I have a static class that I would like to raise an event as part of a try catch block within a static method of that class.
For example in this method I would like to raise a custom event in the catch.
public static void saveMyMessage(String message)
{
try
{
//Do Database stuff
}
catch (Exception e)
{
//Raise custom event here
}
}
Thank you.
Important: be very careful about subscribing to a static event from instances. Static-to-static is fine, but a subscription from a static event to an instance handler is a great (read: very dangerous) way to keep that instance alive forever. GC will see the link, and will not collect the instance unless you unsubscribe (or use something like a WeakReference).
The pattern for creating static events is the same as instance events, just with static:
public static event EventHandler SomeEvent;
To make life easier (re null checking), a useful trick here is to add a trivial handler:
public static event EventHandler SomeEvent = delegate {};
Then you can simply invoke it without the null-check:
SomeEvent(null, EventArgs.Empty);
Note that because delegate instances are immutable, and de-referencing is thread-safe, there is never a race condition here, and no need to lock... who-ever is subscribed when we de-reference gets invoked.
(adjust for your own event-args etc).
This trick applies equally to instance events.
Your event would also need to be static:
public class ErrorEventArgs : EventArgs
{
private Exception error;
private string message;
public ErrorEventArgs(Exception ex, string msg)
{
error = ex;
message = msg;
}
public Exception Error
{
get { return error; }
}
public string Message
{
get { return message; }
}
}
public static class Service
{
public static EventHandler<ErrorEventArgs> OnError;
public static void SaveMyMessage(String message)
{
EventHandler<ErrorEventArgs> errorEvent = OnError;
if (errorEvent != null)
{
errorEvent(null, new ErrorEventArgs(null, message));
}
}
}
And Usage:
public class Test
{
public void OnError(object sender, ErrorEventArgs args)
{
Console.WriteLine(args.Message);
}
}
Test t = new Test();
Service.OnError += t.OnError;
Service.SaveMyMessage("Test message");
Several folks have offered up code examples, just don't fire an event using code such as:
if(null != ExampleEvent)
{
ExampleEvent(/* put parameters here, for events: sender, eventArgs */);
}
as this contains a race condition between when you check the event for null and when you actually fire the event. Instead use a simple variation:
MyEvent exampleEventCopy = ExampleEvent;
if(null != exampleEventCopy)
{
exampleEventCopy(/* put parameters here, for events: sender, eventArgs */);
}
This will copy any event subscribers into the exampleEventCopy, which you can then use as a local-only version of the public event without having to worry about any race conditions (Essentially, it is possible that another thread could pre-empt you right after you have checked the public event for null and proceed to remove all subscribers from the event, causing the subsequent firing of the event to throw an exception, by using a local-only copy, you avoid the possibility of another thread removing subscribers, since there is no way they could access the local variable).
Note: VS2008, C#
Just declare an event as you normally would within the static class, but be sure to mark the event as static:
public static event EventHandler Work;
Then just subscribe to it as you normally would.
Just to add "Delegates are immutable" So, as shown in the example above the following line obtains a copy of the delegate.
EventHandler<ErrorEventArgs> errorEvent = OnError;
The way I did this is the following:
1- define a delegate (this will enable you to have customized arguments):
public delegate void CustomeEventHandler(string str);
2- define an event based on the previously defined delegate:
public static event CustomeEventHandler ReadLine;
3- create an event handler:
static void OnLineRead(string currentLine)
{
if (ReadLine != null)
ReadLine(currentLine);
}
4- raise your event using the event handler (just call it wherever you want the event to be raised).
Related
I have a class, EventContainer.cs, which contains an event, say:
public event EventHandler AfterSearch;
I have another class, EventRaiser.cs. How do I raise (and not handle) the above said event from this class?
The raised event will in turn call the handler of the event in the EventContainer class. Something like this (this is obviously not correct):
EventContainer obj = new EventContainer();
RaiseEvent(obj.AfterSearch);
This is not possible, Events can only be risen from inside the class. If you could do that, it would defeat the purpose of events (being able to rise status changes from inside the class). I think you are misunderstanding the function of events - an event is defined inside a class and others can subscribe to it by doing
obj.AfterSearch += handler; (where handler is a method according to the signature of AfterSearch). One is able to subscribe to the event from the outside just fine, but it can only be risen from inside the class defining it.
It is POSSIBLE, but using clever hack.
Inspired by http://netpl.blogspot.com/2010/10/is-net-type-safe.html
If you don't believe, try this code.
using System;
using System.Runtime.InteropServices;
namespace Overlapping
{
[StructLayout(LayoutKind.Explicit)]
public class OverlapEvents
{
[FieldOffset(0)]
public Foo Source;
[FieldOffset(0)]
public OtherFoo Target;
}
public class Foo
{
public event EventHandler Clicked;
public override string ToString()
{
return "Hello Foo";
}
public void Click()
{
InvokeClicked(EventArgs.Empty);
}
private void InvokeClicked(EventArgs e)
{
var handler = Clicked;
if (handler != null)
handler(this, e);
}
}
public class OtherFoo
{
public event EventHandler Clicked;
public override string ToString()
{
return "Hello OtherFoo";
}
public void Click2()
{
InvokeClicked(EventArgs.Empty);
}
private void InvokeClicked(EventArgs e)
{
var handler = Clicked;
if (handler != null)
handler(this, e);
}
public void Clean()
{
Clicked = null;
}
}
class Test
{
public static void Test3()
{
var a = new Foo();
a.Clicked += AClicked;
a.Click();
var o = new OverlapEvents { Source = a };
o.Target.Click2();
o.Target.Clean();
o.Target.Click2();
a.Click();
}
static void AClicked(object sender, EventArgs e)
{
Console.WriteLine(sender.ToString());
}
}
}
You can write a public method on the class you want the event to fire from and fire the event when it is called. You can then call this method from whatever user of your class.
