Is there any different between declaring event Action<> and event EventHandler<>.
Assuming it doesn't matter what object actually raised an event.
for example:
public event Action<bool, int, Blah> DiagnosticsEvent;
vs
public event EventHandler<DiagnosticsArgs> DiagnosticsEvent;
class DiagnosticsArgs : EventArgs
{
public DiagnosticsArgs(bool b, int i, Blah bl)
{...}
...
}
usage would be almost the same in both cases:
obj.DiagnosticsEvent += HandleDiagnosticsEvent;
There are several things that I don’t like about event EventHandler<> pattern:
Extra type declaration derived from
EventArgs
Compulsory passing of object source –
often no one cares
More code means more code to maintain without any clear advantage.
As a result, I prefer event Action<>
However, only if there are too many type arguments in Action<>, then an extra class would be required.
Based on some of the previous answers, I'm going to break my answer down into three areas.
First, physical limitations of using Action<T1, T2, T2... > vs using a derived class of EventArgs. There are three: First, if you change the number or types of parameters, every method that subscribes to will have to be changed to conform to the new pattern. If this is a public facing event that 3rd party assemblies will be using, and there is any possiblity that the event args would change, this would be a reason to use a custom class derived from event args for consistencies sake (remember, you COULD still use an Action<MyCustomClass>) Second, using Action<T1, T2, T2... > will prevent you from passing feedback BACK to the calling method unless you have a some kind of object (with a Handled property for instance) that is passed along with the Action. Third, you don't get named parameters, so if you're passing 3 bool's an int, two string's, and a DateTime, you have no idea what the meaning of those values are. As a side note, you can still have a "Fire this event safely method while still using Action<T1, T2, T2... >".
Secondly, consistency implications. If you have a large system you're already working with, it's nearly always better to follow the way the rest of the system is designed unless you have an very good reason not too. If you have publicly facing events that need to be maintained, the ability to substitute derived classes can be important. Keep that in mind.
Thirdly, real life practice, I personally find that I tend to create a lot of one off events for things like property changes that I need to interact with (Particularly when doing MVVM with view models that interact with each other) or where the event has a single parameter. Most of the time these events take on the form of public event Action<[classtype], bool> [PropertyName]Changed; or public event Action SomethingHappened;. In these cases, there are two benefits. First, I get a type for the issuing class. If MyClass declares and is the only class firing the event, I get an explicit instance of MyClass to work with in the event handler. Secondly, for simple events such as property change events, the meaning of the parameters is obvious and stated in the name of the event handler and I don't have to create a myriad of classes for these kinds of events.
The main difference will be that if you use Action<> your event will not follow the design pattern of virtually any other event in the system, which I would consider a drawback.
One upside with the dominating design pattern (apart from the power of sameness) is that you can extend the EventArgs object with new properties without altering the signature of the event. This would still be possible if you used Action<SomeClassWithProperties>, but I don't really see the point with not using the regular approach in that case.
The advantage of a wordier approach comes when your code is inside a 300,000 line project.
Using the action, as you have, there is no way to tell me what bool, int, and Blah are. If your action passed an object that defined the parameters then ok.
Using an EventHandler that wanted an EventArgs and if you would complete your DiagnosticsArgs example with getters for the properties that commented their purpose then you application would be more understandable. Also, please comment or fully name the arguments in the DiagnosticsArgs constructor.
I realize that this question is over 10 years old, but it appears to me that not only has the most obvious answer not been addressed, but that maybe its not really clear from the question a good understanding of what goes on under the covers. In addition, there are other questions about late binding and what that means with regards to delegates and lambdas (more on that later).
First to address the 800 lb elephant/gorilla in the room, when to choose event vs Action<T>/Func<T>:
Use a lambda to execute one statement or method. Use event when you
want more of a pub/sub model with multiple
statements/lambdas/functions that will execute (this is a major
difference right off the bat).
Use a lambda when you want to compile statements/functions to expression trees. Use delegates/events when you want to participate in more traditional late binding such as used in reflection and COM interop.
