I'll start with a code example. I have a following class
public class Foo
{
public object DoSomething() {}
}
I also have some code that utilises method DoSomehting from class Foo.
public class Boo
{
privite Foo foo;
public void SomeMethod()
{
...
foo.DoSomething();
...
foo.DoSomething();
}
}
How could I distinguish those two calls foo.DoSomething() inside the Foo class?
What I came up with is to have an identification object passed in parameters for each call to DoSomething. Then in Foo class I would store the ids and compare them when new call is made.
public class Boo
{
privite Foo foo;
public void SomeMethod()
{
...
var idObjA = new IDObj(Guid.NewGuid());
foo.DoSomething(idObjA);
...
var idObjB = new IDObj(Guid.NewGuid());
foo.DoSomething(idObjB);
}
}
Maybe there is a better way to do it, or a pattern that I'm not aware of. I want the utilising code to be the least obscured so calls to the DoSomething method are as simple as possible.
To clarify my intentions. I'm implementing a message service with an ability for the user to check a checkbox on dialog box (e.g. Do not show again, or Apply to all). Code utilising the service can call the same method multiple times, to show an error message for example, but in different context. In other words, when user decided to not show that message again for particular action message box should not appear. Thus I need to know when method was called multiple times in the same context (action)
Maybe you should expand a bit on what exactly you are trying to achieve. If you're using your instantiated class like described above and are just trying to differentiate between the first and second call, you can add a respective toggle field in your Foo class:
public class Foo
{
private bool _firstCall = true;
public object DoSomething() {
if(_firstCall) {
_firstCall = false;
// first call logic
} else {
// second call logic
}
}
}
Based on the extra info in your edit, it sounds like what you perhaps need to be doing is setting a separate property in your Foo class showing whether the "apply to all" or "do not show again" option has been checked for a particular context.
Then when you call DoSomething, it can check that property to know if it should show the dialog or not.
So in the simplest case you might do something like:
public class Foo
{
public bool DoNotShow { get; set; };
public void DoSomething() {
if(this.DoNotShow == true) {
// logic
} else {
// alternative logic
}
}
}
public class Boo
{
privite Foo foo;
public void SomeMethod()
{
...
foo.DoSomething();
foo.DoNotShow = true;
...
foo.DoSomething();
}
}
The value could then be toggled on and off whenever you like.
N.B. You mentioned different "contexts" in which dialogs can be turned on and off.
For this, you could consider either giving this property the ability to store values for different contexts (e.g. in something like a Dictionary, perhaps) and then passing in the current context name to the DoSomething method when it's called. Or even pass in a totally separate "context" object to DoSomething each time, which contains the context name and the boolean indicating whether to show the dialog or not.
Or...using a different instance of Foo for each context might actually be more in line with object-oriented principles (in which case you could probably use the code exactly as per my example above). Again it depends exactly how the class the and the overall application works.
If knowing the line number of the call helps, you could use one of the methods for getting the caller information described here. So for example:
public class Foo
{
public object DoSomething() {
StackFrame frame = new StackFrame(1, true);
var method = frame.GetMethod();
var lineNumber = frame.GetFileLineNumber();
}
}
Like many other posts I've found on SO, I'm trying to get my head around delegates. Hopefully this example is not classed a duplicate because I am asking a specific question about a particular example.
public delegate void HelloFunctionDelegate(string message);
public class Delegate
{
static void Main()
{
HelloFunctionDelegate del = new HelloFunctionDelegate(GoodNight); // delegate will point to the GoodNight method
del("Hello"); // invoke the delegate
}
public static void GoodMorning(string strMessage)
{
Console.WriteLine(strMessage + " and good morning!");
Console.ReadKey();
}
public static void GoodNight(string strMessage)
{
Console.WriteLine(strMessage + " and good night!");
Console.ReadKey();
}
}
So in my example I understand that my delegate is a reference to any function that matches its signature and if I pass in GoodMorning I will see:
Hello and good morning!
and if I pass in GoodNight I will see: Hello and good night!
So its kind of like going through a middle man...
I don't understand is what's the point, why wouldn't I just directly call my GoodMorning / GoodNight methods as and when I need to use them?
Maybe there are better examples for when a delegate is useful, but in this example, why don't I just bypass the middle man?