Of course, this ruins encapsulation and is bad design.
It looks like you're using the Delegate pattern. In this case, the AfterSearch event should be defined on the EventRaiser class, and the EventContainer class should consume the event:
In EventRaiser.cs
public event EventHandler BeforeSearch;
public event EventHandler AfterSearch;
public void ExecuteSearch(...)
{
if (this.BeforeSearch != null)
this.BeforeSearch();
// Do search
if (this.AfterSearch != null)
this.AfterSearch();
}
In EventContainer.cs
public EventContainer(...)
{
EventRaiser er = new EventRaiser();
er.AfterSearch += this.OnAfterSearch;
}
public void OnAfterSearch()
{
// Handle AfterSearch event
}
I stumbled across this problem as well, because i was experimenting with calling PropertyChanged events from outside. So you dont have to implement everything in every class. The solution from halorty wouldn't work using interfaces.
I found a solution working using heavy reflection. It is surely slow and is breaking the principle that events should only be called from inside a class. But it is interesting to find a generic solution to this problem....
It works because every event is a list of invocation methods being called.
So we can get the invocation list and call every listener attached to that event by our own.
Here you go....
class Program
{
static void Main(string[] args)
{
var instance = new TestPropertyChanged();
instance.PropertyChanged += PropertyChanged;
instance.RaiseEvent(nameof(INotifyPropertyChanged.PropertyChanged), new PropertyChangedEventArgs("Hi There from anywhere"));
Console.ReadLine();
}
private static void PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine(e.PropertyName);
}
}
public static class PropertyRaiser
{
private static readonly BindingFlags staticFlags = BindingFlags.Instance | BindingFlags.NonPublic;
public static void RaiseEvent(this object instance, string eventName, EventArgs e)
{
var type = instance.GetType();
var eventField = type.GetField(eventName, staticFlags);
if (eventField == null)
throw new Exception($"Event with name {eventName} could not be found.");
var multicastDelegate = eventField.GetValue(instance) as MulticastDelegate;
if (multicastDelegate == null)
return;
var invocationList = multicastDelegate.GetInvocationList();
foreach (var invocationMethod in invocationList)
invocationMethod.DynamicInvoke(new[] {instance, e});
}
}
public class TestPropertyChanged : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
}
There is good way to do this. Every event in C# has a delegate that specifies the sign of methods for that event. Define a field in your external class with type of your event delegate. get the the reference of that field in the constructor of external class and save it. In main class of your event, send the reference of event for delegate of external class. Now you can easily call the delegate in your external class.
public delegate void MyEventHandler(object Sender, EventArgs Args);
public class MyMain
{
public event MyEventHandler MyEvent;
...
new MyExternal(this.MyEvent);
...
}
public MyExternal
{
private MyEventHandler MyEvent;
public MyExternal(MyEventHandler MyEvent)
{
this.MyEvent = MyEvent;
}
...
this.MyEvent(..., ...);
...
}
Agree with Femaref -- and note this is an important difference between delegates and events (see for example this blog entry for an good discussion of this and other differences).
Depending on what you want to achieve, you might be better off with a delegate.
Not a good programming but if you want to do that any way you can do something like this
class Program
{
static void Main(string[] args)
{
Extension ext = new Extension();
ext.MyEvent += ext_MyEvent;
ext.Dosomething();
}
static void ext_MyEvent(int num)
{
Console.WriteLine(num);
}
}
public class Extension
{
public delegate void MyEventHandler(int num);
public event MyEventHandler MyEvent;
public void Dosomething()
{
int no = 0;
while(true){
if(MyEvent!=null){
MyEvent(++no);
}
}
}
}
I had a similar confusion and honestly find the answers here to be confusing. Although a couple hinted at solutions that I would later find would work.
My solution was to hit the books and become more familiar with delegates and event handlers.
Although I've used both for many years, I was never intimately familiar with them.
http://www.codeproject.com/Articles/20550/C-Event-Implementation-Fundamentals-Best-Practices
gives the best explanation of both delegates and event handlers that I've ever read and clearly explains that a class can be a publisher of events and have other classes consume them.
This article: http://www.codeproject.com/Articles/12285/Implementing-an-event-which-supports-only-a-single discusses how to single-cast events to only one handler since delegates are multicast by definition . A delegate inherits system.MulticastDelegate most including the system delegates are Multicast.
I found that multicast meant that any event handler with the same signature would receive the raised event. Multicast behavior has caused me some sleepless nights as I stepped through code and saw my event seemingly erroneously being sent to handlers that I had no intention of getting this event. Both articles explains this behavior.