As an example of an event, lets wire up a simple and 'standard' set of events using a small console application as follows:
public delegate void FireEvent(int num);
public delegate void FireNiceEvent(object sender, SomeStandardArgs args);
public class SomeStandardArgs : EventArgs
{
public SomeStandardArgs(string id)
{
ID = id;
}
public string ID { get; set; }
}
class Program
{
public static event FireEvent OnFireEvent;
public static event FireNiceEvent OnFireNiceEvent;
static void Main(string[] args)
{
OnFireEvent += SomeSimpleEvent1;
OnFireEvent += SomeSimpleEvent2;
OnFireNiceEvent += SomeStandardEvent1;
OnFireNiceEvent += SomeStandardEvent2;
Console.WriteLine("Firing events.....");
OnFireEvent?.Invoke(3);
OnFireNiceEvent?.Invoke(null, new SomeStandardArgs("Fred"));
//Console.WriteLine($"{HeightSensorTypes.Keyence_IL030}:{(int)HeightSensorTypes.Keyence_IL030}");
Console.ReadLine();
}
private static void SomeSimpleEvent1(int num)
{
Console.WriteLine($"{nameof(SomeSimpleEvent1)}:{num}");
}
private static void SomeSimpleEvent2(int num)
{
Console.WriteLine($"{nameof(SomeSimpleEvent2)}:{num}");
}
private static void SomeStandardEvent1(object sender, SomeStandardArgs args)
{
Console.WriteLine($"{nameof(SomeStandardEvent1)}:{args.ID}");
}
private static void SomeStandardEvent2(object sender, SomeStandardArgs args)
{
Console.WriteLine($"{nameof(SomeStandardEvent2)}:{args.ID}");
}
}
The output will look as follows:
If you did the same with Action<int> or Action<object, SomeStandardArgs>, you would only see SomeSimpleEvent2 and SomeStandardEvent2.
So whats going on inside of event?
If we expand out FireNiceEvent, the compiler is actually generating the following (I have omitted some details with respect to thread synchronization that isn't relevant to this discussion):
private EventHandler<SomeStandardArgs> _OnFireNiceEvent;
public void add_OnFireNiceEvent(EventHandler<SomeStandardArgs> handler)
{
Delegate.Combine(_OnFireNiceEvent, handler);
}
public void remove_OnFireNiceEvent(EventHandler<SomeStandardArgs> handler)
{
Delegate.Remove(_OnFireNiceEvent, handler);
}
public event EventHandler<SomeStandardArgs> OnFireNiceEvent
{
add
{
add_OnFireNiceEvent(value)
}
remove
{
remove_OnFireNiceEvent(value)
}
}
The compiler generates a private delegate variable which is not visible to the class namespace in which it is generated. That delegate is what is used for subscription management and late binding participation, and the public facing interface is the familiar += and -= operators we have all come to know and love : )
You can customize the code for the add/remove handlers by changing the scope of the FireNiceEvent delegate to protected. This now allows developers to add custom hooks to the hooks, such as logging or security hooks. This really makes for some very powerful features that now allows for customized accessibility to subscription based on user roles, etc. Can you do that with lambdas? (Actually you can by custom compiling expression trees, but that's beyond the scope of this response).
To address a couple of points from some of the responses here:
There really is no difference in the 'brittleness' between changing
the args list in Action<T> and changing the properties in a class
derived from EventArgs. Either will not only require a compile
change, they will both change a public interface and will require
versioning. No difference.
With respect to which is an industry standard, that depends on where
this is being used and why. Action<T> and such is often used in IoC
and DI, and event is often used in message routing such as GUI and
MQ type frameworks. Note that I said often, not always.
Delegates have different lifetimes than lambdas. One also has to be
aware of capture... not just with closure, but also with the notion
of 'look what the cat dragged in'. This does affect memory
footprint/lifetime as well as management a.k.a. leaks.
One more thing, something I referenced earlier... the notion of late binding. You will often see this when using framework like LINQ, regarding when a lambda becomes 'live'. That is very different than late binding of a delegate, which can happen more than once (i.e. the lambda is always there, but binding occurs on demand as often as is needed), as opposed to a lambda, which once it occurs, its done -- the magic is gone, and the method(s)/property(ies) will always bind. Something to keep in mind.
