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 :)
The following methods are part of a base class which enables derived classes to specify who should be notified by an event.
protected void RaiseEvent<TEventArgs>(EventHandler<TEventArgs> updateEvent, TEventArgs eventArgs, UpdateReceivers updateReceivers)
where TEventArgs : EventArgs
{
EventHandler<TEventArgs> handler = updateEvent;
if (handler != null)
{
if (updateReceivers.ToAllSubscribers)
{
handler(this, eventArgs);
}
else
{
NotifySpecifiedReceiver(handler, eventArgs, updateReceivers.Receiver);
}
}
}
private void NotifySpecifiedReceiver<TEventArgs>(EventHandler<TEventArgs> handler, TEventArgs eventArgs, object updateReceiver)
where TEventArgs : EventArgs
{
foreach (Delegate #delegate in handler.GetInvocationList())
{
// is the delegates target our receiver?
// but this doesnt work for anonymous methods :(
if (#delegate.Target == updateReceiver)
{
try
{
#delegate.DynamicInvoke(this, eventArgs);
}
catch (Exception ex)
{
}
}
}
}
To notify only a specific receiver, the method is used like this: (the receiver has to be subscribed of course)
event EventHandler<SomethingChangedEventArgs> SomethingChanged;
RaiseEvent(SomethingChanged, args, UpdateReceivers.Single(receiver));
This will only raise the delegates "pointing" to the receiver.
My problem here is when i use a anonymous method to subscribe to the SomethingChanged event its doesnt work when i use this event to notify the object which subscribed to it.
class EventConsumer
{
private EventSource _eventSource;
private void SubscribeEvents()
{
// ReactOnEvent() will not be called because the check [#delegate.Target == updateReceiver] doesnt work for anonymous methods
_eventSource.MyStateChanged += (sender, e) => ReactOnEvent();
_eventSource.PublishCurrentState(this);
}
}
class EventSource
{
// multiple objects are subscribed to this event
event EventHandler<MyStateChangedEventArgs> MyStateChanged;
public void GetCurrentState(object receiver)
{
// receiver ask for the current state, only raise event for him
RaiseEvent(MyStateChanged, args, UpdateReceivers.Single(receiver));
}
}
Is it possible the get the instance containing a anonymous method? or should i use a complete different approach to solve my problem?
You're (presumably) seeing the compiler-generated closure class that contains variables used by the anonymous method.
This class has specifically-named fields that reference information from the parent class.
You can use reflection to find a field in the compiler-generated DisplayClass (the Target value) named <>4__this, and get its value to find the instance of the class that created the delegate.
However, don't do this.
This relies on an internal behavior of the C# compiler that may change at any time.
Also, the fields contained by the closure class depend on where the anonymous method is and what members it references. If the anonymous method doesn't use the class instance, it may not have a this field at all.
When I try to call the event myevent in the function fire, it gives compiler error as
delegate mydelegate does not take 0 arguments.
if i try to give arguments in calling myevent as
myevent(this);
it again shows error as it does not take 1 parameters. what parameters am i supposed to give in the calling myevent? Here's the program code:
namespace useofdelegates
{
public class Class1
{ int i;
public delegate void mydelegate(object sender,EventArgs e);
public event mydelegate myevent;
public void fire()
{
myevent(this); // *** shows compiler error ***
}
}
}
The compiler error is because you are using a delegate with two parameters and calling it with one.
The simple solution is
myevent(this, EventArgs.Empty);
The more regular (and thread-safe) solution, which I think you're getting confused with is
OnMyEvent(EventArgs.Empty);
// and then later
public void OnMyEvent(EventArgs args)
{
mydelegate handler = myevent;
if(handler != null) handler(this, args);
}
The reason for doing it this way is that you should always make sure that there is at least one handler attached to the event. There is a small chance that the handler can be detached between checking for null and calling the handler, so it's good to pass it on to another variable which it cannot be detached from before checking for null.
You need to pass in 2 parameters. the first one being the source of the event. In your example "this" would work. The second one being of type EventArgs or a realted subclass. You can also pass EventArgs.Empty is you do not wish to pass any event especific information to the event handler.
So your method would look like this
fire(this, EventArgs.Empty);
or
fire(this, new MyCustomEVentArgs());
Here is an MSDN link with an example of using events http://msdn.microsoft.com/en-us/library/aa645739(VS.71).aspx
and nother one on creating custom EventArg subclasses to use in your handler. http://www.devarticles.com/c/a/C-Sharp/Creating-Custom-Delegates-and-Events-in-C-sharp/
You have declared mydelegate to require an object and an EventArgs, so you have to pass these in. Of course, you don't have declare your delegate with these arguments. You could just declare it thus:
public delegate void mydelegate();
or just pass this as the sender:
public delegate void mydelegate(object sender);
Just create an empty EventArgs I guess.
myevent(this, new EventArgs());
Note, here is the full code:
namespace useofdelegates
{
public class Class1
{
int i;
public delegate void mydelegate(object sender, EventArgs e);
public event mydelegate myevent;
public void fire()
{
myevent(this, new EventArgs()); // *** shows compiler error ***
}
}
}
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
I had once a situation where I had to override the event handler in some derived class of mine. I could not unfortunately just "override" it since the implementation logic was bound strictly to that particular method implementation in the base class.
My natural idea was to first "unbind" the original event handler from the event and then to bind my own brand-new method. So I tried to use the "-=" operator like:
myObject.SomeEvent -= new EventHandler (BaseClass.SomeEventHandler)
at which point the compiler complainer that it had no access to the private method SomeEventHandler. Since the base class was a part of the library I did not really want to modify its implementation (though simply as it seems by just turning "private" into "protected");
It's so simple to attach an event handler to an event. Why is it so difficult to get rid of one?
Why I wanted it? Because the base implementation was casing some problems (just did not handle our particular case) so I wanted to redefine it. But even after I attached my event handler the base implementation was getting executed anyway. Since it was doing some kind of Redirect, my own implementation was never going to run since the request processing was being broken at server after redirect.
Is there anyway to get rid of an event handler in a base class from a library without modifying its implementation?
It would be just as easy to get rid of the handler if your code can officially see the handler. If absolutely necessary, it is possible to unsubscribe using reflection to get hold of the delegate, but this is messy. It would be a lot cleaner and safer to either a: avoid the requirement, or b: change the code to make the handler protected.
Reflection approach (use at own risk); uncomment the block in the Bar.ctor() to see the change.
using System;
class Foo
{
public event EventHandler SomeEvent;
public Foo()
{
SomeEvent += SecretHandler; // a bad idea, anyway
//(self-subscribed events....)
}
protected virtual void OnSomeEvent()
{
EventHandler handler = SomeEvent;
if (handler != null) handler(this, EventArgs.Empty);
}
private void SecretHandler(object sender, EventArgs args)
{
Console.WriteLine("suscribed");
}
}
class Bar : Foo
{
public Bar()
{
/*
MethodInfo method = typeof(Foo).GetMethod("SecretHandler",
BindingFlags.NonPublic | BindingFlags.Instance);
EventHandler handler = (EventHandler)Delegate.CreateDelegate(
typeof(EventHandler), this, method);
SomeEvent -= handler;
*/
}
public void Test()
{
OnSomeEvent();
}
}
static class Program
{
static void Main()
{
Bar bar = new Bar();
bar.Test();
}
}