The second article shows you one way, and the first article shows you another, by making the delegate and the signature tightly typed.
I personally believe strong typing prevents stupid bugs that can be a pain to find. So I'd vote for the first article, even though I got the second article code working. I was just curious. :-)
I also got curious if I could get #2 articles code to behave like how I interpreted the original question above. Regardless of your chosen approach or if I'm also misinterpreting the original question, my real message is that I still think you would benefit from reading the first article as I did, especially if the questions or answers on this page leave you confused. If you are having multicast nightmares and need a quick solution then article 2 may help you.
I started playing with the second article's eventRaiser class. I made a simple windows form project.
I added the second articles class EventRaiser.cs to my project.
In the Main form's code, I defined a reference to that EventRaiser class at the top as
private EventRaiser eventRaiser = new EventRaiser();
I added a method in the main form code that I wanted to be called when the event was fired
protected void MainResponse( object sender, EventArgs eArgs )
{
MessageBox.Show("got to MainResponse");
}
then in the main form's constructor I added the event assignment:
eventRaiser.OnRaiseEvent += new EventHandler(MainResponse);`
I then created a class that would be instantiated by my main form called "SimpleClass" for lack of creative ingenuity at the moment.
Then I added a button and in the button's click event
I instantiated the SimpleClass code I wanted to raise an event from:
private void button1_Click( object sender, EventArgs e )
{
SimpleClass sc = new SimpleClass(eventRaiser);
}
Note the instance of "eventRaiser" that I passed to SimpleClass.cs. That was defined and instantiated earlier in the Main form code.
In the SimpleClass:
using System.Windows.Forms;
using SinglecastEvent; // see SingleCastEvent Project for info or http://www.codeproject.com/Articles/12285/Implementing-an-event-which-supports-only-a-single
namespace GenericTest
{
public class SimpleClass
{
private EventRaiser eventRaiser = new EventRaiser();
public SimpleClass( EventRaiser ev )
{
eventRaiser = ev;
simpleMethod();
}
private void simpleMethod()
{
MessageBox.Show("in FileWatcher.simple() about to raise the event");
eventRaiser.RaiseEvent();
}
}
}
The only point to the private method I called SimpleMethod was to verify that a privately scoped method could still raise the event, not that I doubted it, but I like to be positive.
I ran the project and this resulted in raising the event from the "simpleMethod" of the "SimpleClass" up to the main form and going to the expected correct method called MainResponse proving that one class can indeed raise an event that is consumed by a different class.
Yes the event has to be raised from within the class that needs it's change broadcast to other classes that care. Receiving classes can be one class or many many classes depending on how strongly typed you defined them or by making them single cast as in 2nd article.
Hope this helps and not muddy the water. Personally I've got a lot of delegates and events to clean up! Multicast demons begone!
The raising class has to get a fresh copy of the EventHandler.
One possible solution below.
using System;
namespace ConsoleApplication1
{
class Program
{
class HasEvent
{
public event EventHandler OnEnvent;
EventInvoker myInvoker;
public HasEvent()
{
myInvoker = new EventInvoker(this, () => OnEnvent);
}
public void MyInvokerRaising() {
myInvoker.Raise();
}
}
class EventInvoker
{
private Func<EventHandler> GetEventHandler;
private object sender;
public EventInvoker(object sender, Func<EventHandler> GetEventHandler)
{
this.sender = sender;
this.GetEventHandler = GetEventHandler;
}
public void Raise()
{
if(null != GetEventHandler())
{
GetEventHandler()(sender, new EventArgs());
}
}
}
static void Main(string[] args)
{
HasEvent h = new HasEvent();
h.OnEnvent += H_OnEnvent;
h.MyInvokerRaising();
}
private static void H_OnEnvent(object sender, EventArgs e)
{
Console.WriteLine("FIRED");
}
}
}
Use public EventHandler AfterSearch;
not
public event EventHandler AfterSearch;
Use a Delegate (an Action or Func) instead of an event. An event is essentially a delegate that can only be triggered from within the class.
I took a slightly different approach in solving this problem. My solution consisted of a winform front end, a main Class Library (DLL) and within that dll, a secondary working class:
WinForm
|------> PickGen Library
|---------> Allocations class
What I decided to do is to create events in the main dll (PickGen) that the Allocations class could call, then those event methods would called the events within the UI.