On the most part, I'd say follow the pattern. I have deviated from it, but very rarely, and for specific reasons. In the case in point, the biggest issue I'd have is that I'd probably still use an Action<SomeObjectType>, allowing me to add extra properties later, and to use the occasional 2-way property (think Handled, or other feedback-events where the subscriber needs to to set a property on the event object). And once you've started down that line, you might as well use EventHandler<T> for some T.
If you follow the standard event pattern, then you can add an extension method to make the checking of event firing safer/easier. (i.e. the following code adds an extension method called SafeFire() which does the null check, as well as (obviously) copying the event into a separate variable to be safe from the usual null race-condition that can affect events.)
(Although I am in kind of two minds whether you should be using extension methods on null objects...)
public static class EventFirer
{
public static void SafeFire<TEventArgs>(this EventHandler<TEventArgs> theEvent, object obj, TEventArgs theEventArgs)
where TEventArgs : EventArgs
{
if (theEvent != null)
theEvent(obj, theEventArgs);
}
}
class MyEventArgs : EventArgs
{
// Blah, blah, blah...
}
class UseSafeEventFirer
{
event EventHandler<MyEventArgs> MyEvent;
void DemoSafeFire()
{
MyEvent.SafeFire(this, new MyEventArgs());
}
static void Main(string[] args)
{
var x = new UseSafeEventFirer();
Console.WriteLine("Null:");
x.DemoSafeFire();
Console.WriteLine();
x.MyEvent += delegate { Console.WriteLine("Hello, World!"); };
Console.WriteLine("Not null:");
x.DemoSafeFire();
}
}
Looking at Standard .NET event patterns we find
The standard signature for a .NET event delegate is:
void OnEventRaised(object sender, EventArgs args);
[...]
The argument list contains two arguments: the sender, and the event arguments. The compile time type of sender is System.Object, even though you likely know a more derived type that would always be correct. By convention, use object.
Below on same page we find an example of the typical event definition which is something like
public event EventHandler<EventArgs> EventName;
Had we defined
class MyClass
{
public event Action<MyClass, EventArgs> EventName;
}
the handler could have been
void OnEventRaised(MyClass sender, EventArgs args);
where sender has the correct (more derived) type.
Related
I am trying to make some events the someone can subscribe to, But i want to give the option to use different types of voids that can be used. See the following for clarification:
public event Action<float, float> MouseMove;
public event Action<Vector2> MouseMove;
public event Action MouseMove;
sending of the events
MouseMove(MouseX, MouseY);
MouseMove(new Vector2(MouseX, MouseY));
MouseMove();
So now in threory the user can subscribe in the following ways:
c.MouseMove += DoWithMouseMove;
And now this user has the ability to create the following types of functions depending on his needs like so:
static void DoWithMouseMove(float x, float y)
{
Console.WriteLine(x +" "+ y);
}
or use the following pattern:
static void DoWithMouseMove()
{
Console.WriteLine("Mouse is moving");
}
But this isnt possible because i get the following error:
already contains a definition for 'MouseMove'.
On the class that contains the events
How can i do this or is this not possible?
If something is unclear or needs further clarification let me know, so i can edit the question!
To my knowledge, it is impossible.
Events are little more then a wrapper around a Delegate. They add a public add and remove function, while turning the backing variable Private.
And I do not know of a way to "mix" delegates in the same delegate variable.
Plus even if you could do that, how would you sensibly raise those events? You as the writer of this class are responsible to raise those events and fill all those Parameters. Ideally using a protected RaiseEvent() function.
It is also wierd why you think you need this. In your example, you would propably implement the first option. A event user that does not care for the values (public event Action MouseMove;) would just not retreive those values. And a user that needs a vector (public event Action<Vector2> MouseMove;) could just build one himself from those values.
Edit: I also second that you should follow the established Event pattern: (object sender, customEventArgsInstance e). Do not throw arguments just randomly into the signature, that makes it hard to proces your event.
