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Event declaration with assignment to empty delegate
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Closed 4 years ago.
While going through C# example codes, I have found this declaration:
public event myEventHandler myEvent = delegate { };
What does this statement mean? It is not inline with other declarations of delegate I have seen on internet.
public event myEventHandler myEvent = delegate { };
Assigns an function to the event when an instance of the class is created by simply delegating it with anonymous function.
When assigning a function to event's invocation list, the following notation is used
myClass.myEvent += delegate { };
However the following assignment is also valid inside the parent class, but it basically discards all the existing invocation list unless it is used in ctor.
myEvent = delegate {};
Is there anything wrong with subscribing to event with
MyPopup.CustomPopupPlacementCallback = popupFixCentered;
instead of:
MyPopup.CustomPopupPlacementCallback += popupFixCentered;
For example, if I am changing to different callback from another method I want to make sure I have only one callback subscribed without needing to -= the correct one.
Well, it sounds like you didn't try it. If you did, you would get the following compilation error:
The event 'XXX' can only appear on the left hand side of += or -= (except when used from within the type 'YourClass')
The error is pretty clear: you can only use the += and -= operators on the event.
If you try to assign to the event from within the class that defines the event, then it will "work". But the reason it appears to be able to assign to an event in that case is because it's actually not accessing the event. It's accessing an auto-generated private delegate instance that you may not realize is actually there.
Quoting from Chris Burrows' article on the subject:
outside of the class or struct that defines a field-like event E, binding to the name E resolves to the event itself, on which the only legal operation is calling an accessor; inside the class or struct that defines a field-like event E, binding to the name E resolves to the private delegate field.
To understand this, you need to visualize that when you define an event such as:
public event EventHandler MyEvent;
... what you don't see, is that it actually gets translated into something like this (I'm copying this from Jon Skeet's article on events and delegates. Also note that the exact code it gets translated into has changed between versions of C#, so it may be a bit different, but the general idea is the same):
private EventHandler _myEvent;
public event EventHandler MyEvent
{
add
{
lock (this)
{
_myEvent += value;
}
}
remove
{
lock (this)
{
_myEvent -= value;
}
}
}
So when you access MyEvent from outside the class, you can only invoke the add and remove methods through the += and -= operators.
But from within the class, accessing MyEvent means something different. It actually becomes a reference to that private _myEvent delegate variable that you can't see, but it is there. Because this is a delegate type, then you can use the assignment (=) operator on it.
So, to achieve what you want, you could define a public method in the same class that defines the event, and use that method to set your new event handler.
Something like this:
public class MyClass
{
public event EventHandler MyEvent;
public void setSingleEventHandler(EventHandler eventHandler)
{
this.MyEvent = eventHandler;
}
}
But if you are going to do that, then it defeats the purpose of the event type. If you only want to invoke a single event handler at most at any given time, then defining it this way (without using the event keyword) makes more sense:
public class MyClass
{
public EventHandler MyEvent { get; set; }
}
References
Jon Skeet article: Delegates and Events
Chris Burrows article: (also check out the rest of the series): Events get a little overhaul in C# 4, Part II: Semantic Changes and +=/-=
I just tested it. Yes, you can use the = operator to assign to an event. (Edit: Apparently only from within the same class)
delegate void Foo();
event Foo bar;
Method()
{
bar = () => { Console.WriteLine("1"); };
bar();
bar = () => { Console.WriteLine("2"); };
bar();
}
Produces the output:
1
2
But if you try to assign from outside the class, it will give you an error.
You can get around this by using a java-style set method:
SetBar(Foo foo)
{
bar = foo;
}
Only time I'd ever recommend java convention for external access of properties :)
Typically we use this code:
private EventHandler _updateErrorIcons;
public event EventHandler UpdateErrorIcons
{
add { _updateErrorIcons += value; }
remove { _updateErrorIcons -= value; }
}
Is there a similar shortcut like with automatic properties?
Something like:
public event EventHandler UpdateErrorIcons { add; remove; }
Yep. Get rid of the { add; remove; } part and the backing delegate field and you're golden:
public event EventHandler UpdateErrorIcons;
That's it!
Let me just add that before you asked this question, I hadn't even thought about the fact that the auto-implemented version of events is inconsistent with that of properties. Personally, I would actually prefer it if auto-implemented events worked the way you first attempted in your question. It would be more consistent, and it would also serve as a mental reminder that events are not identical to delegate fields, just like properties are not identical to regular fields.
Honestly, I think you're the rare exception where you actually knew about the custom syntax first. A lot of .NET developers have no clue there's an option to implement your own add and remove methods at all.
Update: Just for your own peace of mind, I have confirmed using Reflector that the default implementation of events in C# 4 (i.e., the implementation that gets generated when you go the auto-implemented route) is equivalent to this:
private EventHandler _updateErrorIcons;
public event EventHandler UpdateErrorIcons
{
add
{
EventHandler current, original;
do
{
original = _updateErrorIcons;
EventHandler updated = (EventHandler)Delegate.Combine(original, value);
current = Interlocked.CompareExchange(ref _updateErrorIcons, updated, original);
}
while (current != original);
}
remove
{
// Same deal, only with Delegate.Remove instead of Delegate.Combine.