So, allocations raises an event in PickGen which takes the parameter values and raises the event in the form. From a code standpoint, this is in the lowest class:
public delegate void AllocationService_RaiseAllocLog(string orderNumber, string message, bool logToDatabase);
public delegate void AllocationService_RaiseAllocErrorLog(string orderNumber, string message, bool logToDatabase);
public class AllocationService { ...
public event AllocationService_RaiseAllocLog RaiseAllocLog;
public event AllocationService_RaiseAllocErrorLog RaiseAllocErrorLog;
then later in the subclass code:
RaiseAllocErrorLog(SOHNUM_0, ShipmentGenerated + ": Allocated line QTY was: " + allocatedline.QTY_0 + ", Delivered was: " + QTY_0 + ". Problem batch.", false);
In the main DLL Class library I have these two event methods:
private void PickGenLibrary_RaiseAllocLog(string orderNumber, string message, bool updateDB)
{
RaiseLog(orderNumber, message, false);
}
private void PickGenLibrary_RaiseAllocErrorLog(string orderNumber, string message, bool updateDB)
{
RaiseErrorLog(orderNumber, message, false);
}
and I make the connection here when I create the allocation object:
AllocationService allsvc = new AllocationService(PickResult);
allsvc.RaiseAllocLog += new AllocationService_RaiseAllocLog(PickGenLibrary_RaiseAllocLog);
allsvc.RaiseAllocErrorLog += new AllocationService_RaiseAllocErrorLog(PickGenLibrary_RaiseAllocErrorLog);
and I also then have delegates that are set up to tie the main class with the winform code:
public delegate void JPPAPickGenLibrary_RaiseLog(string orderNumber, string message, bool logToDatabase);
public delegate void JPPAPickGenLibrary_RaiseErrorLog(string orderNumber, string message, bool logToDatabase);
It may not be the most elegant way to do it, but in the end, it does work and without being too obscure.
A nested class with an instance of the outer class provided in the constructor can access even private members of the outer class. As explained more here: stackoverflow question on inner classes.
This includes the ability to raise events in the outer class. This EventRaisers class could be internal, or otherwise controlled somehow, because it could technically otherwise be created by any script with a reference to the outer class instance.
Very simple example. i like to do it this way using EventHandler.
class Program
{
static void Main(string[] args)
{
MyExtension ext = new MyExtension();
ext.MyEvent += ext_MyEvent;
ext.Dosomething();
Console.ReadLine();
}
static void ext_MyEvent(object sender, int num)
{
Console.WriteLine("Event fired.... "+num);
}
}
public class MyExtension
{
public event EventHandler<int> MyEvent;
public void Dosomething()
{
int no = 1;
if (MyEvent != null)
MyEvent(this, ++no);
}
}
}
I may be misunderstanding something fundamental here as I'm new to these concepts so please bear with me.
I'm currently removing methods from an event like so:
scheduleView.TouchDown -= scheduleView_TouchDown;
And then on other occasions - adding the methods:
scheduleView.TouchDown += scheduleView_TouchDown;
It all works fine so far, and I can understand it's possible to add several methods, like so:
scheduleView.TouchDown += scheduleView_TouchDown;
scheduleView.TouchDown += scheduleView_AnotherTouchDownEventHandler;
But how would I then later check what methods were wired up to this event?
Interestingly, you can't (at least, from the outside). An event is only obliged to offer 2 accessors - add and remove. There are other accessor methods defined in the CLI spec, but they aren't used in C# or anywhere else AFAIK. The key point: we can't ask an event what is subscribed (and indeed, we shouldn't need to know). All you can do is: add or remove.
If you are worried about double-subscribing, then note that if you try to unsubscribe and you haven't actually subscribed, then under every sane implementation this is simply a no-op; which means you can do:
// make sure we are subscribed once but **only** once
scheduleView.TouchDown -= scheduleView_TouchDown;
scheduleView.TouchDown += scheduleView_TouchDown;
From the perspective of the code raising the event, you rarely need to know who - simply:
// note I'm assuming a "field-like event" implementation here; otherwise,
// change this to refer to the backing-field, or the delegate from the
// event-handler-list
var handler = TouchDown;
if(handler != null) handler(this, EventArgs.Empty); // or similar
There is also a way to break the delegate list into individual subscribers, but it is very rarely needed:
var handler = TouchDown;
if(handler != null) {
foreach(EventHandler subscriber in handler.GetInvocationList()) {
subscriber(this, EventArgs.Empty);
}
}
The main uses for this are:
when you want to perform exception-handling on a per-subscriber basis
when the delegate returns a value or changes state, and you need to handle that on a per-subscriber basis
Yes: If you are within the class that publishes the Event, you can just access the delegate, and you can call the GetInvocationList method to get a list of the subscribers.
No: If you are working outside the class, as the delegate is not exposed to you. You could use reflection to get at it, but that would be a hack, at best.
In the type that declares the event, you can use GetInvocationList() to find out which delegates are subscribed:
public class EventProvider
{
public event EventHandler SomeEvent;
protected virtual void OnSomeEvent(EventArgs args)
{
if (SomeEvent != null)
{
var delegates = SomeEvent.GetInvocationList();
foreach (var del in delegates)
{
Console.WriteLine("{0} has subscribed to SomeEvent", del.Method.Name);
}
SomeEvent(this, args);
}
}
public void RaiseSomeEvent()
{
OnSomeEvent(EventArgs.Empty);
}
}
public class Program
{
public static void Main()
{
EventProvider provider = new EventProvider();
provider.SomeEvent += Callback1;
provider.SomeEvent += Callback2;
provider.RaiseSomeEvent();
}
public static void Callback1(object sender, EventArgs e)
{
Console.WriteLine("Callback 1!");
}
public static void Callback2(object sender, EventArgs e)
{
Console.WriteLine("Callback 2!");
}
}
This produces the following output:
Callback1 has subscribed to SomeEvent
Callback2 has subscribed to SomeEvent
Callback 1!
