I am trying to create my own custom ToolTip class where I register the Loading event in the constructor. How do I unsubscribe from this event? When I do it in the finalizer/destructor, it causes an exception especially when I open a new window of the app and close it:
COM object that has been separated from its underlying RCW cannot be used.
Here is my code:
public class CustomToolTip: ToolTip
{
public CustomToolTip()
{
this.Loading += CustomToolTip_Loading;
}
protected void CustomToolTip_Loading(Windows.UI.Xaml.FrameworkElement sender, object args)
{
// my logic here
}
~CustomToolTip()
{
this.Loading -= CustomToolTip_Loading;
}
}
This exception also happens when I create a custom Flyout control and try to unsubscribe the Opening event in the finalizer.
My code:
public class CustomFlyout: Flyout
{
public CustomFlyout()
{
this.Opening += CustomFlyout_Opening;
}
private void CustomFlyout_Opening(object sender, object e)
{
// my logic here
}
~CustomFlyout()
{
this.Opening -= CustomFlyout_Opening;
}
}
Any help will be greatly appreciated.
You should not define a finalizer in your class unless you deal directly with unmanaged resources which you certainly don't here.
If you only want to handle the event once regardless of how many times your control is loaded, you could simply unregister the event handler when the Loading event has been raised:
public class CustomToolTip : ToolTip
{
public CustomToolTip()
{
this.Loading += CustomToolTip_Loading;
}
protected void CustomToolTip_Loading(Windows.UI.Xaml.FrameworkElement sender, object args)
{
this.Loading -= CustomToolTip_Loading;
//...
}
}
Otherwise, there is an Unloading event:
private void CustomToolTip_Unloaded(object sender, RoutedEventArgs e)
{
this.Loading -= CustomToolTip_Loading;
this.Unloaded -= CustomToolTip_Unloaded;
}
Also note that unregister from an event to avoid memory leaks is only important when the publisher lives longer than the subscriber.
As the T.Schwarz said, it would never get to executing the finalizer if there are still references to the object. Therefore, you need to remove eventHandlers somewhere else, such as the Dispose method.
public class CustomToolTip : ToolTip,IDisposable
{
……
public void Dispose()
{
this.Loading -= CustomToolTip_Loading;
}
}
Related
What is the best way or best practice to propagate events from a base class (and handle them at the base as well) to the inherited or implementing application - because I want all the code to execute from the base to the MyClientListener to the Winform ?
I have a [WCF DuplexClient] class which other [WCF ClientListener] classes will derive from. I want to make it reusable for all of my services. I have an event InnerChannel_Faulted - in this base class I have an initializer in the base class which subscribes to events and the base class will generally handle these as far as the WCF side of things goes. I want to also be able to have my specific ClientListener implementation to be able to provide additional functionality - behavior to those events - mostly for the Winforms Application.
Is my thinking right here - or do I need to regurgitate the events up the food chain so they will be available to the Winforms app ?
I have made the handlers in the base class like this:
class MyClient<T> :DuplexClientBase<T> where T : class
{
protected virtual void InitializeClient()
{
base.InnerChannel.Faulted += InnerChannel_Faulted;
}
protected virtual void InnerChannel_Faulted(object sender, EventArgs e)
{
// ... do something()
}
}
class MyListener : MyClient<MyListenerService>
{
public MyListener(){ // do stuff}
// .. other methods
}
WINDOWFORMAPP : FORM
{
private MyListener mylistener = new MyListener();
WINDOWFORMAPP()
{
// somehow subscribe to
mylistener.InnerChannel_Faulted +=
}
private override void InnerChannel_Faulted(object sender, EventArgs e)
{
// DoSomething to GUI - notifications GUI elements etc..