}
}
Note that the above utilizes lock-free synchronization to effectively serialize add and remove calls. So if you're using the latest C# compiler, you don't need to implement add/remove yourself even for synchronization.
public event EventHandler UpdateErrorIcons;
is just fine
you can use
yourObbject.UpdateErrorIcons += YourFunction;
add {} and remove {} are used only in special cases where you need to handle event hookups manually. Us mere mortals normally just use public event EventHandler UpdateErrorIcons; where "EventHandler" is the delegate of choice.
For instance:
public delegate void MyEventDelegate(object sender, string param1);
public event MyEventDelegate MyEvent;
Note that because MyEvent is null if it doesn't have any listeners you need to check if it is null before invoking it. A standard method for doing this check is:
public void InvokeMyEvent(string param1)
{
MyEventDelegate myEventDelegate = MyEvent;
if (myEventDelegate != null)
myEventDelegate(this, param1);
}
A key element in this check is to make a copy of the object in question first and then work only on the copy. If not you could get a rare race condition where another thread unhooks between your if and your call.
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
This question already has answers here:
How to ensure an event is only subscribed to once
(8 answers)
Closed 8 years ago.
Duplicate of: How to ensure an event is only subscribed to once
and Has an event handler already been added?
I have a singleton that provides some service and my classes hook into some events on it, sometimes a class is hooking twice to the event and then gets called twice.
I'm looking for a classical way to prevent this from happening. somehow I need to check if I've already hooked to this event...
How about just removing the event first with -= , if it is not found an exception is not thrown
/// -= Removes the event if it has been already added, this prevents multiple firing of the event
((System.Windows.Forms.WebBrowser)sender).Document.Click -= new System.Windows.Forms.HtmlElementEventHandler(testii);
((System.Windows.Forms.WebBrowser)sender).Document.Click += new System.Windows.Forms.HtmlElementEventHandler(testii);
Explicitly implement the event and check the invocation list. You'll also need to check for null:
using System.Linq; // Required for the .Contains call below:
...
private EventHandler foo;
public event EventHandler Foo
{
add
{
if (foo == null || !foo.GetInvocationList().Contains(value))
{
foo += value;
}
}
remove
{
foo -= value;
}
}
Using the code above, if a caller subscribes to the event multiple times, it will simply be ignored.
I've tested each solution and the best one (considering performance) is:
private EventHandler _foo;
public event EventHandler Foo {
add {
_foo -= value;
_foo += value;
}
remove {
_foo -= value;
}
}
No Linq using required. No need to check for null before cancelling a subscription (see MS EventHandler for details). No need to remember to do the unsubscription everywhere.
You really should handle this at the sink level and not the source level. That is, don't prescribe event handler logic at the event source - leave that to the handlers (the sinks) themselves.
As the developer of a service, who are you to say that sinks can only register once? What if they want to register twice for some reason? And if you are trying to correct bugs in the sinks by modifying the source, it's again a good reason for correcting these issues at the sink-level.
I'm sure you have your reasons; an event source for which duplicate sinks are illegal is not unfathomable. But perhaps you should consider an alternate architecture that leaves the semantics of an event intact.
You need to implement the add and remove accessors on the event, and then check the target list of the delegate, or store the targets in a list.
In the add method, you can use the Delegate.GetInvocationList method to obtain a list of the targets already added to the delegate.
Since delegates are defined to compare equal if they're linked to the same method on the same target object, you could probably run through that list and compare, and if you find none that compares equal, you add the new one.
Here's sample code, compile as console application:
using System;
using System.Linq;
namespace DemoApp
{
public class TestClass
{
private EventHandler _Test;
public event EventHandler Test
{
add
{
if (_Test == null || !_Test.GetInvocationList().Contains(value))
_Test += value;
}
remove
{
_Test -= value;
}
}
public void OnTest()
{
if (_Test != null)
_Test(this, EventArgs.Empty);
}
}
class Program
{
static void Main()
{
TestClass tc = new TestClass();
tc.Test += tc_Test;
tc.Test += tc_Test;
tc.OnTest();
Console.In.ReadLine();
}
static void tc_Test(object sender, EventArgs e)
{
Console.Out.WriteLine("tc_Test called");
}
}
}
Output:
tc_Test called
(ie. only once)
Microsoft's Reactive Extensions (Rx) framework can also be used to do "subscribe only once".
Given a mouse event foo.Clicked, here's how to subscribe and receive only a single invocation:
Observable.FromEvent<MouseEventArgs>(foo, nameof(foo.Clicked))
.Take(1)
.Subscribe(MyHandler);
...
private void MyHandler(IEvent<MouseEventArgs> eventInfo)
{
// This will be called just once!
var sender = eventInfo.Sender;
var args = eventInfo.EventArgs;
}
In addition to providing "subscribe once" functionality, the RX approach offers the ability to compose events together or filter events. It's quite nifty.
Create an Action instead of an event. Your class may look like:
public class MyClass
{
// sender arguments <----- Use this action instead of an event
public Action<object, EventArgs> OnSomeEventOccured;
public void SomeMethod()
{
if(OnSomeEventOccured!=null)
OnSomeEventOccured(this, null);
}
}
have your singleton object check it's list of who it notifies and only call once if duplicated. Alternatively if possible reject event attachment request.
In silverlight you need to say e.Handled = true; in the event code.
void image_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
e.Handled = true; //this fixes the double event fire problem.
string name = (e.OriginalSource as Image).Tag.ToString();
DoSomething(name);
}
Please tick me if this helps.