Callback 2!
I've created an event handler that simply returns a list of objects that I receive from a web service when the call completes.
Now I went ahead and ran the app in debug mode and found out that the first time the event is called it works perfectly, but immediately after it completes the event is being fired for a second time. I've checked and am absolutely sure I am not calling the event more than once in the receiver class.
This is my first shot at creating custom event handlers inside my applications so I am not entirely sure the implementation is 100% accurate.
Any ideas of what might be causing this? Is the way I created the event handler accurate?
This is the DataHelper class
public class DataHelper
{
public delegate void DataCalledEventHandler(object sender, List<DataItem> dateItemList);
public event DataCalledEventHandler DataCalled;
public DataHelper()
{
}
public void CallData()
{
List<DataItem> dataItems = new List<DataItem>();
//SOME CODE THAT RETURNS DATA
DataCalled(this, dataItems);
}
}
This is where I subscribed to my event:
protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
GetNewDataItems();
}
private void GetNewDataItems()
{
try
{
DataHelper dataHelper = new DataHelper();
dataHelper.CallData();
dataHelper.DataCalled += new DataHelper.DataCalledEventHandler(dataHelper_DataCalled);
}
catch
{
//Handle any errors
}
}
}
void dataHelper_DataCalled(object sender, List<DataItem> dataItemsList)
{
//Do something with results
//NOTE: THIS IS WHERE THE EXCEPTION OCCURS WHEN EVENT IS FIRED FOR SECOND TIME
}
Probably you added the delegate twice, is it possible?
In this case the problem is not in who calls the delegate but in who adds the delegate to the event.
Probably you did something like...
private Class1 instance1;
void callback(...)
{
}
void myfunction()
{
this.instance1.DataCalled += this.callback;
this.instance1.DataCalled += this.callback;
}
If not, try to add a breakpoint where you subscribe to the event and see if it is called twice.
As a side note, you should always check for null when calling an event, if there is no subscriber you can get a NullReferenceException.
I would also suggest you to use a variable to store the event delegate to avoid the risk of multithreading failure.
public void CallData()
{
List<DataItem> dataItems = new List<DataItem>();
var handler = this.DataCalled;
if (handler != null)
handler(this, dataItems);
}
Edit: since now I see the code, is obvious that each time you call the GetNewDataItems method you are subsribing every time to the event.
Do in such a way you subscribe only once, for example, in constructor, or store your variable somewhere or deregister the event when you finish.
This code contains also a probable memory leak: every time you add a delegate you keep alive both the instance that contains the event and the instance that contains the subscribed method, at least, until both are unreferenced.
You can try to do something like this...
void dataHelper_DataCalled(object sender, List<DataItem> dataItemsList)
{
// Deregister the event...
(sender as Class1).DataCalled -= dataHelper_DataCalled;
//Do something with results
}
In this way however you must ensure that if there is not an exception during the event registration the event will be fired or you have again memory leaks.
Instead of an event perhaps you need just a delegate. Of course you should set your delegate field to null when you want to release the delegate.
// in data helper class
private DataHelper.DataCalledEventHandler myFunctor;
public void CallData(DataHelper.DataCalledEventHandler functor)
{
this.myFunctor = functor;
//SOME CODE THAT RETURNS DATA
}
// when the call completes, asynchronously...
private void WhenTheCallCompletes()
{
var functor = this.myFunctor;
if (functor != null)
{
this.myFunctor = null;
List<DataItem> dataItems = new List<DataItem>();
functor(this, dataItems);
}
}
// in your function
... dataHelper.CallData(this.dataHelper_DataCalled); ...
The below lines on your code should be flipped. That is
These lines
dataHelper.CallData();
dataHelper.DataCalled += new DataHelper.DataCalledEventHandler(dataHelper_DataCalled);
Should be:
dataHelper.DataCalled += new DataHelper.DataCalledEventHandler(dataHelper_DataCalled);
dataHelper.CallData();
Because you first need to attach the event handler and then call other methods on the object which can raise the event
I have some C# code that updates some properties of an object. I have an event handler defined to help me respond when the update process is done. Unfortunately, I've learned that this event is getting fired multiple times. I suspect this is happening because the event handler is being set at the wrong time. Currently, it is being set like the following:
myObject.Update_Succeeded += new EventHandler(myObject_Update_Succeeded);
Due to the complexity of the code, I'm having a difficult time of tracking down where it should be set. I would like to only set the event handler it hasn't been previously set. Because of this, I want to do something like this:
ClearEventHandlers(myObject);
or
myObject.Update_Succeeded = null;
myObject.Update_Succeeded += new EventHandler(myObject_Update_Succeeded);
Is there a way to accomplish what I'm trying?
Thank you!
Yes, you can customize the add/remove accessors of your event. This article describes these accessors. But you can do something like:
class MyClass
{
private EventHandler _myEvent;
public event EventHandler MyEvent
{
[MethodImpl(MethodImplOptions.Synchronized)]
add
{
_myEvent = (EventHandler)Delegate.Combine(_myEvent, value);
}
[MethodImpl(MethodImplOptions.Synchronized)]
remove
{
_myEvent = (EventHandler)Delegate.Remove(_myEvent, value);
}
}
public void ClearMyEvent() {
_myEvent = null;
}
...