Since you are asking concretely about this example and not in general: There is no point to doing that in this particular piece of code. It teaches you the mechanics of delegates but it does not teach you the point of using them.
In short, the point is that some piece of code can take a reference to a method without knowing what method it will actually receive. It can later call that delegate at will. That enables more abstractions than otherwise possible.
Consider you have the following delegate:
public delegate void CarEvent(Car car);
And then you have an implementation like the following:
public class Car : DataRecord
{
// An event to execute when the record is deleted
public CarEvent OnDelete { get; set; }
public void Delete()
{
this.DeleteRecord(); // Deletes this record from ex. the database
if (OnDelete)
{
OnDelete(this); // Executes the event
}
}
}
By using a delegate you can subscribe different methods to the OnDelete allowing you to do different things when the record is deleted.
Ex. you can make it so when the record is deleted it's deleted from a "ListView" that holds it.
public class CarList : ListView
{
public CarList()
: base()
{
foreach (var car in CarRecords.LoadCars())
{
var listViewItem = new ListViewItem(car);
car.OnDelete = this.DeleteCarFromList;
this.Items.Add(listViewItem);
}
}
private void DeleteCarFromList(Car deletedCar)
{
this.Items.Remove(deletedCar);
}
}
Of course the above is a rough example and there is a lot more things and different kind of situations where you can use delegates and most notably if you want to use them for events you should consider implementing them using the event keyword. - https://msdn.microsoft.com/en-us/library/awbftdfh.aspx
All in all you want to use delegates when the behavior may differ depending on the overall implementation of something. Like you might want to do one thing in one situation and something else in another situation, but they should both over-all do the same thing.
If you do not need different behaviors based on implementation then there's no need to use delegates. You'd always want to call a method directly if possible.
I hope this explained it okay.
If, lets say I have a FormA with a ListView and an Update() function. Then I also have one Math-Class with a function A() wich does some magic... Can a delegate be used to call Update() from A()? Or Is there a better way? I've realized it's risky to update a gui form from another class.... Thanks in advance!
Yes. Its not that risky as long as the Math class has no knowledge of what its actually calling. You just give it a rough idea by pointing it to the desired function from your Form:
public class MathClass {
public Action FunctionToCall { get; set; }
public void DoSomeMathOperation() {
// do something here.. then call the function:
FunctionToCall();
}
}
In your form you would do this:
// Form.cs
public void Update() {
// this is your update function
}
public void DoMathStuff() {
MathClass m = new MathClass() { FunctionToCall = Update };
m.DoSomeMathOperation(); // MathClass will end up calling the Update method above.
}
Your MathClass calls Update, but it has no knowledge of the object that told it to call Update or where Update is.. making it safer than tightly coupling your objects together.
I'm currently programming a library for simple games, mostly intended to be used by myself.
Now I've run into a problem. I have a class called "Game" which looks like this:
class Game
{
private List<Entity> entities;
private int counter;
public Game()
{
entities = new List<Entity>();
counter = 0;
}
public AddEntity(Entity entity_)
{
entities.Add(entity_);
// problem! how to inform entity_ that it was added?
counter++;
}
}
Entity is a class which each object that acts in the game must be derived from. Its contents don't really matter. What I am looking for is a way for Game to inform the newly added entity_ class about its owner (current Game instance) and its id (which is what "counter" is for). Now I have been thinking about using an interface which would have a method "OnAdd(Game owner_, int id_)" for that as that would definitely work, but I wanted to make sure there is no preferred way over that. So that is my question:
Is there a better solution for my problem than interfaces? The Entity instance does not know what Game instance it is being added to, and using methods for event-like purposes doesn't feel right in my eyes. I could be wrong if course.
If your Entity has a property of type Game, it's easy to solve this without even using events:
... code ...
entities.Add(entity_);
entity_.AddToGame(this);
... code ...
and then in the AddToGame method, you would do whatever you would do in your event handler, which now is not necessary.
Yes seems correct. But need to add a difference.
Interface - Once changes, you need to track down all implementation.
Abstract Class - Virtual functions can be declared so no need to keep track for the implementations
There are lots of solutions.
You could of course call a method after the .Add:
entity_.HasBeenAdded();
Another solution would be to "reverse the add". Something like:
Entity class:
public Entity
{
...
public void AddTo(IList<Entity> list)
{
if (list == null)
{
throw new ArgumentNullException();
}
list.Add(this);
// Do some logic here, as you now know your object
// has been added.