For a simple example, if I had some sort of button UI class, could I write a function that takes an expression that points to its Click event handler:
SomeMethod<SomeButtonClass>(button => button.Click);
I'm trying to eliminate some magic strings currently being used for a system to make events awaitable. The code in question is derived from a blog post by Frank Krueger (a worthwhile read, if you want some background).
public static Task<TEventArgs> GetEventAsync<TEventArgs>(this object eventSource, string eventName) where TEventArgs : EventArgs {
//...
Type type = eventSource.GetType();
EventInfo ev = type.GetEvent(eventName);
//...
}
While the specifics inside probably aren't important, the full method allows you to use an Event triggering as the completion source for a Task, making it easier to manage with await. For some class that raises an event, you can tie into a Task based on that event with a simple call.
Task<EventArgs> eventTask = someEventCausingObject.GetEventAsync<EventArgs>("SomeEventHandler");
// traditionally used as someEventCausingObject.SomeEventHandler += ...;
await eventTask;
// Proceed back here when SomeEventHandler event is raised.
I have been using this happily for a couple projects, but it has its drawbacks, one of the biggest being the use of hard-coded event name strings. This makes event name changes turn into runtime exceptions, and determining usage of the event is difficult.
I started trying to make a version that would allow the EventHandler to be passed in as part of an Expression with the goal of something like this:
await someEventCausingObject.GetEventAsync<EventCausingClass, EventArgs>(x => x.SomeEventHandler);
...with the corresponding method signature...
public static Task<TEventArgs> GetEventAsync<TSource, TEventArgs>(this TSource eventSource, Expression<Func<TSource, EventHandler>> eventHandlerExpression) where TEventArgs : EventArgs {
//...
}
Unfortunately, the lambda expression in the calling code causes a compile error:
Error CS0070: The event `SomeEventHandler' can only appear on the left hand side of += or -= when used outside of the type `EventCausingClass'.
This makes some sense given how event handlers are typically used, but I was hoping to find a better solution going forward than the pre-specified string name. It seems searches for combinations of "expression" and "eventhandler" all tend to be polluted with people describing lambda expressions for beginning += event handler assignment. I'm hoping I am missing something obvious here.
No, it is not possible to target an event. Basically event is not a real type member, but just C# syntax which produces add_EventName and remove_EventName methods pair.
You could try refer to these internal methods name, but it's not possible in C# - http://msdn.microsoft.com/en-us/library/z47a7kdw.aspx
There are many similar questions in SO, with the same answer NO - like this one from Jon Skeet https://stackoverflow.com/a/4756021/2170171
If you're real crazy, you can try something like
private static void Subscribe(Action addHandler)
{
var IL = addHandler.Method.GetMethodBody().GetILAsByteArray();
// Magic here, in which we understand ClassName and EventName
???
}
with usage like
Subscribe(() => new Button().Click += null);
You could try using Cecil http://www.mono-project.com/Cecil for analyzing IL, or implement your own logic as it should not be too hard for predictable line of code.
I don't think that it is good solution though, as it just replaces one headache (proper event naming) with another one (proper Subscribe calling). Though, it will help with rename stuff.
I just realized static events exist - and I'm curious how people use them. I wonder how the relative comparison holds up to static vs. instance methods. For instance, a static method is basically a global function. But I've always associated events with instances of objects and I'm having trouble thinking of them at the global level.
Here some code to refer to if it helps an explanation:
void Main()
{
var c1 = new C1();
c1.E1 += () => Console.WriteLine ("E1");
C1.E2 += () => Console.WriteLine ("E2");
c1.F1();
}
// <<delegate>>+D()
public delegate void D();
// +<<event>>E1
// +<<class>><<event>>E2
// +F()
// <<does>>
// <<fire>>E1
// <<fire>>E2
public class C1
{
public void F1()
{
OnE1();
OnE2();
}
public event D E1;
private void OnE1()
{
if(E1 != null)
{
E1();
}
}
static public event D E2;
static private void OnE2()
{
if(E2 != null)
{
E2();
}
}
}
Be wary of static events. Remember that, when an object subscribes to an event, a reference to that object is held by the publisher of the event. That means that you have to be very careful about explicitly unsubscribing from static events as they will keep the subscriber alive forever, i.e., you may end up with the managed equivalent of a memory leak.
Much of OOP can be thought of in terms of message passing.