}
You should be able to remove a handler using the subtract operator like below
myObject.Update_Succeeded -= new EventHandler(myObject_Update_Succeeded);
Or check this out for a way to remove all event handler if you are in doubt
How to remove all event handlers from a control
Proper way should be to detach the handler from each event after you no longer use it:
public class MyObjectListener
{
private readonly MyObject _object;
public class MyObjectListener(MyObject obj)
{
_object = obj;
Attach();
}
// adds event handlers
private void Attach()
{
obj.UpdateSucceeded += UpdateSuceededHandler;
obj.UpdateFailed += UpdateFailedHandler;
}
// removes event handlers
private void Detach()
{
obj.UpdateSucceeded -= UpdateSuceededHandler;
obj.UpdateFailed -= UpdateFailedHandler;
}
...
}
The only thing you need to decide is where to call the Detach method. For example, you can call it in the handler itself:
private void UpdateSuceededHandler(object sender, Data args)
{
Detach();
// do something when it succeeds
}
private void UpdateFailedHandler(object sender, Data args)
{
Detach();
// do something when it fails
}
Or, you could allow users of MyObjectListener to tell it that it no longer needs to listen to the attached object:
public void StopListening()
{
Detach();
}
An object which raises an event should not allow its listeners to modify the event invocation list. Each event listener should subscribe or unsubscribe its own event handlers only.
You better set event handler on the initialization of your object i.e. in your Constructor.
I have a class, EventContainer.cs, which contains an event, say:
public event EventHandler AfterSearch;
I have another class, EventRaiser.cs. How do I raise (and not handle) the above said event from this class?
The raised event will in turn call the handler of the event in the EventContainer class. Something like this (this is obviously not correct):
EventContainer obj = new EventContainer();
RaiseEvent(obj.AfterSearch);
This is not possible, Events can only be risen from inside the class. If you could do that, it would defeat the purpose of events (being able to rise status changes from inside the class). I think you are misunderstanding the function of events - an event is defined inside a class and others can subscribe to it by doing
obj.AfterSearch += handler; (where handler is a method according to the signature of AfterSearch). One is able to subscribe to the event from the outside just fine, but it can only be risen from inside the class defining it.
It is POSSIBLE, but using clever hack.
Inspired by http://netpl.blogspot.com/2010/10/is-net-type-safe.html
If you don't believe, try this code.
using System;
using System.Runtime.InteropServices;
namespace Overlapping
{
[StructLayout(LayoutKind.Explicit)]
public class OverlapEvents
{
[FieldOffset(0)]
public Foo Source;
[FieldOffset(0)]
public OtherFoo Target;
}
public class Foo
{
public event EventHandler Clicked;
public override string ToString()
{
return "Hello Foo";
}
public void Click()
{
InvokeClicked(EventArgs.Empty);
}
private void InvokeClicked(EventArgs e)
{
var handler = Clicked;
if (handler != null)
handler(this, e);
}
}
public class OtherFoo
{
public event EventHandler Clicked;
public override string ToString()
{
return "Hello OtherFoo";
}
public void Click2()
{
InvokeClicked(EventArgs.Empty);
}
private void InvokeClicked(EventArgs e)
{
var handler = Clicked;
if (handler != null)
handler(this, e);
}
public void Clean()
{
Clicked = null;
}
}
class Test
{
public static void Test3()
{
var a = new Foo();
a.Clicked += AClicked;
a.Click();
var o = new OverlapEvents { Source = a };
o.Target.Click2();
o.Target.Clean();
o.Target.Click2();
a.Click();
}
static void AClicked(object sender, EventArgs e)
{
Console.WriteLine(sender.ToString());
}
}
}
You can write a public method on the class you want the event to fire from and fire the event when it is called. You can then call this method from whatever user of your class.
Of course, this ruins encapsulation and is bad design.
It looks like you're using the Delegate pattern. In this case, the AfterSearch event should be defined on the EventRaiser class, and the EventContainer class should consume the event:
In EventRaiser.cs
public event EventHandler BeforeSearch;
public event EventHandler AfterSearch;
public void ExecuteSearch(...)
{
if (this.BeforeSearch != null)
this.BeforeSearch();
// Do search
if (this.AfterSearch != null)
this.AfterSearch();
}
In EventContainer.cs
public EventContainer(...)
{
EventRaiser er = new EventRaiser();
er.AfterSearch += this.OnAfterSearch;
}
public void OnAfterSearch()
{
// Handle AfterSearch event
}
I stumbled across this problem as well, because i was experimenting with calling PropertyChanged events from outside. So you dont have to implement everything in every class. The solution from halorty wouldn't work using interfaces.
I found a solution working using heavy reflection. It is surely slow and is breaking the principle that events should only be called from inside a class. But it is interesting to find a generic solution to this problem....
It works because every event is a list of invocation methods being called.
So we can get the invocation list and call every listener attached to that event by our own.