// then call.
mylistener.InnerChannel_Faulted()
}
}
It's not standard to subscribe to the event from the same class or subclass. The usual approach is to structure the code as:
public class MyClass {
public event EventHandler SomeAction;
private void DoStuff() {
bool fireAction = false;
//....
if (fireAction) {
EventArgs e = ...; // can be more specific if needed
OnSomeAction(e);
}
}
protected virtual void OnSomeAction(EventArgs e) {
if (SomeAction != null)
SomeAction(this, e);
}
}
public class MySubclass : MyClass {
protected override void OnSomeAction(EventArgs e) {
// code before event is triggered
base.OnSomeAction(e); // fires event to listeners
// code after event is triggered
}
}
Then in your form:
public class MyForm : Form {
MyClass mc = new MyClass();
public MyForm() {
mc.SomeAction += mc_SomeAction;
}
private void mc_SomeAction(Object sender, EventArgs e) {
//...
}
}
I'm trying to come up with a generic way to intercept C# events for processing (e.g. logging, filtering, distributing asynchronously etc.), before passing the event back on to its original consumer.
So to give an example, in the normal flow of things you'd have
class EventProducer
{
public event EventHandler<int> OnEvent;
internal void FireEvent()
{
if (OnEvent != null)
OnEvent(this, 1);
}
}
class EventConsumer
{
public EventConsumer(EventProducer producer)
{
producer.OnEvent += eventHandler;
}
public void eventHandler(object sender, int e)
{
Debug.WriteLine("Consumed " + e);
}
}
In my case, I'd want to insert an additional step between the two.. something like
//event consumer
class EventConsumer
{
//use this to handle one particular event (EventProducer.OnEvent)
InterceptEventHandler<int> EventHandler;
public EventConsumer(EventProducer producer)
{
//just one line of code to insert an event handler to do what I need
//but whoops - cant refer to OnEvent outside of the producer
EventHandler = new InterceptEventHandler<int>(producer.OnEvent, eventHandler);
}
public void eventHandler(object sender, int e)
{
Debug.WriteLine("Consumed " + e);
}
}
//intercepts events, does something with them, then forwards them to original consumer
class InterceptEventHandler<T>
{
public EventHandler<T> Callback;
public InterceptEventHandler(EventHandler<T> eventHook, EventHandler<T> callback)
{
//save callback
Callback = callback;
//subscribe ourselves to the specified event
eventHook += interceptHandler;
}
public void interceptHandler(object sender, T e)
{
//do something with the event here
Debug.WriteLine("Intercepted " + e.ToString());
//then pass callback back to original consumer
Callback(sender, e);
}
}
The problem (as commented above) is that the code doesn't compile because the events themselves are not accessible from outside the producer class. I understand this is because the event handling is implemented as a private class, so can't see any obvious way around this.
Is there any way to intercept and chain events that would allow me to keep the standard event syntax in producer classes?
I realise that I can abandon events, and just use my own equivalent producer/consumer code, but i'll lose the normal benefits of events (e.g. more readable/maintainable, proper syntax highlighting/autocomplete etc.)
EDIT - I also tried fiddling around with the MulticastDelegate class, but while I got the code to compile, I couldn't get the events to be subscribed (effectively OnEvent was always null).
Rather than accepting an EventHandler which is of course not the type of an event, accept an Action<EventHandler> which represents an action that subscribes to the event:
public class InterceptEventHandler
{
public static void Attach<T>(Action<EventHandler<T>> eventHook,
EventHandler<T> callback)
{
eventHook((sender, args) =>
{
doStuffOnFire(sender, args);
callback(sender, args);
});
}
private static void doStuffOnFire<T>(object sender, T e)
{
//...
}
}
You would then call it like so:
public EventConsumer(EventProducer producer)
{
InterceptEventHandler.Attach<int>(
handler => producer.OnEvent += handler,
eventHandler);
}
All implementation of IHttpModule I've seen looks following:
class HttpCompressionModule : IHttpModule
{
public void Init(HttpApplication application)
{
application.SomeEvent += OnSomeEvent;
}
private void OnSomeEvent(Object source, EventArgs e)
{
// ...
}
public void Dispose()
{
// nothing here !!!
