Anybody know why this is not possible?
If the event is just a MulticastDelegate instance you should be able to reference it in the code. the compiler says EventA can only be on left side of -= or +=.
public delegate void MyDelegate();
public event MyDelegate EventA;
public void addHandlerToEvent(MulticastDelegate md,Delegate d){
md+=d;
}
///
addHandlerToEvent(EventA,new MyDelegate(delegate(){}));
An event is not a multicast delegate, just like a property is not a field.
C#'s event syntax wraps multicast delegates to make life easier by providing syntactic sugar for adding and removing handler delegates. Your event definition would be compiled to the following:
private MyDelegate _EventA;
public event MyDelegate EventA
{
[MethodImpl(MethodImplOptions.Synchronized)]
add
{
_EventA = (MyDelegate)Delegate.Combine(_EventA, value);
}
[MethodImpl(MethodImplOptions.Synchronized)]
remove
{
_EventA = (MyDelegate)Delegate.Remove(_EventA, value);
}
}
The add and remove methods within the expanded event definition are then called when you use operator += and operator -= on the event.
This is done in order to hide the internals of the multicast delegate, but expose a simple way to implement publishing/subscribing to events. Being able to get the underlying delegate (outside of the class where it's defined) would break that intentional encapsulation.
An event is a member which binds a pair of add/remove methods(*), each of which accepts a delegate corresponding to the event signature. By default, the C# compiler will for each event auto-define a MultiCast delegate field along with add and remove members which will take the passed-in delegate and add or remove them to/from that MulticastDelegate field. The statement myEvent += someMethod;, when performed outside the class defining the event, is the required syntactic shorthand for, essentially myEvent.AddHandler(someMethod), and myEvent -= someMethod; for myEvent.RemoveHandler(someMethod), except that there is no way in C# to call the add/remove methods except using the += and -= notation.
Things are a little tricky when using the += and -= notation within the class which defines an event, since the field defined by the auto-generated event code has the same name as the event, and the behavior of myEvent += someMethod; will vary with different versions of C#.
(*) Technically a trio, since an event also includes a 'raise' method, but in practice that method is essentially never used.
Events are backed by multicast delegates, yes, but their purpose is to provide an implementation of the observer pattern, where observers (delegates in this case) may only be registered (+=) or unregistered (-=). If normal access to the backing delegate were possible outside the class itself, then it would be possible for client code to interfere with an unrelated delegate that was registered elsewhere, which could mess things up. It's also somewhat beside the point of the observer pattern to look at which other things are observing the event in question.
If you need to perform this kind of manipulation of the backing delegate, it must be done within the class (where it's treated as a regular delegate rather than an event).
You can also implement the backing delegate explicitly and provide an accessor to register/unregister with it:
private EventHandler SomeEvent;
public event EventHandler
{
add
{
SomeEvent += value;
}
remove
{
SomeEvent -= value;
}
}
This way you can provide direct access to the delegate if you need to. It's still best to provide public access to it for the purposes of subscribing/unsubscribing as an event though (rather than a raw delegate), otherwise you can run into thread-safety problems with data races on the delegates.
Related
Basically what the title says.
Whats the difference between those two (I am currently using the first one)
private EventHandler _progressEvent;
and
private event EventHandler _progressEvent;
I have a method
void Download(string url, EventHandler progressEvent) {
doDownload(url)
this._progressEvent = progressEvent;
}
The doDownload method would call the
_progressEvent(this, new EventArgs());
It works fine, so far. But I feel I am doing something horribly wrong.
The first defines a delegate, the second defines an event. The two are related, but they're typically used very differently.
In general, if you're using EventHandler or EventHandler<T>, this would suggest that you're using an event. The caller (for handling progress) would typically subscribe to the event (not pass in a delegate), and you'd raise the event if you have subscribers.
If you want to use a more functional approach, and pass in a delegate, I would choose a more appropriate delegate to use. In this case, you're not really providing any information in the EventArgs, so perhaps just using System.Action would be more appropriate.