}
}
...
entity_.AddTo(entities);
What are the differences between delegates and an events? Don't both hold references to functions that can be executed?
An Event declaration adds a layer of abstraction and protection on the delegate instance. This protection prevents clients of the delegate from resetting the delegate and its invocation list and only allows adding or removing targets from the invocation list.
To understand the differences you can look at this 2 examples
Example with Delegates (in this case, an Action - that is a kind of delegate that doesn't return a value)
public class Animal
{
public Action Run {get; set;}
public void RaiseEvent()
{
if (Run != null)
{
Run();
}
}
}
To use the delegate, you should do something like this:
Animal animal= new Animal();
animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running") ;
animal.RaiseEvent();
This code works well but you could have some weak spots.
For example, if I write this:
animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running");
animal.Run = () => Console.WriteLine("I'm sleeping") ;
with the last line of code, I have overridden the previous behaviors just with one missing + (I have used = instead of +=)
Another weak spot is that every class which uses your Animal class can invoke the delegate directly. For example, animal.Run() or animal.Run.Invoke() are valid outside the Animal class.
To avoid these weak spots you can use events in c#.
Your Animal class will change in this way:
public class ArgsSpecial : EventArgs
{
public ArgsSpecial (string val)
{
Operation=val;
}
public string Operation {get; set;}
}
public class Animal
{
// Empty delegate. In this way you are sure that value is always != null
// because no one outside of the class can change it.
public event EventHandler<ArgsSpecial> Run = delegate{}
public void RaiseEvent()
{
Run(this, new ArgsSpecial("Run faster"));
}
}
to call events
Animal animal= new Animal();
animal.Run += (sender, e) => Console.WriteLine("I'm running. My value is {0}", e.Operation);
animal.RaiseEvent();
Differences:
You aren't using a public property but a public field (using events, the compiler protects your fields from unwanted access)
Events can't be assigned directly. In this case, it won't give rise to the previous error that I have showed with overriding the behavior.
No one outside of your class can raise or invoke the event. For example, animal.Run() or animal.Run.Invoke() are invalid outside the Animal class and will produce compiler errors.
Events can be included in an interface declaration, whereas a field cannot
Notes:
EventHandler is declared as the following delegate:
public delegate void EventHandler (object sender, EventArgs e)
it takes a sender (of Object type) and event arguments. The sender is null if it comes from static methods.
This example, which uses EventHandler<ArgsSpecial>, can also be written using EventHandler instead.
Refer here for documentation about EventHandler
In addition to the syntactic and operational properties, there's also a semantical difference.
Delegates are, conceptually, function templates; that is, they express a contract a function must adhere to in order to be considered of the "type" of the delegate.
Events represent ... well, events. They are intended to alert someone when something happens and yes, they adhere to a delegate definition but they're not the same thing.
Even if they were exactly the same thing (syntactically and in the IL code) there will still remain the semantical difference. In general I prefer to have two different names for two different concepts, even if they are implemented in the same way (which doesn't mean I like to have the same code twice).
Here is another good link to refer to.
http://csharpindepth.com/Articles/Chapter2/Events.aspx
Briefly, the take away from the article - Events are encapsulation over delegates.
Quote from article:
Suppose events didn't exist as a concept in C#/.NET. How would another class subscribe to an event? Three options:
A public delegate variable
A delegate variable backed by a property
A delegate variable with AddXXXHandler and RemoveXXXHandler methods
Option 1 is clearly horrible, for all the normal reasons we abhor public variables.
Option 2 is slightly better, but allows subscribers to effectively override each other - it would be all too easy to write someInstance.MyEvent = eventHandler; which would replace any existing event handlers rather than adding a new one. In addition, you still need to write the properties.
Option 3 is basically what events give you, but with a guaranteed convention (generated by the compiler and backed by extra flags in the IL) and a "free" implementation if you're happy with the semantics that field-like events give you. Subscribing to and unsubscribing from events is encapsulated without allowing arbitrary access to the list of event handlers, and languages can make things simpler by providing syntax for both declaration and subscription.
What a great misunderstanding between events and delegates!!! A delegate specifies a TYPE (such as a class, or an interface does), whereas an event is just a kind of MEMBER (such as fields, properties, etc). And, just like any other kind of member an event also has a type. Yet, in the case of an event, the type of the event must be specified by a delegate. For instance, you CANNOT declare an event of a type defined by an interface.