A method call is a message from the caller to the callee (carrying the parameters) and a message back with the return value.
An event is a message from the source to the subscriber. There are thus potentially two instances involved, the one sending the message and the one receiving it.
With a static event, there is no sending instance (just a type, which may or may not be a class). There still can be a recipient instance encoded as the target of the delegate.
In case you're not familiar with static methods
You're probably already familiar with static methods. In case you're not, An easy-to-understand difference is that you don't need to create an instance of an object toi use a static method, but you DO need to create an instance of an object to call a non-static method.
A good example is the System.IO.Directory and System.IO.DirectoryInfo classes.
The Directory class offers static methods, while the DirectoryInfo class does not.
There are two articles describing them here for you to see the difference for yourself.
http://visualcsharptutorials.com/2011/01/system-io-directory-class/
http://visualcsharptutorials.com/2011/01/system-io-directoryinfo-class/
Now on to static events...
However, static events are seldom seen in the wild. There are very few cases that I can think opf where I'd actually want to use one, but there is a CodeProject article that does show one potential use.
http://www.codeproject.com/KB/cs/staticevent.aspx
The key thought here is taken from the explanation (bold added by me to point out the relevant text):
We saw this property as a separate object and we made sure that there
is only one instance of it at a time. And all instances of
transactions knew where to find it when needed. There is a fine
difference though. The transactions will not need to know about the
changes happening on the exchange rate, rather they will use the last
changed value at the time that they use it by requesting the current
value. This is not enough when, for example, we want to implement an
application where the user interface reacts immediately on changes in
the UI characteristics like font, as if it has to happen at
real-time. It would be very easy if we could have a static property
in the Font class called currentFont and a static method to change
that value and a static event to all instances to let them know when
they need to update their appearance.
As .NET developers we're trained to work with a disconnected model. Think of ADO.NET compared to classic ADO. IN a VB6 app, you could use data controls that would allow the following functionality: If you were running the app on your PC, the data in your grid would update when someone on another PC edited the data.
This isn't something that .NET developers are used to. We're very used to the disconnected model. Static events enable a more "connected" experience. (even if that experience is something we're not used to any more.)
for some insight check this link http://www.codeproject.com/KB/cs/staticevent.aspx
static event can be used
when no instance exists
to do some multicast event for all existing instances...
when you have a static class which can fire events...
BUT one should use them with cuation... see discussion http://groups.google.com/group/microsoft.public.dotnet.languages.csharp/browse_thread/thread/2ac862f346b24a15/8420fbd9294ab12a%238420fbd9294ab12a?sa=X&oi=groupsr&start=1&num=2
more info
http://msdn.microsoft.com/en-us/library/8627sbea.aspx
http://dylanbeattie.blogspot.com/2008/05/firing-static-events-from-instance.html
http://www.nivisec.com/2008/09/static-events-dont-release.html
Static members are not "global," they are simply members of the class, not of class instances. This is as true for events as it is for methods, properties, fields, etc.
I can't give an example for using a static event, because I generally don't find static members to be useful in most cases. (They tend to hint at anti-patterns, like Singleton.)
I'm looking to implement the Observer pattern in VB.NET or C# or some other first-class .NET language. I've heard that delegates can be used for this, but can't figure out why they would be preferred over plain old interfaces implemented on observers. So,
Why should I use delegates instead of defining my own interfaces and passing around references to objects implementing them?
Why might I want to avoid using delegates, and go with good ol'-fashioned interfaces?
When you can directly call a method, you don't need a delegate.
A delegate is useful when the code calling the method doesn't know/care what the method it's calling is -- for example, you might invoke a long-running task and pass it a delegate to a callback method that the task can use to send notifications about its status.
Here is a (very silly) code sample:
enum TaskStatus
{
Started,
StillProcessing,
Finished
}
delegate void CallbackDelegate(Task t, TaskStatus status);
class Task
{
public void Start(CallbackDelegate callback)
{
callback(this, TaskStatus.Started);
// calculate PI to 1 billion digits
for (...)