That being said, an event approach appears more appropriate here, from the little code shown. For details on using events, see Events in the C# Programming Guide.
Your code, using events, would likely look something like:
// This might make more sense as a delegate with progress information - ie: percent done?
public event EventHandler ProgressChanged;
public void Download(string url)
{
// ... No delegate here...
When you call your progress, you'd write:
var handler = this.ProgressChanged;
if (handler != null)
handler(this, EventArgs.Empty);
The user of this would write this as:
yourClass.ProgressChanged += myHandler;
yourClass.Download(url);
For private, there is no difference between the two, but for public you would want to use event.
The event keyword is an access modifier like private internal and protected.
It is used for restricting access to multicast delegates. https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/event
Going from most visible to least visible we have
public EventHandler _progressEvent;
//can be assigned to from outside the class (using = and multicast using +=)
//can be invoked from outside the class
public event EventHandler _progressEvent;
//can be bound to from outside the class (using +=), but can't be assigned (using =)
//can only be invoked from inside the class
private EventHandler _progressEvent;
//can be bound from inside the class (using = or +=)
//can be invoked inside the class
private event EventHandler _progressEvent;
//Same as private. given event restricts the use of the delegate from outside
// the class - i don't see how private is different to private event
public class TestA
{
public event Action<int> updateEvent;
...
if(updateEvent != null)
{
Actin<int> tempEvent = updateEvent;
updateEvent(1);
}
}
public class TestB
{
public Action<int> updateEvent;
...
if(updateEvent != null)
{
Actin<int> tempEvent = updateEvent;
updateEvent(1);
}
}
however event Action<T> is eventQueue au
use same and resutl same. What is more good? (i like TestB. Because simple using.)
The event modifier is useful to prevent an assignment. What does it mean? If that's a callback with many handlers attached you may want to prevent that someone simply write:
myObject.UpdateEvent = null or something like that. With event they can't do that. Take a look at Learning C# 3.0: Master the fundamentals of C# 3.0.
The code you use to call the event isn't bad but not so useful, if you assign the delegate to temporary variable you may want to be somehow thread-safe (someone could unhook after your null check) so you should do it before the check:
Action<int> tempEvent = updateEvent;
if (tempEvent != null)
tempEvent(1);
A little side note: if you use events you may want to follow .NET Framework patterns and guidelines, take a look at MSDN
To the compiler, the difference is that the event cannot be invoked except by the declaring class (like a private member). You can use either in the same way.
To design, events should represent their namesake, i.e. events. Think of a Delegate as just the type or signature of methods. You would declare a class to have an event (of type Action), and you would add (subscribe) to that event other Actions (or methods that can be cast to an Action).
Furthermore, it is common practice to create events of a type that inherits from EventHandler<T>
An event is just a delegate with some access wrappers.
The problem with your TestB is that because the updateEvent delegate is public, ANYONE can fire the delegate, not just the class owning it.
Defining it as an Event as in your TestA, client code can only subscribe or unsubscribe to the event, they cannot fire it.
ps. You do not need your tempEvent delegates.
public delegate void SecondChangedHandler(
object clock,
TimeInfoEventArgs timeInformation);
public event SecondChangedHandler SecondChanged;
I have written a clock based on this article.
Now if i remove the event keyword i get the same result, so what does event really do?
It's compiled differently. It makes it so someone can't do
mySecondChangedHandler.SecondChanged = SomeMethod(...); //overwrite
mySecondChangedHandler.SecondChanged(...); //invoke
but only
mySecondChangedHandler.SecondChanged += SomeMethod(...);
mySecondChangedHandler.SecondChanged -= SomeMethod(...);
The event keyword creates a private delegate field, and a pair of public event accessors called add_EventName and remove_EventName. (details)
This means that writing EventName inside the class returns the delegate instance, allowing you to call or inspect the event handlers.
Outside the class, EventName doesn't really exist; all you can do is write EventName += something and EventName -= something, and the compiler will convert it into calls to the accessors. (like a property)
For more details, see this series of blog posts.