Concluding, we can make the following Observation: the type of an event MUST be defined by a delegate. This is the main relation between an event and a delegate and is described in the section II.18 Defining events of ECMA-335 (CLI) Partitions I to VI:
In typical usage, the TypeSpec (if present) identifies a delegate whose signature matches the arguments passed to the event’s fire method.
However, this fact does NOT imply that an event uses a backing delegate field. In truth, an event may use a backing field of any different data structure type of your choice. If you implement an event explicitly in C#, you are free to choose the way you store the event handlers (note that event handlers are instances of the type of the event, which in turn is mandatorily a delegate type---from the previous Observation). But, you can store those event handlers (which are delegate instances) in a data structure such as a List or a Dictionary or any other else, or even in a backing delegate field. But don’t forget that it is NOT mandatory that you use a delegate field.
NOTE: If you have access to C# 5.0 Unleashed, read the "Limitations on Plain Use of Delegates" in Chapter 18 titled "Events" to understand better the differences between the two.
It always helps me to have a simple, concrete example. So here's one for the community. First I show how you can use delegates alone to do what Events do for us. Then I show how the same solution would work with an instance of EventHandler. And then I explain why we DON'T want to do what I explain in the first example. This post was inspired by an article by John Skeet.
Example 1: Using public delegate
Suppose I have a WinForms app with a single drop-down box. The drop-down is bound to an List<Person>. Where Person has properties of Id, Name, NickName, HairColor. On the main form is a custom user control that shows the properties of that person. When someone selects a person in the drop-down the labels in the user control update to show the properties of the person selected.
Here is how that works. We have three files that help us put this together:
Mediator.cs -- static class holds the delegates
Form1.cs -- main form
DetailView.cs -- user control shows all details
Here is the relevant code for each of the classes:
class Mediator
{
public delegate void PersonChangedDelegate(Person p); //delegate type definition
public static PersonChangedDelegate PersonChangedDel; //delegate instance. Detail view will "subscribe" to this.
public static void OnPersonChanged(Person p) //Form1 will call this when the drop-down changes.
{
if (PersonChangedDel != null)
{
PersonChangedDel(p);
}
}
}
Here is our user control:
public partial class DetailView : UserControl
{
public DetailView()
{
InitializeComponent();
Mediator.PersonChangedDel += DetailView_PersonChanged;
}
void DetailView_PersonChanged(Person p)
{
BindData(p);
}
public void BindData(Person p)
{
lblPersonHairColor.Text = p.HairColor;
lblPersonId.Text = p.IdPerson.ToString();
lblPersonName.Text = p.Name;
lblPersonNickName.Text = p.NickName;
}
}
Finally we have the following code in our Form1.cs. Here we are Calling OnPersonChanged, which calls any code subscribed to the delegate.
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Mediator.OnPersonChanged((Person)comboBox1.SelectedItem); //Call the mediator's OnPersonChanged method. This will in turn call all the methods assigned (i.e. subscribed to) to the delegate -- in this case `DetailView_PersonChanged`.
}
Ok. So that's how you would get this working without using events and just using delegates. We just put a public delegate into a class -- you can make it static or a singleton, or whatever. Great.
BUT, BUT, BUT, we do not want to do what I just described above. Because public fields are bad for many, many reason. So what are our options? As John Skeet describes, here are our options:
A public delegate variable (this is what we just did above. don't do this. i just told you above why it's bad)
Put the delegate into a property with a get/set (problem here is that subscribers could override each other -- so we could subscribe a bunch of methods to the delegate and then we could accidentally say PersonChangedDel = null, wiping out all of the other subscriptions. The other problem that remains here is that since the users have access to the delegate, they can invoke the targets in the invocation list -- we don't want external users having access to when to raise our events.
A delegate variable with AddXXXHandler and RemoveXXXHandler methods
This third option is essentially what an event gives us. When we declare an EventHandler, it gives us access to a delegate -- not publicly, not as a property, but as this thing we call an event that has just add/remove accessors.