{
callback(this, TaskStatus.StillProcessing);
}
callback(this, TaskStatus.Finished);
}
}
class Program
{
static void Main(string[] args)
{
Task t = new Task();
t.Start(new CallbackDelegate(MyCallbackMethod));
}
static void MyCallbackMethod(Task t, TaskStatus status)
{
Console.WriteLine("The task status is {0}", status);
}
}
As you can see, the Task class doesn't know or care that -- in this case -- the delegate is to a method that prints the status of the task to the console. The method could equally well send the status over a network connection to another computer. Etc.
You're an O/S, and I'm an application. I want to tell you to call one of my methods when you detect something happening. To do that, I pass you a delegate to the method of mine which I want you to call. I don't call that method of mine myself, because I want you to call it when you detect the something. You don't call my method directly because you don't know (at your compile-time) that the method exists (I wasn't even written when you were built); instead, you call whichever method is specified by the delegate which you receive at run-time.
Well technically, you don't have to use delegates (except when using event handlers, then it's required). You can get by without them. Really, they are just another tool in the tool box.
The first thing that comes to mind about using them is Inversion Of Control. Any time you want to control how a function behaves from outside of it, the easiest way to do it is to place a delegate as a parameter, and have it execute the delegate.
You're not thinking like a programmer.
The question is, Why would you call a function directly when you could call a delegate?
A famous aphorism of David Wheeler
goes: All problems in computer science
can be solved by another level of
indirection.
I'm being a bit tongue-in-cheek. Obviously, you will call functions directly most of the time, especially within a module. But delegates are useful when a function needs to be invoked in a context where the containing object is not available (or relevant), such as event callbacks.
There are two places that you could use delegates in the Observer pattern. Since I am not sure which one you are referring to, I will try to answer both.
The first is to use delegates in the subject instead of a list of IObservers. This approach seems a lot cleaner at handling multicasting since you basically have
private delegate void UpdateHandler(string message);
private UpdateHandler Update;
public void Register(IObserver observer)
{
Update+=observer.Update;
}
public void Unregister(IObserver observer)
{
Update-=observer.Update;
}
public void Notify(string message)
{
Update(message);
}
instead of
public Subject()
{
observers = new List<IObserver>();
}
public void Register(IObserver observer)
{
observers.Add(observer);
}
public void Unregister(IObserver observer)
{
observers.Remove(observer);
}
public void Notify(string message)
{
// call update method for every observer
foreach (IObserver observer in observers)
{
observer.Update(message);
}
}
Unless you need to do something special and require a reference to the entire IObserver object, I would think the delegates would be cleaner.
The second case is to use pass delegates instead of IObervers for example
public delegate void UpdateHandler(string message);
private UpdateHandler Update;
public void Register(UpdateHandler observerRoutine)
{
Update+=observerRoutine;
}
public void Unregister(UpdateHandler observerRoutine)
{
Update-=observerRoutine;
}
public void Notify(string message)
{
Update(message);
}
With this, Observers don't need to implement an interface. You could even pass in a lambda expression. This changes in the level of control is pretty much the difference. Whether this is good or bad is up to you.
A delegate is, in effect, passing around a reference to a method, not an object... An Interface is a reference to a subset of the methods implemented by an object...
If, in some component of your application, you need access to more than one method of an object, then define an interface representing that subset of the objects' methods, and assign and implement that interface on all classes you might need to pass to this component... Then pass the instances of these classes by that interface instead of by their concrete class..
If, otoh, in some method, or component, all you need is one of several methods, which can be in any number of different classes, but all have the same signature, then you need to use a delegate.
I'm repeating an answer I gave to this question.
I've always like the Radio Station metaphor.
When a radio station wants to broadcast something, it just sends it out. It doesn't need to know if there is actually anybody out there listening. Your radio is able to register itself with the radio station (by tuning in with the dial), and all radio station broadcasts (events in our little metaphor) are received by the radio who translates them into sound.
Without this registration (or event) mechanism. The radio station would have to contact each and every radio in turn and ask if it wanted the broadcast, if your radio said yes, then send the signal to it directly.
Your code may follow a very similar paradigm, where one class performs an action, but that class may not know, or may not want to know who will care about, or act on that action taking place. So it provides a way for any object to register or unregister itself for notification that the action has taken place.
