Develop Reference

Having trouble changing event handler

It's for my irc bot, and I am trying to change the message receiver event to be linked to the method in my other class.
private static void client_Connected(object sender, EventArgs e)
{
gamebot.LocalUser.JoinedChannel += LocalUser_JoinedChannel;
gamebot.LocalUser.MessageReceived += LocalUser_MessageReceived;
}
// private static void newmessage(object sender, IrcChannelEventArgs e)
// {
// e.Channel.MessageReceived += Hangman.MessageReceivedHangman;
// }
private static void LocalUser_JoinedChannel(object sender, IrcChannelEventArgs e)
{
e.Channel.MessageReceived += Channel_MessageReceived;
Console.WriteLine("Joined " + e.Channel + "\n");
}
Just not sure how to get the channeleventargs outside of a method, so I can change the event. The commented method shows sort of what i need.
public static void MessageReceivedHangman(object sender, IrcMessageEventArgs e)
{
That is the method in a different class i would like to have execute when a message is received.
Thanks for the help and sorry if this is a really stupid question I'm pretty new to all of this still.

It's hard to know what would be best here, as you have provided so little context. All we really know is that you have one class (call it class A) handling specific events, and another class (call it class B) that wants to be able to handle events the first class already knows about.
Based on that, there are at least a couple of possibilities that might work for you.
Option #1:
Expose the "joined" event so that the second class can receive the same notifications and subscribe to the channel's event:
class JoinedChannelEventArgs : EventArgs
{
public Channel Channel { get; private set; }
public JoinedChannelEventArgs(Channel channel) { Channel = channel; }
}
class A
{
public static event EventHandler<JoinedChannelEventArgs> JoinedChannel;
private static void LocalUser_JoinedChannel(object sender, IrcChannelEventArgs e)
{
e.Channel.MessageReceived += Channel_MessageReceived;
Console.WriteLine("Joined " + e.Channel + "\n");
EventHandler<JoinedChannelEventArgs> handler = JoinedChannel;
if (handler != null)
{
handler(null, new JoinedChannelEventArgs(e.Channel);
}
}
}
class B
{
static void SomeMethod()
{
A.JoinedChannel += A_JoinedChannel;
}
private static void A_JoinedChannel(object sender, JoinedChannelEventArgs e)
{
e.Channel += MessageReceivedHangman;
}
}
Option #2:
Expose the "message received" event instead:
class A
{
public static event EventHandler<IrcMessageEventArgs> AnyChannelMessageReceived;
public static void Channel_MessageReceived(object sender, IrcMessageEventArgs e)
{
// Whatever other code you had here, would remain
EventHandler<IrcMessageEventArgs> handler = AnyChannelMessageReceived;
if (handler != null)
{
handler(null, e);
}
}
}
class B
{
static void SomeMethod()
{
A.AnyChannelMessageReceived += MessageReceivedHangman;
}
}
It's not clear from your post whether having the sender of the original event is important. If it is, then IMHO Option #1 is better, as it provides direct access to the event. However, you could modify Option #2 so that it passed sender to the handler (in Channel_MessageReceived()), instead of the null that's in the example (the null is more idiomatic for a static event, but not mandatory).
If neither of those options work for you, please provide better context. See https://stackoverflow.com/help/mcve and https://stackoverflow.com/help/how-to-ask.

C# generic event handling - chaining/proxy

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);
}

How to implement an event system

Im trying to implement an Event System for a game, where there are classes that can fire or handle an event wheter or not they implement these interfaces:
public interface IGameEvent<T> where T : EventArgs
{
event EventHandler<T> OnEvent;
}
public interface IGameHandler<T> where T : EventArgs
{
void OnEvent(object sender, T e);
}
everything looked great until i realized that no class can implement more than 1 IGameEvent
because it would cause duplicate declaration,
Here is an example:
public class Event
{
public KeyPressEvent OnKeyPress;
public UpdateEvent OnUpdate;
public void AddHadler<T>(IGameEvent<T> eEvent , IGameHandler<T> eHandler) where T : EventArgs
{
eEvent.OnEvent += eHandler.OnEvent;
}
public void RemoveHandler<T>(IGameEvent<T> eEvent, IGameHandler<T> eHandler) where T : EventArgs
{
eEvent.OnEvent -= eHandler.OnEvent;
}
}
KeyPressEvent:
public class KeyPressEvent : IGameEvent<KeyPressEvent.KeyPressedEventArgs>
{
public class KeyPressedEventArgs : EventArgs
{
public KeyPressedEventArgs(Keys key)
{
Key = key;
}
public Keys Key { get; private set; }
}
public event EventHandler<KeyPressedEventArgs> OnEvent;
private void OnCheckForKeyPressed() //Example
{
if (OnEvent != null)
OnEvent(this, new KeyPressedEventArgs(Keys.Space));
}
}
Would be better to manually store the suscribers in a list in the EventSystem ?
How slower or faster than this approach that would be?
Thanks!

The common approach is to implement events aggregator pattern. Google search will provide you with tons of different variations. For example, it might look like this (what i am currently using):
interface IEventsAggrgator
{
//fires an event, reperesented by specific message
void Publish<TMessage>(TMessage message);
//adds object to the list of subscribers
void Subscribe(object listener);
//remove object from the list of subscribers
void Unsubscribe(object listener);
}
interface IHandler<TMessage>
{
//implement this in subscribers to handle specific messages
void Handle(TMessage message);
}

Collection of Events

I am using events as a publisher/subscriber pattern in c#. However I dont know at design time how many publishers my program will be using. I would like to dynamically add events to either a class directly, or more plausibly to a collection/dictionary containing the events.
Are either of these scenarios possible using C#?

Create a mediator that your publishers publish to and that your subscribers subscribe to. For example:
public class Mediator
{
public static readonly Mediator Current = new Mediator();
public event EventHandler<EventArgs> EventRaised;
public void RaiseEvent(object sender, EventArgs eventArgs)
{
if (EventRaised!=null)
EventRaised(sender, eventArgs);
}
}
public class PublisherEventArgs : EventArgs
{
public string SomeData { get; set; }
}
public class Publisher
{
public void Publish(string data)
{
Mediator.Current.RaiseEvent(this, new PublisherEventArgs() { SomeData = data} );
}
}
public class Subscriber
{
public Subscriber()
{
Mediator.Current.EventRaised += HandlePublishedEvent;
}
private void HandlePublishedEvent(object sender, EventArgs e)
{
if(e is PublisherEventArgs)
{
string data = ((PublisherEventArgs)e).SomeData;
// todo: do something here
}
}
}
Make sure you implement IDisposable on your subscriber (its not in my example) so that it unsubscribes from the Mediator during dispose.

You'll need to disconnect the event from it's source in order to do this. This is commonly done with an event aggregator; it manages clients that want to publish events as well as those that want to subscribe to events. All of this decouples the publishers from the listeners and allows you to do what you are describing.
Prism has an event aggregator out-of-the-box that you can use in the form of the IEventAggregator interface.

C# event handling (compared to Java)

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));
}
}
}