}
}
I am wondering why is the Dispose method always empty? Shouldn't we unsubscribe the event which we subscribe in the Init method?
The lifecycle of an HttpModule is tightly integrated with the lifecycle of an HttpApplication. Instances of HttpModule are generated when the application is started and destroyed when the application is disposed of.
In this case there is no point in unsubscribing from the event because the publisher (HttpApplication) is being disposed of anyway. Of course, in a situation where the publisher wasn't being disposed of, unhooking the event handler would be the right thing to do.
The dispose method won't be empty if you need to instantiate IDisposable objects inside your module.
class HttpCompressionModule : IHttpModule
{
private IDisposalbe _myResource;
public void Init(HttpApplication application)
{
_myResource = new MyDisposableResource();
application.SomeEvent += OnSomeEvent;
}
private void OnSomeEvent(Object source, EventArgs e)
{
// ...
myResource.DoSomething();
}
public void Dispose()
{
_myResource.Dispose();
}
}
I need to be able to trigger a event whenever an object is added to a Queue<Delegate>.
I created a new class that extends Queue:
public delegate void ChangedEventHandler(object sender, EventArgs e);
public class QueueWithChange<Delegate> : Queue<Delegate>
{
public event ChangedEventHandler Changed;
protected virtual void OnChanged(EventArgs e) {
if (Changed != null)
{
Changed(this, e);
}
}
}
And then attached the event from another class, like such:
QueueWithChange<TimerDelegate> eventQueue = new QueueWithChange<TimerDelegate>();
//
eventQueue.Changed += new ChangedEventHandler(delegate(object s, EventArgs ex) {
//This event is not being triggered, so this code is unreachable atm...and that is my problem
if (eventQueue.Count > 0)
{
eventQueue.Dequeue().Invoke(new DispatcherTimer() { Interval = TimeSpan.FromMilliseconds(5) });
actionTimer.Stop();
}
});
But whenever I enqueue an object (eventQueue.Enqueue(something)), the attached event is not being fired.
What am I missing here?
If you mean the non-generic Queue class, then you can just override Enqueue:
public override void Enqueue(object obj)
{
base.Enqueue(obj);
OnChanged(EventArgs.Empty);
}
However, if you mean the generic Queue<T> class, then note that there is no suitable virtual method to override. You might do better to encapsulate the queue with your own class:
(** important edit: removed base-class!!! **)
class Foo<T>
{
private readonly Queue<T> queue = new Queue<T>();
public event EventHandler Changed;
protected virtual void OnChanged()
{
if (Changed != null) Changed(this, EventArgs.Empty);
}
public virtual void Enqueue(T item)
{
queue.Enqueue(item);
OnChanged();
}
public int Count { get { return queue.Count; } }
public virtual T Dequeue()
{
T item = queue.Dequeue();
OnChanged();
return item;
}
}
However, looking at your code, it seems possible that you are using multiple threads here. If that is the case, consider a threaded queue instead.
I just did write up on what I call a TriggeredQueue. It's inspired the answer by Marc Gravell.
You can find my post here: http://joesauve.com/triggeredqueuet
And the Gist here: http://gist.github.com/jsauve/b2e8496172fdabd370c4
It has four events:
WillEnqueue
WillDequeue
DidEnqueue
DidDequeue
You can hook into any of these like so:
YourQueue.WillEnqueue += (sender, e) => {
// kick off some process
};
YourQueue.DidEnqueue += (sender, e) => {
// kick off some process
// e.Item provides access to the enqueued item, if you like
};
YourQueue.WillDequeue += (sender, e) => {
// kick off some process
};
YourQueue.DidDequeue += (sender, e) => {
// kick off some process
// e.Item provides access to the dequeued item, if you like
};
One neat trick is that you can use the DidDequeue method to kick off some process to ensure that the queue is full by making a web request or loading some data from a filesystem, etc. I use this class in Xamarin mobile apps to ensure that data and images are pre-cached in order to provide a smooth user experience, instead of loading images AFTER they scroll onto the screen (like you might see in Facebook and countless other apps).
try
public new void Enqueue(Delegate d)
{
base.Enqueue(d);
OnChanged(EventArgs.Empty);
}
You have to override Enqueue, to call OnChanged.