Delegates are strong typing for function/method interfaces.
If your language takes the position that there should be strong typing, and that it has first-class functions (both of which C# does), then it would be inconsistent to not have delegates.
Consider any method that takes a delegate. If you didn't have a delegate, how would you pass something to it? And how would the the callee have any guarantees about its type?
I've heard some "events evangelists" talk about this and they say that as more decoupled events are, the better it is.
Preferably, the event source should never know about the event listeners and the event listener should never care about who originated the event. This is not how things are today because in the event listener you normally receive the source object of the event.
With this said, delegates are the perfect tool for this job. They allow decoupling between event source and event observer because the event source doesn't need to keep a list of all observer objects. It only keeps a list of "function pointers" (delegates) of the observers.
Because of this, I think this is a great advantage over Interfaces.
Look at it the other way. What advantage would using a custom interface have over using the standard way that is supported by the language in both syntax and library?
Granted, there are cases where it a custom-tailored solution might have advantages, and in such cases you should use it. In all other cases, use the most canonical solution available. It's less work, more intuitive (because it's what users expect), has more support from tools (including the IDE) and chances are, the compiler treats them differently, resulting in more efficient code.
Don't reinvent the wheel (unless the current version is broken).
Actually there was an interesting back-and-forth between Sun and Microsoft about delegates. While Sun made a fairly strong stance against delegates, I feel that Microsoft made an even stronger point for using delegates. Here are the posts:
http://java.sun.com/docs/white/delegates.html
http://msdn.microsoft.com/en-us/vjsharp/bb188664.aspx
I think you'll find these interesting reading...
i think it is more related to syntatic sugar and a way to organize your code, a good use would be to handle several methods related to a common context which ones belong to a object or a static class.
it is not that you are forced to use them, you can programme sth with and without them, but maybe using them or not might affect how organized, readable and why not cool the code would be, maybe bum some lines in your code.
Every example given here is a good one where you could implement them, as someone said it, is just another feature in the language you can play with.
greetings
Here is something that i can write down as a reason of using delegate.
The following code is written in C# And please follow the comments.
public delegate string TestDelegate();
protected void Page_Load(object sender, EventArgs e)
{
TestDelegate TD1 = new TestDelegate(DiaplayMethodD1);
TestDelegate TD2 = new TestDelegate(DiaplayMethodD2);
TD2 = TD1 + TD2; // Make TD2 as multi-cast delegate
lblDisplay.Text = TD1(); // invoke delegate
lblAnotherDisplay.Text = TD2();
// Note: Using a delegate allows the programmer to encapsulate a reference
// to a method inside a delegate object. Its like the function pointer
// in C or C++.
}
//the Signature has to be same.
public string DiaplayMethodD1()
{
//lblDisplay.Text = "Multi-Cast Delegate on EXECUTION"; // Enable on multi-cast
return "This is returned from the first method of delegate explanation";
}
// The Method can be static also
public static string DiaplayMethodD2()
{
return " Extra words from second method";
}
Best Regards,
Pritom Nandy,
Bangladesh
Here is an example that might help.
There is an application that uses a large set of data. A feature is needed that allows the data to be filtered. 6 different filters can be specified.
The immediate thought is to create 6 different methods that each return the data filtered. For example
public Data FilterByAge(int age)
public Data FilterBySize(int size)
.... and so on.
This is fine but is a very limited and produces rubbish code because it's closed for expansion.
A better way is to have a single Filter method and to pass information on how the data should be filtered. This is where a delegate can be used. The delegate is a function that can be applied to the data in order to filter it.
public Data Filter(Action filter)
then the code to use this becomes
Filter(data => data.age > 30);
Filter(data => data.size = 19);
The code data => blah blah becomes a delegate. The code becomes much more flexible and remains open.
I'm understanding how events work in C# (am a fair newbie in this field). What I'm trying to understand is why we use events.
Do you know a well coded / architected app which uses events?
To provide a concrete normal world example....
You have a form, the form has a listbox. There's a nice happy class for the listbox. When the user selects something from the listbox, you want to know, and modify other things on the form.
Without events:
You derive from the listbox, overriding things to make sure that your parent is the form you expect to be on.
