valentinvalentina

Answer 7

//This delegate can be used to point to methods
//which return void and take a string.
public delegate void MyDelegate(string foo);

//This event can cause any method which conforms
//to MyEventHandler to be called.
public event MyDelegate MyEvent;

//Here is some code I want to be executed
//when SomethingHappened fires.
void MyEventHandler(string foo)
{
    //Do some stuff
}

//I am creating a delegate (pointer) to HandleSomethingHappened
//and adding it to SomethingHappened's list of "Event Handlers".
myObj.MyEvent += new MyDelegate (MyEventHandler);

Clavicembalo

Answer 8

publisher: where the events happen. Publisher should specify which delegate the class is using and generate necessary arguments, pass those arguments and itself to the delegate.

subscriber: where the response happen. Subscriber should specify methods to respond to events. These methods should take the same type of arguments as the delegate. Subscriber then add this method to publisher's delegate.

Therefore, when the event happen in publisher, delegate will receive some event arguments (data, etc), but publisher has no idea what will happen with all these data. Subscribers can create methods in their own class to respond to events in publisher's class, so that subscribers can respond to publisher's events.

Lucilucia

Answer 9

I recently made an example of how to use events in c#, and posted it on my blog. I tried to make it as clear as possible, with a very simple example. In case it might help anyone, here it is: http://www.konsfik.com/using-events-in-csharp/

It includes description and source code (with lots of comments), and it mainly focuses on a proper (template - like) usage of events and event handlers.

Some key points are:

• Events are like "sub - types of delegates", only more constrained (in a good way). In fact an event's declaration always includes a delegate (EventHandlers are a type of delegate).

• Event Handlers are specific types of delegates (you may think of them as a template), which force the user to create events which have a specific "signature". The signature is of the format: (object sender, EventArgs eventarguments).

• You may create your own sub-class of EventArgs, in order to include any type of information the event needs to convey. It is not necessary to use EventHandlers when using events. You may completely skip them and use your own kind of delegate in their place.

• One key difference between using events and delegates, is that events can only be invoked from within the class that they were declared in, even though they may be declared as public. This is a very important distinction, because it allows your events to be exposed so that they are "connected" to external methods, while at the same time they are protected from "external misuse".

Babin

Answer 10

Another thing to know about, in some cases, you have to use the Delegates/Events when you need a low level of coupling !

If you want to use a component in several place in application, you need to make a component with low level of coupling and the specific unconcerned LOGIC must be delegated OUTSIDE of your component ! This ensures that you have a decoupled system and a cleaner code.

In SOLID principle this is the "D", (Dependency inversion principle).

Also known as "IoC", Inversion of control.

You can make "IoC" with Events, Delegates and DI (Dependency Injection).

It's easy to access a method in a child class. But more difficult to access a method in a parent class from child. You have to pass the parent reference to the child ! (or use DI with Interface)

Delegates/Events allows us to communicate from the child to the parent without reference !

In this diagram above, I do not use Delegate/Event and the parent component B has to have a reference of the parent component A to execute the unconcerned business logic in method of A. (high level of coupling)

With this approach, I would have to put all the references of all components that use component B ! :(

In this diagram above, I use Delegate/Event and the component B doesn't have to known A. (low level of coupling)

And you can use your component B anywhere in your application !

Fibroblast

Answer 11

DELEGATES, EVENTS(EVENT HANDLERS/EVENT LISTENERS), CONCEPTS(MULTICASTING/BROADCASTING), ACTION & FUNC

This will be a long one but its the simplest explanation, the problem this is such a nuisance of a topic is because people are just using different words to explain the same thing

First of all, you should know a few things

DELEGATES: It's nothing but a list of methods, why create a list? because when your code is being executed, that list is taken and every method there is executed one by one, just don't listen to textbook definitions take this and you will be all right

also called :

• a pointer to a function
• a wrapper for a method that can send and receive methods just like a variable

to create a delegate you go

[[access modifier] delegate [return type] [delegate name]([parameters])]

example: public delegate int demo(int a);

now to execute all these methods stored in a list called delegate, you go

1. demo.invoke(a);
2. demo(a);   ..... both are valid

using the dot and explicitly saying invoke shines in async programming where you use beginInvoke, but that is out of the scope of this topic

there is one more thing called "Creating an object of the delegate/instantiate Delegate" which is pretty much as it sounds but just to avoid confusion it goes like (for the above example )

example : demo del = new demo(); (or) Public demo del = null;

to add any method to the list called delegate you go += and you also need to remove it once the "requirements of the methods are met" you go -=