I am currently having a hardtime understanding and implementing events in C# using delagates. I am used to the Java way of doing things:
Define an interface for a listener type which would contain a number of method definitions
Define adapter class for that interface to make things easier if I'm not interested in all the events defined in a listener
Define Add, Remove and Get[] methods in the class which raises the events
Define protected fire methods to do the dirty work of looping through the list of added listeners and calling the correct method
This I understand (and like!) - I know I could do this exactly the same in c#, but it seems that a new (better?) system is in place for c#. After reading countless tutorials explaining the use of delegates and events in c# I still am no closer to really understanding what is going on :S
In short, for the following methods how would I implement the event system in c#:
void computerStarted(Computer computer);
void computerStopped(Computer computer);
void computerReset(Computer computer);
void computerError(Computer computer, Exception error);
^ The above methods are taken from a Java application I once made which I'm trying to port over to c#.
Many many thanks!
You'd create four events, and methods to raise them, along with a new EventArgs-based class to indicate the error:
public class ExceptionEventArgs : EventArgs
{
private readonly Exception error;
public ExceptionEventArgs(Exception error)
{
this.error = error;
}
public Error
{
get { return error; }
}
}
public class Computer
{
public event EventHandler Started = delegate{};
public event EventHandler Stopped = delegate{};
public event EventHandler Reset = delegate{};
public event EventHandler<ExceptionEventArgs> Error = delegate{};
protected void OnStarted()
{
Started(this, EventArgs.Empty);
}
protected void OnStopped()
{
Stopped(this, EventArgs.Empty);
}
protected void OnReset()
{
Reset(this, EventArgs.Empty);
}
protected void OnError(Exception e)
{
Error(this, new ExceptionEventArgs(e));
}
}
Classes would then subscribe to the event using either a method or a an anonymous function:
someComputer.Started += StartEventHandler; // A method
someComputer.Stopped += delegate(object o, EventArgs e)
{
Console.WriteLine("{0} has started", o);
};
someComputer.Reset += (o, e) => Console.WriteLine("{0} has been reset");
A few things to note about the above:
The OnXXX methods are protected so that derived classes can raise the events. This isn't always necessary - do it as you see fit.
The delegate{} piece on each event declaration is just a trick to avoid having to do a null check. It's subscribing a no-op event handler to each event
The event declarations are field-like events. What's actually being created is both a variable and an event. Inside the class you see the variable; outside the class you see the event.
See my events/delegates article for much more detail on events.
You'll have to define a single delegate for that
public delegate void ComputerEvent(object sender, ComputerEventArgs e);
ComputerEventArgs would be defined like this:
public class ComputerEventArgs : EventArgs
{
// TODO wrap in properties
public Computer computer;
public Exception error;
public ComputerEventArgs(Computer aComputer, Exception anError)
{
computer = aComputer;
error = anError;
}
public ComputerEventArgs(Computer aComputer) : this(aComputer, null)
{
}
}
The class that fires the events would have these:
public YourClass
{
...
public event ComputerEvent ComputerStarted;
public event ComputerEvent ComputerStopped;
public event ComputerEvent ComputerReset;
public event ComputerEvent ComputerError;
...
}
This is how you assign handlers to the events:
YourClass obj = new YourClass();
obj.ComputerStarted += new ComputerEvent(your_computer_started_handler);
Your handler is:
private void ComputerStartedEventHandler(object sender, ComputerEventArgs e)
{
// do your thing.
}
The main difference is that in C# the events are not interface-based. Instead, the event publisher declares the delegate which you can think of as a function pointer (although not exactly the same :-)). The subscriber then implements the event prototype as a regular method and adds a new instance of the delegate to the event handler chain of the publisher. Read more about delegates and events.