You override a ListSelected method or something, that manipulates other things on your parent form.
With events:
Your form listens for the event to indicate a user selected something, and manipulates other things on the form.
The difference being that in the without events case you've created a single-purpose class, and also one that is tightly bound to the environment it expects to be in. In the with events case, the code that manipulates your form is localized into your form, and the listbox is just, well, a listbox.
What would be very useful is a non trivial example of an app which uses events (guess it really helps testing too?)
Thoughts so far are:
Why use Events or publish / subscribe?
Any number of classes can be notified when an event is raised.
The subscribing classes do not need to know how the Metronome (see code below) works, and the Metronome does not need to know what they are going to do in response to the event
The publisher and the subscribers are decoupled by the delegate. This is highly desirable as it makes for more flexible and robust code. The metronome can change how it detects time without breaking any of the subscribing classes. The subscribing classes can change how they respond to time changes without breaking the metronome. The two classes spin independently of one another, which makes for code that is easier to maintain.
class Program
{
static void Main()
{
// setup the metronome and make sure the EventHandler delegate is ready
Metronome metronome = new Metronome();
// wires up the metronome_Tick method to the EventHandler delegate
Listener listener = new Listener(metronome);
ListenerB listenerB = new ListenerB(metronome);
metronome.Go();
}
}
public class Metronome
{
// a delegate
// so every time Tick is called, the runtime calls another method
// in this case Listener.metronome_Tick and ListenerB.metronome_Tick
public event EventHandler Tick;
// virtual so can override default behaviour in inherited classes easily
protected virtual void OnTick(EventArgs e)
{
// null guard so if there are no listeners attached it wont throw an exception
if (Tick != null)
Tick(this, e);
}
public void Go()
{
while (true)
{
Thread.Sleep(2000);
// because using EventHandler delegate, need to include the sending object and eventargs
// although we are not using them
OnTick(EventArgs.Empty);
}
}
}
public class Listener
{
public Listener(Metronome metronome)
{
metronome.Tick += new EventHandler(metronome_Tick);
}
private void metronome_Tick(object sender, EventArgs e)
{
Console.WriteLine("Heard it");
}
}
public class ListenerB
{
public ListenerB(Metronome metronome)
{
metronome.Tick += new EventHandler(metronome_Tick);
}
private void metronome_Tick(object sender, EventArgs e)
{
Console.WriteLine("ListenerB: Heard it");
}
}
Full article I'm writing on my site: http://www.programgood.net/
nb some of this text is from http://www.akadia.com/services/dotnet_delegates_and_events.html
Cheers.
You can implement the Observer Pattern in C# with Events and Delegates.
Here is a link to an article that describes such: http://blogs.msdn.com/bashmohandes/archive/2007/03/10/observer-pattern-in-c-events-delegates.aspx
At the most basic conceptual level, Events are what let the computer react to what you do, rather than you being required to react to what the computer does. When you're sitting in front of your PC with several applications running (including the operating system), and several clickable objects available in each context for you to choose among, Events are what happens when you choose one and all the pieces involved can be properly notified.
Even moving your mouse around kicks off a stream of events (to move the cursor, for instance).
You could always build your own way of sending/receiving events, subscribing/unsubscribing to event sources. But the language gives you a simple/standard way of doing it, so that is a good reason to use language "events" instead of your own events technique.
Also, using the language "events" allows you to do all kinds of cool things using reflection because it is standardized.
As to why using an event technique at all. There are all kinds of real-life examples where this is quite usefull and simpler to use events. Events are almost similar in their usefullness than Windows Messages are.
If you are confused on why use events when we can achieve the same functionality with delegates, then the answer is :
the client (the class where subscription occurs) cannot directly call the invoke method on the delegate when using events
instanceOfClassWithDelegate.sampleDelegate.Invoke(); // not possible with events
the client cannot list or clear the functions assigned to the delegate when using events
instanceOfClassWithDelegate.sampleDelegate = null; // not possible when using events
events are a better way to encapsulate logic as is implicitly understood
The entire credit to this answer goes to this blog : https://dev.to/willydavidjr/why-use-events-instead-of-delegates-in-c-40k8