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Async event handler callback through event arguments

I have a problem when I need to collect event argument data that has been changed through event hadnler that contains async calls. As you can see in code below, MessageReceiver raises event and collects data from property "Change", and continues processing. Problem is when event handler is async because it calles asny method with "await". In method OnMyEvent code continues immediately after calling the handler, it does not wait for handler to finish, so "Change" property is not set... How to make things more "in sync"?
public class MyEventArgs:EventArgs
{
public bool Change { get; set; }
}
public delegate void MyEventHandler(object sender, MyEventArgs e);
public class MessageReceiver
{
public event MyEventHandler MyEvent;
public void RaiseEvent()
{
OnMyEvent();
}
protected void OnMyEvent()
{
MyEventArgs e = new MyEventArgs();
if (MyEvent != null)
MyEvent(this, e);
if (e.Change)
Console.WriteLine("Change occured");
}
}
public class Form
{
MessageReceiver mr;
public Form(MessageReceiver MR)
{
mr = MR;
mr.MyEvent += Mr_MyEvent;
}
private async void Mr_MyEvent(object sender, MyEventArgs e)
{
string s = await GetString();
e.Change = true;
}
public async Task<string> GetString()
{
return await Task.Factory.StartNew(() =>
{
return Guid.NewGuid().ToString();
}
);
}
}
public class Program
{
static void Main(string[] args)
{
MessageReceiver mr = new MessageReceiver();
Form frm = new Form(mr);
mr.RaiseEvent();
}
}

I discuss a variety of approaches for asynchronous events on my blog. I recommend the deferral approach, which you can implement using the DeferralManager and IDeferralSource types from my Nito.AsyncEx.Coordination library:
public class MyEventArgs: EventArgs, IDeferralSource
{
private readonly DeferralManager _deferralManager;
public MyEventArgs(DeferralManager deferralManager)
{
_deferralManager = deferralManager;
}
public bool Change { get; set; }
public IDisposable GetDeferral() { return _deferralManager.GetDeferral(); }
}
public class MessageReceiver
{
protected async Task OnMyEventAsync()
{
if (MyEvent != null)
{
DeferralManager deferralManager = new DeferralManager();
MyEventArgs e = new MyEventArgs(deferralManager);
MyEvent(this, e);
await deferralManager.WaitForDeferralsAsync();
}
if (e.Change)
Console.WriteLine("Change occured");
}
}
public class Form
{
private async void Mr_MyEvent(object sender, MyEventArgs e)
{
using (e.GetDeferral())
{
string s = await GetString();
e.Change = true;
}
}
}