The event keyword does two things
It supplied permissions. Only the class can raise the event, however any external method can invoke the raw delegate
It provides metadata which can be used for designers and the like
The event keywords means only methods on the instance that hosts the SecondChanged field can invoke it. External attempts will fail.
The event keyword creates a pair of accessors for a delegate. These are effectively two methods (add and remove) that are called when you subscribe or unsubscribe from the event.
In your case, you're creating a "field-like event". The compiler makes a delegate behind the scenes, and allows you to subscribe and unsubscribe from it's invocation list. This means that you have all of the functionality of a delegate, but you're restricting access so that that outside world can "handle" the event, but not raise the event (invoke the delegate).
You can, however, also explicitly create your own accessors for an event, and these can do other things (though that's not typically recommended unless there is a good reason to do so).
Quote from C# lang reference about keyword event,
Events are a special kind of multicast delegate that can only be invoked from within the class or struct where they are declared (the publisher class).
I have an event Load
public delegate void OnLoad(int i);
public event OnLoad Load;
I subscribe to it with a method:
public void Go()
{
Load += (x) => { };
}
Is it possible to retrieve this method using reflection? How?
In this particular case you could, with reflection. However, in general, you can't. Events encapsulate the idea of subscribers subscribing and unsubscribing - and that's all. A subscriber isn't meant to find out what other subscribers there are.
A field-like event as you've just shown is simply backed by a field of the relevant delegate type, with autogenerated add/remove handlers which just use the field. However, there's nothing to say they have to be implemented like that. For example, an event can store its subscribers in an EventHandlerList, which is efficient if you have several events in a class and only a few of them are likely to be subscribed to.
Now I suppose you could try to find the body of the "add" handler, decompile it and work out how the event handlers are being stored, and fetch them that way... but please don't. You're creating a lot of work, just to break encapsulation. Just redesign your code so that you don't need to do this.
EDIT: I've been assuming that you're talking about getting the subscribers from outside the class declaring the event. If you're inside the class declaring the event, then it's easy, because you know how the event is being stored.
At that point, the problem goes from "fetching the subscribers of an event" to "fetching the individual delegates making up a multicast delegate" - and that's easy. As others have said, you can call Delegate.GetInvocationList to get an array of delegates... and then use the Delegate.Method property to get the method that that particular delegate targets.
Now, let's look again at your subscription code:
public void Go()
{
Load += (x) => { };
}
The method that's used to create the delegate here isn't Go... it's a method created by the C# compiler. It will have an "unspeakable name" (usually with angle brackets) so will look something like this:
[CompilerGenerated]
private static void <Go>b__0(int x)
{
}
Now, is that actually what you want to retrieve? Or were you really looking to find out which method performed the subscription, rather than which method was used as the subscribed handler?
If you call Load.GetInvocationList() you will be handed back an array of Delegate types. From the these types, you can access the MethodInfo.
You could use the GetInvocationList method which will give you all the subscribers.
Are there any reason why you should not declare an event backing field protected? For example to prevent having to create OnSomeEvent methods for all your events. For example like this:
protected SomeEventHandler someEvent;
readonly object someEventLock = new object();
public event SomeEventHandler SomeEvent
{
add
{
lock (someEventLock)
someEvent += value;
}
remove
{
lock (someEventLock)
someEvent -= value;
}
}
Of course the decending classes would have to remember to lock when raising the event, etc., but anyways.
Any reasons why this should not be done?
Encapsulation. It's exactly the point you make about subclasses having to remember to lock - which means exposing the lock field as well, etc. That's implementation detail which should be encapsulated by the class.
Creating an OnSomeEvent method means that subclasses don't need to know the details of how you're dealing with the event - they just have a way of raising it. This also reduces code duplication: there's one way of raising the event, rather than having that code all over the place.
I prefer all my fields to be private, unless they're public static readonly fields of immutable types (like string.Empty) - and even then I tend to prefer properties.