(requirements of the methods are met mean you no longer need the method to be active or aka "listening") if you don't remove it, it could cause a "memory leak" meaning your computers ram will be eaten alive, technically allocated memory will not be released

example: say there is a method

public int calculate (int c)

to add this method to delegate you go

1.  del = calculate;
2.  del += calculate; .... all are valid

to remove 

del -= calculate

first of all notice the similarities between the delegate and the method, the return type(output) and the input/parameters are the same, and that is a rule you just cannot add any random or a bunch of methods in a delegate it needs to follow the input-output rule

now why are there 2 different ways to do one thing, the only thing different is the assignment operators (+, =), this introduces a new topic called

EVENTS

which is nothing but a constrained version of a Delegate, It's still a List of methods don't confuse when people explain these terminologies, they change the name, so stick with this to understand

what is the constraint? you cannot do this del = calculate; what's the harm in it, say a bunch of methods are added to the Delegate(List), you do that 👆 all are wiped out and only a single method "calculate" remains, so to prevent that Events are used, Event Syntax

Public Event demo del = null;

One more thing you cannot do with events is invoke the delegate directly like demo.invoke since its public it can be accessed and invoked but with events, it can't

now you just add the methods to the event (a special type of delegate)

when to use an event vs a delegate, depends on your situation but pragmatically events are popular

few more keywords

MULTICASTING: nothing but adding more than one method to a delegate BROADCASTING: adding more than one method to an event

PUBLISHER: the one that executes the method (term used in broadcasting), only a single entity SUBSCRIBER: The methods that are being executed, can be multiple

LISTENER: the same thing as a subscriber but the term is used in multicasting

EVENT HANDLER: same thing as a subscriber/event listener so what the difference? it's basically the same thing, some say an eventlistener detect for the event to occur and the event handler "handles" or execute the code, ITS THE SAME THING PRACTICALLY!

action and func are just delegates that have been created and instantiated so 2 lines of code in a word, the difference is just in return types

ACTION: does not return anything while taking 0 or more than 1 input

FUNC: returns one thing and takes in parameters

if you don't do good with reading here is the best video on this topic

https://www.youtube.com/playlist?list=PLFt_AvWsXl0dliMtpZC8Qd_ru26785Ih_

Perspex

Answer 12

I agree with KE50 except that I view the 'event' keyword as an alias for 'ActionCollection' since the event holds a collection of actions to be performed (ie. the delegate).

using System;

namespace test{

class MyTestApp{
    //The Event Handler declaration
    public delegate void EventAction();

    //The Event Action Collection 
    //Equivalent to 
    //  public List<EventAction> EventActions=new List<EventAction>();
    //        
    public event EventAction EventActions;

    //An Action
    public void Hello(){
        Console.WriteLine("Hello World of events!");
    }
    //Another Action
    public void Goodbye(){
        Console.WriteLine("Goodbye Cruel World of events!");
    }

    public static void Main(){
        MyTestApp TestApp = new MyTestApp();

        //Add actions to the collection
        TestApp.EventActions += TestApp.Hello;
        TestApp.EventActions += TestApp.Goodbye;

        //Invoke all event actions
        if (TestApp.EventActions!= null){
            //this peculiar syntax hides the invoke 
            TestApp.EventActions();
            //using the 'ActionCollection' idea:
            // foreach(EventAction action in TestApp.EventActions)
            //     action.Invoke();
        }
    }

}   

}

Jillane

Answer 13

Great technical answers in the post! I have nothing technically to add to that.

One of the main reasons why new features appear in languages and software in general is marketing or company politics! :-) This must not be under estimated!

I think this applies to certain extend to delegates and events too! i find them useful and add value to the C# language, but on the other hand the Java language decided not to use them! they decided that whatever you are solving with delegates you can already solve with existing features of the language i.e. interfaces e.g.

Now around 2001 Microsoft released the .NET framework and the C# language as a competitor solution to Java, so it was good to have NEW FEATURES that Java doesn't have.

Ramburt