Redirecting Events to handlers inside custom controls

I have mad a custom control. and i need to redirect the Event handler. i have cut the code down dramatically to try and articulate what im trying to do.
public class RemoteDesktop : WindowsFormsHost
{
public event OnConnectingEventHandler OnConnecting;
public delegate void OnConnectingEventHandler(object sender, EventArgs Arguments);
public event OnDisconnectingEventHandler OnDisconnecting;
public delegate void OnDisconnectingEventHandler(Object sender, IMsTscAxEvents_OnDisconnectedEvent Arguments);
private AxMsRdpClient7NotSafeForScripting RDPUserControl = new AxMsRdpClient7NotSafeForScripting();
public RemoteDesktop()
{
this.RDPUserControl.BeginInit();
this.RDPUserControl.SuspendLayout();
base.Child = RDPUserControl;
this.RDPUserControl.ResumeLayout();
this.RDPUserControl.EndInit();
}
}
public class RemoteDesktopViewModel
{
public RemoteDesktopViewModel()
{
RemoteDesktop newRDC = new RemoteDesktop();
newRDC.OnConnecting += new RemoteDesktop.OnConnectingEventHandler(newRDC_OnConnecting);
}
void newRDC_OnConnecting(object sender, EventArgs Arguments)
{
//DoStuff
}
}
basically it all works, i can connect and disconnect to the remote computer however i cannot get the fired events to occur in my view model.
Can anyone help me figure out how i can point my events correctly.
Thank you.
Thanks to some help i have the resolution
Step 1:
Declare delegates outside the class (within the namespace)
Step 2:
Declare the events to be called for the control.
Step 3: use the event handlers of the controls to reise the delegates you created
Completed Code
public delegate void OnConnectingEventHandler(object sender, EventArgs Arguments);
public delegate void OnDisconnectingEventHandler(Object sender,IMsTscAxEvents_OnDisconnectedEvent Arguments);
public class RemoteDesktop : WindowsFormsHost
{
public event OnConnectingEventHandler IsConnecting;
public event OnDisconnectingEventHandler IsDisconnecting;
private AxMsRdpClient7NotSafeForScripting RDPUserControl = new AxMsRdpClient7NotSafeForScripting();
public RemoteDesktop()
{
this.RDPUserControl.BeginInit();
this.RDPUserControl.SuspendLayout();
base.Child = RDPUserControl;
this.RDPUserControl.ResumeLayout();
this.RDPUserControl.EndInit();
RDPUserControl.OnConnecting += RemoteDesktop_OnConnecting;
RDPUserControl.OnDisconnected += RDPUserControl_OnDisconnected;
}
void RDPUserControl_OnDisconnected(object sender, IMsTscAxEvents_OnDisconnectedEvent e)
{
IsDisconnecting(sender, e);
}
void RemoteDesktop_OnConnecting(object sender, EventArgs Arguments)
{
IsConnecting(sender, Arguments);
}
}
public class RemoteDesktopViewModel
{
public RemoteDesktopViewModel()
{
RemoteDesktop newRDC = new RemoteDesktop();
newRDC.IsConnecting += new RemoteDesktop.OnConnectingEventHandler(newRDC_OnConnecting);
}
void newRDC_OnConnecting(object sender, EventArgs Arguments)
{
//DoStuff
}
}

//at the constractor of the class
OnConnecting+=RDC_OnConnecting;
then you can write your logic in method:newRDC_OnConnecting. make sure OnConnectingEventHandler have same method signatures with newRDC_OnConnecting.

How to publish and subscribe to events across different classes

Goal: To change a image on a form when either udp or tcp uses its send method
Problem: I have no idea how to get the event, eventhandler and delegates set up correctly
Send Interface
interface ISendData
{
void Send();
}
Tcp Connection class
//Need some type of delegate??
public class TCPconnection : ISendData
{
void Send()
{
//how invoke/fire a send Event?
}
}
UDP Connection class
//Need some type of delegate??
public class UDPConnection : ISendData
{
void Send()
{
//how invoke/fire a send event?
}
}
the winform which 'should' subscribe to seeing the fired events
public class myForm
{
private DataWatcher datawatcher = new DataWatcher();
private Image statusIndicator = null;
public myform()
{
initComponents();
datawatcher.DataSendActive += new DataWatcherSendHandler(DataSending);
datawatcher.DataSendInactive += new DataWatcherSendHandler(NoDataSending);
}
public void DataSending(object sender, DataWatcherArgs e)
{
statusIndicator = Properties.resources.greenLight;
}
public void NoDataSending(object sender, DataWatcherArgs e)
{
statusIndicator = Properties.resources.redLight;
}
}
The Event/Event handler?? But I really have no Idea what I'm doing here to make this work
public delegate void EventHandler(object sender, EventArgs e);
class DataWatcher
{
public event EventHandler DataSendActive;
public event EventHandler DataSendInactive;
protected virtual void onDataSendActive(System.EventArgs e)
{
if (DataSendActive != null)
{
DataSendActive(this, e);
}
}
protected virtual void onDataSendInactive(System.EventArgs e)
{
if (DataSendInactive != null)
{
DataSendInactive(this, e);
}
}
}