You can also read short comparison of C# vs. Java events here.
First of all, there is a standard method signature in .Net that is typically used for events. The languages allow any sort of method signature at all to be used for events, and there are some experts who believe the convention is flawed (I mostly agree), but it is what it is and I will follow it for this example.
Create a class that will contain the event’s parameters (derived from EventArgs).
public class ComputerEventArgs : EventArgs
{
Computer computer;
// constructor, properties, etc.
}
Create a public event on the class that is to fire the event.
class ComputerEventGenerator // I picked a terrible name BTW.
{
public event EventHandler<ComputerEventArgs> ComputerStarted;
public event EventHandler<ComputerEventArgs> ComputerStopped;
public event EventHandler<ComputerEventArgs> ComputerReset;
...
}
Call the events.
class ComputerEventGenerator
{
...
private void OnComputerStarted(Computer computer)
{
EventHandler<ComputerEventArgs> temp = ComputerStarted;
if (temp != null) temp(this, new ComputerEventArgs(computer)); // replace "this" with null if the event is static
}
}
Attach a handler for the event.
void OnLoad()
{
ComputerEventGenerator computerEventGenerator = new ComputerEventGenerator();
computerEventGenerator.ComputerStarted += new EventHandler<ComputerEventArgs>(ComputerEventGenerator_ComputerStarted);
}
Create the handler you just attached (mostly by pressing the Tab key in VS).
private void ComputerEventGenerator_ComputerStarted(object sender, ComputerEventArgs args)
{
if (args.Computer.Name == "HAL9000")
ShutItDownNow(args.Computer);
}
Don't forget to detach the handler when you're done. (Forgetting to do this is the biggest source of memory leaks in C#!)
void OnClose()
{
ComputerEventGenerator.ComputerStarted -= ComputerEventGenerator_ComputerStarted;
}
And that's it!
EDIT: I honestly can't figure out why my numbered points all appear as "1." I hate computers.
there are several ways to do what you want. The most direct way would be to define delegates for each event in the hosting class, e.g.
public delegate void ComputerStartedDelegate(Computer computer);
protected event ComputerStartedDelegate ComputerStarted;
public void OnComputerStarted(Computer computer)
{
if (ComputerStarted != null)
{
ComputerStarted.Invoke(computer);
}
}
protected void someMethod()
{
//...
computer.Started = true; //or whatever
OnComputerStarted(computer);
//...
}
any object may 'listen' for this event simply by:
Computer comp = new Computer();
comp.ComputerStarted += new ComputerStartedDelegate(
this.ComputerStartedHandler);
protected void ComputerStartedHandler(Computer computer)
{
//do something
}
The 'recommended standard way' of doing this would be to define a subclass of EventArgs to hold the Computer (and old/new state and exception) value(s), reducing 4 delegates to one. In this case that would be a cleaner solution, esp. with an Enum for the computer states in case of later expansion. But the basic technique remains the same:
the delegate defines the signature/interface for the event handler/listener
the event data member is a list of 'listeners'
listeners are removed using the -= syntax instead of +=
In c# events are delegates. They behave in a similar way to a function pointer in C/C++ but are actual classes derived from System.Delegate.
In this case, create a custom EventArgs class to pass the Computer object.
public class ComputerEventArgs : EventArgs
{
private Computer _computer;
public ComputerEventArgs(Computer computer) {
_computer = computer;
}
public Computer Computer { get { return _computer; } }
}
Then expose the events from the producer:
public class ComputerEventProducer
{
public event EventHandler<ComputerEventArgs> Started;
public event EventHandler<ComputerEventArgs> Stopped;
public event EventHandler<ComputerEventArgs> Reset;
public event EventHandler<ComputerEventArgs> Error;
/*
// Invokes the Started event */
private void OnStarted(Computer computer) {
if( Started != null ) {
Started(this, new ComputerEventArgs(computer));
}
}
// Add OnStopped, OnReset and OnError
}
The consumer of the events then binds a handler function to each event on the consumer.
public class ComputerEventConsumer
{
public void ComputerEventConsumer(ComputerEventProducer producer) {
producer.Started += new EventHandler<ComputerEventArgs>(ComputerStarted);
// Add other event handlers
}
private void ComputerStarted(object sender, ComputerEventArgs e) {
}
}
When the ComputerEventProducer calls OnStarted the Started event is invoked which in turn will call the ComputerEventConsumer.ComputerStarted method.