Let's see what the same program looks like, but now using an Event instead of the public delegate (I've also changed our Mediator to a singleton):
Example 2: With EventHandler instead of a public delegate
Mediator:
class Mediator
{
private static readonly Mediator _Instance = new Mediator();
private Mediator() { }
public static Mediator GetInstance()
{
return _Instance;
}
public event EventHandler<PersonChangedEventArgs> PersonChanged; //this is just a property we expose to add items to the delegate.
public void OnPersonChanged(object sender, Person p)
{
var personChangedDelegate = PersonChanged as EventHandler<PersonChangedEventArgs>;
if (personChangedDelegate != null)
{
personChangedDelegate(sender, new PersonChangedEventArgs() { Person = p });
}
}
}
Notice that if you F12 on the EventHandler, it will show you the definition is just a generic-ified delegate with the extra "sender" object:
public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);
The User Control:
public partial class DetailView : UserControl
{
public DetailView()
{
InitializeComponent();
Mediator.GetInstance().PersonChanged += DetailView_PersonChanged;
}
void DetailView_PersonChanged(object sender, PersonChangedEventArgs e)
{
BindData(e.Person);
}
public void BindData(Person p)
{
lblPersonHairColor.Text = p.HairColor;
lblPersonId.Text = p.IdPerson.ToString();
lblPersonName.Text = p.Name;
lblPersonNickName.Text = p.NickName;
}
}
Finally, here's the Form1.cs code:
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Mediator.GetInstance().OnPersonChanged(this, (Person)comboBox1.SelectedItem);
}
Because the EventHandler wants and EventArgs as a parameter, I created this class with just a single property in it:
class PersonChangedEventArgs
{
public Person Person { get; set; }
}
Hopefully that shows you a bit about why we have events and how they are different -- but functionally the same -- as delegates.
You can also use events in interface declarations, not so for delegates.
Delegate is a type-safe function pointer. Event is an implementation of publisher-subscriber design pattern using delegate.
An event in .net is a designated combination of an Add method and a Remove method, both of which expect some particular type of delegate. Both C# and vb.net can auto-generate code for the add and remove methods which will define a delegate to hold the event subscriptions, and add/remove the passed in delegagte to/from that subscription delegate. VB.net will also auto-generate code (with the RaiseEvent statement) to invoke the subscription list if and only if it is non-empty; for some reason, C# doesn't generate the latter.
Note that while it is common to manage event subscriptions using a multicast delegate, that is not the only means of doing so. From a public perspective, a would-be event subscriber needs to know how to let an object know it wants to receive events, but it does not need to know what mechanism the publisher will use to raise the events. Note also that while whoever defined the event data structure in .net apparently thought there should be a public means of raising them, neither C# nor vb.net makes use of that feature.
To define about event in simple way:
Event is a REFERENCE to a delegate with two restrictions
Cannot be invoked directly
Cannot be assigned values directly (e.g eventObj = delegateMethod)
Above two are the weak points for delegates and it is addressed in event. Complete code sample to show the difference in fiddler is here https://dotnetfiddle.net/5iR3fB .
Toggle the comment between Event and Delegate and client code that invokes/assign values to delegate to understand the difference
Here is the inline code.
/*
This is working program in Visual Studio. It is not running in fiddler because of infinite loop in code.