There are many conventions used to do this. Here's my little implementation.
public enum ActivityState
{
Sending,
Receiving,
Idle
}
public interface IDataTransferManager
{
// This event will fire when the activity state changes.
// note that Action<T> is introduced in .NET 3.5
// if you're using .NET 2.0, you can use a delegate.
event Action<ActivityState> DataActivityStateChange;
void Send(byte[] data);
//byte[] Receive();
// ... more methods ... //
}
Now the TcpConnection class will implement this.
public class TcpConnection : IDataTransferManager
{
public event Action<ActivityState> DataActivityStateChange;
public void Send(byte[] data)
{
// we're sending data. fire the change event
FireDataActivityStateChange(ActivityState.Sending);
//TODO: send the data
// we're done sending. Fire the change event
FireDataActivityStateChange(ActivityState.Idle);
}
private void FireDataActivityStateChange(ActivityState state)
{
// helper method, so I don't have to check the event
// to avoid null reference exceptions.
if (DataActivityStateChange != null)
DataActivityStateChange(state);
}
}
Here's the setup for your Form.
class MyForm // :Form
{
IDataTransferManager dataManager;
public MyForm()
{ // here, usually an instance will be passed in,
// so there's only one instance throughout the application.
// let's new up an instance for explanation purposes.
dataManager = new TcpConnection();
dataManager.DataActivityStateChange += (state) =>
{
// NOTE: if you don't like inline,
// you can point this labda to a method.
switch (state)
{
case ActivityState.Sending:
// change the image to the spinning toilet ball
break;
case ActivityState.Receiving:
// change the image to the spinning toilet ball, but reverse :P
break;
case ActivityState.Idle:
// hide it ?
break;
}
};
}
}

Here is a simple example of how you could implement an event for sending and not sending and subscribe to it
public class Connection
{
//Set up an event
public event EventHandler DataSending;
public event EventHandler DataNotSending
//This method will trigger the event for sending
private void OnDataSending()
{
if (DataSending!= null) { DataSending(this, EventArgs.Empty); }
}
//this method will trigger the event for finished sending
private void OnDataNotSending()
{
if (DataNotSending!= null) { DataNotSending(this, EventArgs.Empty); }
}
//This method performs your send logic
public void Send()
{
//Call your method that tells the event to be raised
OnDataSending();
//Then put your send code
OnDataNotSending(); //we're done!
}
}
This is how you use it in a consuming program
public class myForm
{
//This method is the one that sets up the
//instance and subscribes to the event
public myForm()
{
Connection con = new Connection();
con.DataSending += new EventHandler(con_DataSending);
con.DataNotSending += new EventHander(con_DataNotSending);
}
void con_DataSending(object sender, EventArgs e)
{
//Put your subscription logic here.
//Whatever you want to do in response to a send
}
void con_DataNotSending(object sender, EventArgs e)
{
//Put your subscription logic here.
//Respond to it not sending
}
}

How can I bubble events from the Repository through the Service up to the UI?

I have a repository that uses components that report events.
I want to show the reported events in the front end.
This is the repository:
public interface IXmlRepository
{
//irrelevant stuff removed
event EventHandler TraceEventHandler;
}
public class XmlPanelRepository : IXmlRepository
{
public XmlPanelRepository()
{
public event EventHandler TraceEventHandler;
var panelCom = new PanelCom(); // this is a COM object that connects to a device
// when something happens in the COM object it reports it.
panelCom.Trace += panelCom_Trace;
// I want to bubble the trace events up to my UI.
TraceEventHandler += TraceEventHandler_Tracing;
}
private void TraceEventHandler_Tracing(object sender, EventArgs e)
{
// what do I do here?
}
void panelCom_Trace(string message)
{
if (TraceEventHandler!= null) TraceEventHandler.Invoke(this, new EventArgs());
}
}
My UI uses a Service to interface with the repository. The service is defined as:
public interface IXmlConfigurationService
{
//irrelevant stuff removed
event EventHandler TraceEventHandler;
}
public class XmlConfigurationService : IXmlConfigurationService
{
public event EventHandler TraceEventHandler;
public XmlConfigurationService(IXmlRepository configurationRepository)
{
_configurationRepository.TraceEventHandler += ConfigurationRepository_TraceEventHandler;
}
void ConfigurationRepository_TraceEventHandler(object sender, EventArgs e)
{
// this never gets hit.
if (TraceEventHandler != null) TraceEventHandler.Invoke(sender, e);
}
}
If I can get this working, I presume I can follow the same steps to get the UI displaying event reports.
How can I get the Service to report the events that are occurring in the repository?