The delegate declares a function signature, and when it's used as an event on a class it also acts as a collection of enlisted call targets. The += and -= syntax on an event is used to adding a target to the list.
Given the following delegates used as events:
// arguments for events
public class ComputerEventArgs : EventArgs
{
public Computer Computer { get; set; }
}
public class ComputerErrorEventArgs : ComputerEventArgs
{
public Exception Error { get; set; }
}
// delegates for events
public delegate void ComputerEventHandler(object sender, ComputerEventArgs e);
public delegate void ComputerErrorEventHandler(object sender, ComputerErrorEventArgs e);
// component that raises events
public class Thing
{
public event ComputerEventHandler Started;
public event ComputerEventHandler Stopped;
public event ComputerEventHandler Reset;
public event ComputerErrorEventHandler Error;
}
You would subscribe to those events with the following:
class Program
{
static void Main(string[] args)
{
var thing = new Thing();
thing.Started += thing_Started;
}
static void thing_Started(object sender, ComputerEventArgs e)
{
throw new NotImplementedException();
}
}
Although the arguments could be anything, the object sender and EventArgs e is a convention that's used very consistently. The += thing_started will first create an instance of the delegate pointing to target method, then add it to the event.
On the component itself you would typically add methods to fire the events:
public class Thing
{
public event ComputerEventHandler Started;
public void OnStarted(Computer computer)
{
if (Started != null)
Started(this, new ComputerEventArgs {Computer = computer});
}
}
You must test for null in case no delegates have been added to the event. When you make the method call however all delegates which have been added will be called. This is why for events the return type is void - there is no single return value - so to feed back information you would have properties on the EventArgs which the event handlers would alter.
Another refinement would be to use the generic EventHandler delegate rather than declaring a concrete delegate for each type of args.
public class Thing
{
public event EventHandler<ComputerEventArgs> Started;
public event EventHandler<ComputerEventArgs> Stopped;
public event EventHandler<ComputerEventArgs> Reset;
public event EventHandler<ComputerErrorEventArgs> Error;
}
Thank you all so much for your answers! Finally I'm starting to understand what is going on. Just one thing; It seems that if each event had a different number/type of arguments I'd need to create a different :: EventArgs class to deal with it:
public void computerStarted(Computer computer);
public void computerStopped(Computer computer);
public void computerReset(Computer computer);
public void breakPointHit(Computer computer, int breakpoint);
public void computerError(Computer computer, Exception exception);
This would require three classses to deal with the events!? (Well two custom, and one using the default EventArgs.Empty class)
Cheers!
Ok, FINAL clarification!: So this is pretty much the best I can do code-wise to implement those events?
public class Computer {
public event EventHandler Started;
public event EventHandler Stopped;
public event EventHandler Reset;
public event EventHandler<BreakPointEvent> BreakPointHit;
public event EventHandler<ExceptionEvent> Error;
public Computer() {
Started = delegate { };
Stopped = delegate { };
Reset = delegate { };
BreakPointHit = delegate { };
Error = delegate { };
}
protected void OnStarted() {
Started(this, EventArgs.Empty);
}
protected void OnStopped() {
Stopped(this, EventArgs.Empty);
}
protected void OnReset() {
Reset(this, EventArgs.Empty);
}
protected void OnBreakPointHit(int breakPoint) {
BreakPointHit(this, new BreakPointEvent(breakPoint));
}
protected void OnError(System.Exception exception) {
Error(this, new ExceptionEvent(exception));
}
}
}