This code demonstrates the difference between event and delegate
Event is an delegate reference with two restrictions for increased protection
1. Cannot be invoked directly
2. Cannot assign value to delegate reference directly
Toggle between Event vs Delegate in the code by commenting/un commenting the relevant lines
*/
public class RoomTemperatureController
{
private int _roomTemperature = 25;//Default/Starting room Temperature
private bool _isAirConditionTurnedOn = false;//Default AC is Off
private bool _isHeatTurnedOn = false;//Default Heat is Off
private bool _tempSimulator = false;
public delegate void OnRoomTemperatureChange(int roomTemperature); //OnRoomTemperatureChange is a type of Delegate (Check next line for proof)
// public OnRoomTemperatureChange WhenRoomTemperatureChange;// { get; set; }//Exposing the delegate to outside world, cannot directly expose the delegate (line above),
public event OnRoomTemperatureChange WhenRoomTemperatureChange;// { get; set; }//Exposing the delegate to outside world, cannot directly expose the delegate (line above),
public RoomTemperatureController()
{
WhenRoomTemperatureChange += InternalRoomTemperatuerHandler;
}
private void InternalRoomTemperatuerHandler(int roomTemp)
{
System.Console.WriteLine("Internal Room Temperature Handler - Mandatory to handle/ Should not be removed by external consumer of ths class: Note, if it is delegate this can be removed, if event cannot be removed");
}
//User cannot directly asign values to delegate (e.g. roomTempControllerObj.OnRoomTemperatureChange = delegateMethod (System will throw error)
public bool TurnRoomTeperatureSimulator
{
set
{
_tempSimulator = value;
if (value)
{
SimulateRoomTemperature(); //Turn on Simulator
}
}
get { return _tempSimulator; }
}
public void TurnAirCondition(bool val)
{
_isAirConditionTurnedOn = val;
_isHeatTurnedOn = !val;//Binary switch If Heat is ON - AC will turned off automatically (binary)
System.Console.WriteLine("Aircondition :" + _isAirConditionTurnedOn);
System.Console.WriteLine("Heat :" + _isHeatTurnedOn);
}
public void TurnHeat(bool val)
{
_isHeatTurnedOn = val;
_isAirConditionTurnedOn = !val;//Binary switch If Heat is ON - AC will turned off automatically (binary)
System.Console.WriteLine("Aircondition :" + _isAirConditionTurnedOn);
System.Console.WriteLine("Heat :" + _isHeatTurnedOn);
}
public async void SimulateRoomTemperature()
{
while (_tempSimulator)
{
if (_isAirConditionTurnedOn)
_roomTemperature--;//Decrease Room Temperature if AC is turned On
if (_isHeatTurnedOn)
_roomTemperature++;//Decrease Room Temperature if AC is turned On
System.Console.WriteLine("Temperature :" + _roomTemperature);
if (WhenRoomTemperatureChange != null)
WhenRoomTemperatureChange(_roomTemperature);
System.Threading.Thread.Sleep(500);//Every second Temperature changes based on AC/Heat Status
}
}
}
public class MySweetHome
{
RoomTemperatureController roomController = null;
public MySweetHome()
{
roomController = new RoomTemperatureController();
roomController.WhenRoomTemperatureChange += TurnHeatOrACBasedOnTemp;
//roomController.WhenRoomTemperatureChange = null; //Setting NULL to delegate reference is possible where as for Event it is not possible.
//roomController.WhenRoomTemperatureChange.DynamicInvoke();//Dynamic Invoke is possible for Delgate and not possible with Event
roomController.SimulateRoomTemperature();
System.Threading.Thread.Sleep(5000);
roomController.TurnAirCondition (true);
roomController.TurnRoomTeperatureSimulator = true;
}
public void TurnHeatOrACBasedOnTemp(int temp)
{
if (temp >= 30)
roomController.TurnAirCondition(true);
if (temp <= 15)
roomController.TurnHeat(true);
}
public static void Main(string []args)
{
MySweetHome home = new MySweetHome();
}
}
For people live in 2020, and want a clean answer...
Definitions:
delegate: defines a function pointer.
event: defines
(1) protected interfaces, and
(2) operations(+=, -=), and
(3) advantage: you don't need to use new keyword anymore.
Regarding the adjective protected:
// eventTest.SomeoneSay = null; // Compile Error.
// eventTest.SomeoneSay = new Say(SayHello); // Compile Error.
Also notice this section from Microsoft: https://learn.microsoft.com/en-us/dotnet/standard/events/#raising-multiple-events
Code Example:
with delegate:
public class DelegateTest
{
public delegate void Say(); // Define a pointer type "void <- ()" named "Say".
private Say say;
public DelegateTest() {
say = new Say(SayHello); // Setup the field, Say say, first.
say += new Say(SayGoodBye);
say.Invoke();
}
public void SayHello() { /* display "Hello World!" to your GUI. */ }
public void SayGoodBye() { /* display "Good bye!" to your GUI. */ }
}
with event:
public class EventTest
{
public delegate void Say();
public event Say SomeoneSay; // Use the type "Say" to define event, an
// auto-setup-everything-good field for you.
public EventTest() {
SomeoneSay += SayHello;
SomeoneSay += SayGoodBye;
SomeoneSay();
}
public void SayHello() { /* display "Hello World!" to your GUI. */ }
public void SayGoodBye() { /* display "Good bye!" to your GUI. */ }
}
Reference:
Event vs. Delegate - Explaining the important differences between the Event and Delegate patterns in C# and why they're useful.: https://dzone.com/articles/event-vs-delegate