If I understand correctly, you would need to fulfil your event. This is akin to the Observable fulfilling all listening observers in the Observer Pattern. But for events written like this:
public XmlPanelRepository()
{
public event EventHandler TraceEventHandler;
var panelCom = new PanelCom(); // this is a COM object that connects to a device
// when something happens in the COM object it reports it.
panelCom.Trace += panelCom_Trace;
// I want to bubble the trace events up to my UI.
TraceEventHandler += TraceEventHandler_Tracing;
}
private void TraceEventHandler_Tracing(object sender, EventArgs e)
{
if (TraceEventHandler != null)
{
TraceEventHandler(this, e);
}
}
However, perhaps you should name the event something other than TraceEventHandler because you are now exposing an event (which usually is expressed in the past tense).
It is also worth noting that you can potentially change the event args as you bubble up (if you want to). EventHandler(TEventArgs) can help with this.

You're forgetting to use delegates...
Try something like this:
public interface IXmlRepository
{
//irrelevant stuff removed
event EventHandler TraceEventHandler;
}
public class XmlPanelRepository : IXmlRepository
{
public delegate void EventHandler(string parameter1, string parameter2);
public event EventHandler TraceEventHandler;
public XmlPanelRepository()
{
var panelCom = new PanelCom(); // this is a COM object that connects to a device
// when something happens in the COM object it reports it.
panelCom.Trace += panelCom_Trace;
}
void panelCom_Trace(string message)
{
if (EventHandler != null)
EventHandler("Event was hit, here's the message:", message);
}
}
public interface IXmlConfigurationService
{
//irrelevant stuff removed
}
public class XmlConfigurationService : IXmlConfigurationService
{
public XmlConfigurationService(IXmlRepository configurationRepository)
{
_configurationRepository.TraceEventHandler += ConfigurationRepository_EventHandler;
}
void ConfigurationRepository_EventHandler(string parameter1, string parameter2);)
{
// Do something with your parameters.
Response.Write(parameter1 + parameter2);
}
}

Super-simple example of C# observer/observable with delegates

I recently started digging into C# but I can't by my life figure out how delegates work when implementing the observer/observable pattern in the language.
Could someone give me a super-simple example of how it is done? I have googled this, but all of the examples I found were either too problem-specific or too "bloated".

The observer pattern is usually implemented with events.
Here's an example:
using System;
class Observable
{
public event EventHandler SomethingHappened;
public void DoSomething() =>
SomethingHappened?.Invoke(this, EventArgs.Empty);
}
class Observer
{
public void HandleEvent(object sender, EventArgs args)
{
Console.WriteLine("Something happened to " + sender);
}
}
class Test
{
static void Main()
{
Observable observable = new Observable();
Observer observer = new Observer();
observable.SomethingHappened += observer.HandleEvent;
observable.DoSomething();
}
}
See the linked article for a lot more detail.
Note that the above example uses C# 6 null-conditional operator to implement DoSomething safely to handle cases where SomethingHappened has not been subscribed to, and is therefore null. If you're using an older version of C#, you'd need code like this:
public void DoSomething()
{
var handler = SomethingHappened;
if (handler != null)
{
handler(this, EventArgs.Empty);
}
}

In this model, you have publishers who will do some logic and publish an "event."
Publishers will then send out their event only to subscribers who have subscribed to receive the specific event.
In C#, any object can publish a set of events to which other applications can subscribe.
When the publishing class raises an event, all the subscribed applications are notified.
The following figure shows this mechanism.
Simplest Example possible on Events and Delegates in C#:
code is self explanatory, Also I've added the comments to clear out the code.
using System;
public class Publisher //main publisher class which will invoke methods of all subscriber classes
{
public delegate void TickHandler(Publisher m, EventArgs e); //declaring a delegate
public TickHandler Tick; //creating an object of delegate
public EventArgs e = null; //set 2nd paramter empty
public void Start() //starting point of thread
{
while (true)
{
System.Threading.Thread.Sleep(300);
if (Tick != null) //check if delegate object points to any listener classes method
{
Tick(this, e); //if it points i.e. not null then invoke that method!
}
}
}
}
public class Subscriber1 //1st subscriber class
{
public void Subscribe(Publisher m) //get the object of pubisher class
{
m.Tick += HeardIt; //attach listener class method to publisher class delegate object
}
private void HeardIt(Publisher m, EventArgs e) //subscriber class method
{
System.Console.WriteLine("Heard It by Listener");
}
}
public class Subscriber2 //2nd subscriber class
{
public void Subscribe2(Publisher m) //get the object of pubisher class
{
m.Tick += HeardIt; //attach listener class method to publisher class delegate object
}
private void HeardIt(Publisher m, EventArgs e) //subscriber class method
{
System.Console.WriteLine("Heard It by Listener2");
}
}
class Test
{
static void Main()
{
Publisher m = new Publisher(); //create an object of publisher class which will later be passed on subscriber classes
Subscriber1 l = new Subscriber1(); //create object of 1st subscriber class
Subscriber2 l2 = new Subscriber2(); //create object of 2nd subscriber class
l.Subscribe(m); //we pass object of publisher class to access delegate of publisher class
l2.Subscribe2(m); //we pass object of publisher class to access delegate of publisher class
m.Start(); //starting point of publisher class
}
}
Output:
Heard It by Listener
Heard It by Listener2
Heard It by Listener
Heard It by Listener2
Heard It by Listener
.
.
.
(infinite times)