Here you go....
class Program
{
static void Main(string[] args)
{
var instance = new TestPropertyChanged();
instance.PropertyChanged += PropertyChanged;
instance.RaiseEvent(nameof(INotifyPropertyChanged.PropertyChanged), new PropertyChangedEventArgs("Hi There from anywhere"));
Console.ReadLine();
}
private static void PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine(e.PropertyName);
}
}
public static class PropertyRaiser
{
private static readonly BindingFlags staticFlags = BindingFlags.Instance | BindingFlags.NonPublic;
public static void RaiseEvent(this object instance, string eventName, EventArgs e)
{
var type = instance.GetType();
var eventField = type.GetField(eventName, staticFlags);
if (eventField == null)
throw new Exception($"Event with name {eventName} could not be found.");
var multicastDelegate = eventField.GetValue(instance) as MulticastDelegate;
if (multicastDelegate == null)
return;
var invocationList = multicastDelegate.GetInvocationList();
foreach (var invocationMethod in invocationList)
invocationMethod.DynamicInvoke(new[] {instance, e});
}
}
public class TestPropertyChanged : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
}
There is good way to do this. Every event in C# has a delegate that specifies the sign of methods for that event. Define a field in your external class with type of your event delegate. get the the reference of that field in the constructor of external class and save it. In main class of your event, send the reference of event for delegate of external class. Now you can easily call the delegate in your external class.
public delegate void MyEventHandler(object Sender, EventArgs Args);
public class MyMain
{
public event MyEventHandler MyEvent;
...
new MyExternal(this.MyEvent);
...
}
public MyExternal
{
private MyEventHandler MyEvent;
public MyExternal(MyEventHandler MyEvent)
{
this.MyEvent = MyEvent;
}
...
this.MyEvent(..., ...);
...
}
Agree with Femaref -- and note this is an important difference between delegates and events (see for example this blog entry for an good discussion of this and other differences).
Depending on what you want to achieve, you might be better off with a delegate.
Not a good programming but if you want to do that any way you can do something like this
class Program
{
static void Main(string[] args)
{
Extension ext = new Extension();
ext.MyEvent += ext_MyEvent;
ext.Dosomething();
}
static void ext_MyEvent(int num)
{
Console.WriteLine(num);
}
}
public class Extension
{
public delegate void MyEventHandler(int num);
public event MyEventHandler MyEvent;
public void Dosomething()
{
int no = 0;
while(true){
if(MyEvent!=null){
MyEvent(++no);
}
}
}
}
I had a similar confusion and honestly find the answers here to be confusing. Although a couple hinted at solutions that I would later find would work.
My solution was to hit the books and become more familiar with delegates and event handlers.
Although I've used both for many years, I was never intimately familiar with them.
http://www.codeproject.com/Articles/20550/C-Event-Implementation-Fundamentals-Best-Practices
gives the best explanation of both delegates and event handlers that I've ever read and clearly explains that a class can be a publisher of events and have other classes consume them.
This article: http://www.codeproject.com/Articles/12285/Implementing-an-event-which-supports-only-a-single discusses how to single-cast events to only one handler since delegates are multicast by definition . A delegate inherits system.MulticastDelegate most including the system delegates are Multicast.
I found that multicast meant that any event handler with the same signature would receive the raised event. Multicast behavior has caused me some sleepless nights as I stepped through code and saw my event seemingly erroneously being sent to handlers that I had no intention of getting this event. Both articles explains this behavior.
The second article shows you one way, and the first article shows you another, by making the delegate and the signature tightly typed.
I personally believe strong typing prevents stupid bugs that can be a pain to find. So I'd vote for the first article, even though I got the second article code working. I was just curious. :-)
I also got curious if I could get #2 articles code to behave like how I interpreted the original question above. Regardless of your chosen approach or if I'm also misinterpreting the original question, my real message is that I still think you would benefit from reading the first article as I did, especially if the questions or answers on this page leave you confused. If you are having multicast nightmares and need a quick solution then article 2 may help you.
I started playing with the second article's eventRaiser class. I made a simple windows form project.
I added the second articles class EventRaiser.cs to my project.
In the Main form's code, I defined a reference to that EventRaiser class at the top as
private EventRaiser eventRaiser = new EventRaiser();
I added a method in the main form code that I wanted to be called when the event was fired
protected void MainResponse( object sender, EventArgs eArgs )
{
MessageBox.Show("got to MainResponse");
}
then in the main form's constructor I added the event assignment:
eventRaiser.OnRaiseEvent += new EventHandler(MainResponse);`
I then created a class that would be instantiated by my main form called "SimpleClass" for lack of creative ingenuity at the moment.
Then I added a button and in the button's click event
I instantiated the SimpleClass code I wanted to raise an event from:
private void button1_Click( object sender, EventArgs e )
{
SimpleClass sc = new SimpleClass(eventRaiser);
}
Note the instance of "eventRaiser" that I passed to SimpleClass.cs. That was defined and instantiated earlier in the Main form code.
In the SimpleClass:
using System.Windows.Forms;
using SinglecastEvent; // see SingleCastEvent Project for info or http://www.codeproject.com/Articles/12285/Implementing-an-event-which-supports-only-a-single
namespace GenericTest
{
public class SimpleClass
{
private EventRaiser eventRaiser = new EventRaiser();
public SimpleClass( EventRaiser ev )
{
eventRaiser = ev;
simpleMethod();
}
private void simpleMethod()
{
MessageBox.Show("in FileWatcher.simple() about to raise the event");
eventRaiser.RaiseEvent();
}
}
}
The only point to the private method I called SimpleMethod was to verify that a privately scoped method could still raise the event, not that I doubted it, but I like to be positive.
I ran the project and this resulted in raising the event from the "simpleMethod" of the "SimpleClass" up to the main form and going to the expected correct method called MainResponse proving that one class can indeed raise an event that is consumed by a different class.
Yes the event has to be raised from within the class that needs it's change broadcast to other classes that care. Receiving classes can be one class or many many classes depending on how strongly typed you defined them or by making them single cast as in 2nd article.
Hope this helps and not muddy the water. Personally I've got a lot of delegates and events to clean up! Multicast demons begone!
The raising class has to get a fresh copy of the EventHandler.
One possible solution below.
using System;
namespace ConsoleApplication1
{
class Program
{
class HasEvent
{
public event EventHandler OnEnvent;
EventInvoker myInvoker;
public HasEvent()
{
myInvoker = new EventInvoker(this, () => OnEnvent);
}
public void MyInvokerRaising() {
myInvoker.Raise();
}
}
class EventInvoker
{
private Func<EventHandler> GetEventHandler;
private object sender;
public EventInvoker(object sender, Func<EventHandler> GetEventHandler)
{
this.sender = sender;
this.GetEventHandler = GetEventHandler;
}
public void Raise()
{
if(null != GetEventHandler())
{
GetEventHandler()(sender, new EventArgs());
}
}
}
static void Main(string[] args)
{
HasEvent h = new HasEvent();
h.OnEnvent += H_OnEnvent;
h.MyInvokerRaising();
}
private static void H_OnEnvent(object sender, EventArgs e)
{
Console.WriteLine("FIRED");
}
}
}
Use public EventHandler AfterSearch;
not
public event EventHandler AfterSearch;
Use a Delegate (an Action or Func) instead of an event. An event is essentially a delegate that can only be triggered from within the class.
I took a slightly different approach in solving this problem. My solution consisted of a winform front end, a main Class Library (DLL) and within that dll, a secondary working class:
WinForm
|------> PickGen Library
|---------> Allocations class
What I decided to do is to create events in the main dll (PickGen) that the Allocations class could call, then those event methods would called the events within the UI.
So, allocations raises an event in PickGen which takes the parameter values and raises the event in the form. From a code standpoint, this is in the lowest class:
public delegate void AllocationService_RaiseAllocLog(string orderNumber, string message, bool logToDatabase);
public delegate void AllocationService_RaiseAllocErrorLog(string orderNumber, string message, bool logToDatabase);
public class AllocationService { ...
public event AllocationService_RaiseAllocLog RaiseAllocLog;
public event AllocationService_RaiseAllocErrorLog RaiseAllocErrorLog;
then later in the subclass code:
RaiseAllocErrorLog(SOHNUM_0, ShipmentGenerated + ": Allocated line QTY was: " + allocatedline.QTY_0 + ", Delivered was: " + QTY_0 + ". Problem batch.", false);
In the main DLL Class library I have these two event methods:
private void PickGenLibrary_RaiseAllocLog(string orderNumber, string message, bool updateDB)
{
RaiseLog(orderNumber, message, false);
}
private void PickGenLibrary_RaiseAllocErrorLog(string orderNumber, string message, bool updateDB)
{
RaiseErrorLog(orderNumber, message, false);
}
and I make the connection here when I create the allocation object:
AllocationService allsvc = new AllocationService(PickResult);
allsvc.RaiseAllocLog += new AllocationService_RaiseAllocLog(PickGenLibrary_RaiseAllocLog);
allsvc.RaiseAllocErrorLog += new AllocationService_RaiseAllocErrorLog(PickGenLibrary_RaiseAllocErrorLog);
and I also then have delegates that are set up to tie the main class with the winform code:
public delegate void JPPAPickGenLibrary_RaiseLog(string orderNumber, string message, bool logToDatabase);
public delegate void JPPAPickGenLibrary_RaiseErrorLog(string orderNumber, string message, bool logToDatabase);
It may not be the most elegant way to do it, but in the end, it does work and without being too obscure.
A nested class with an instance of the outer class provided in the constructor can access even private members of the outer class. As explained more here: stackoverflow question on inner classes.
This includes the ability to raise events in the outer class. This EventRaisers class could be internal, or otherwise controlled somehow, because it could technically otherwise be created by any script with a reference to the outer class instance.
Very simple example. i like to do it this way using EventHandler.
class Program
{
static void Main(string[] args)
{
MyExtension ext = new MyExtension();
ext.MyEvent += ext_MyEvent;
ext.Dosomething();
Console.ReadLine();
}
static void ext_MyEvent(object sender, int num)
{
Console.WriteLine("Event fired.... "+num);
}
}
public class MyExtension
{
public event EventHandler<int> MyEvent;
public void Dosomething()
{
int no = 1;
if (MyEvent != null)
MyEvent(this, ++no);
}
}
}