Here's a simple example:
public class ObservableClass
{
private Int32 _Value;
public Int32 Value
{
get { return _Value; }
set
{
if (_Value != value)
{
_Value = value;
OnValueChanged();
}
}
}
public event EventHandler ValueChanged;
protected void OnValueChanged()
{
if (ValueChanged != null)
ValueChanged(this, EventArgs.Empty);
}
}
public class ObserverClass
{
public ObserverClass(ObservableClass observable)
{
observable.ValueChanged += TheValueChanged;
}
private void TheValueChanged(Object sender, EventArgs e)
{
Console.Out.WriteLine("Value changed to " +
((ObservableClass)sender).Value);
}
}
public class Program
{
public static void Main()
{
ObservableClass observable = new ObservableClass();
ObserverClass observer = new ObserverClass(observable);
observable.Value = 10;
}
}
Note:
This violates a rule in that I don't unhook the observer from the observable, this is perhaps good enough for this simple example, but make sure you don't keep observers hanging off of your events like that. A way to handle this would be to make ObserverClass IDisposable, and let the .Dispose method do the opposite of the code in the constructor
No error-checking performed, at least a null-check should be done in the constructor of the ObserverClass

I've tied together a couple of the great examples above (thank you as always to Mr. Skeet and Mr. Karlsen) to include a couple of different Observables and utilized an interface to keep track of them in the Observer and allowed the Observer to to "observe" any number of Observables via an internal list:
namespace ObservablePattern
{
using System;
using System.Collections.Generic;
internal static class Program
{
private static void Main()
{
var observable = new Observable();
var anotherObservable = new AnotherObservable();
using (IObserver observer = new Observer(observable))
{
observable.DoSomething();
observer.Add(anotherObservable);
anotherObservable.DoSomething();
}
Console.ReadLine();
}
}
internal interface IObservable
{
event EventHandler SomethingHappened;
}
internal sealed class Observable : IObservable
{
public event EventHandler SomethingHappened;
public void DoSomething()
{
var handler = this.SomethingHappened;
Console.WriteLine("About to do something.");
if (handler != null)
{
handler(this, EventArgs.Empty);
}
}
}
internal sealed class AnotherObservable : IObservable
{
public event EventHandler SomethingHappened;
public void DoSomething()
{
var handler = this.SomethingHappened;
Console.WriteLine("About to do something different.");
if (handler != null)
{
handler(this, EventArgs.Empty);
}
}
}
internal interface IObserver : IDisposable
{
void Add(IObservable observable);
void Remove(IObservable observable);
}
internal sealed class Observer : IObserver
{
private readonly Lazy<IList<IObservable>> observables =
new Lazy<IList<IObservable>>(() => new List<IObservable>());
public Observer()
{
}
public Observer(IObservable observable) : this()
{
this.Add(observable);
}
public void Add(IObservable observable)
{
if (observable == null)
{
return;
}
lock (this.observables)
{
this.observables.Value.Add(observable);
observable.SomethingHappened += HandleEvent;
}
}
public void Remove(IObservable observable)
{
if (observable == null)
{
return;
}
lock (this.observables)
{
observable.SomethingHappened -= HandleEvent;
this.observables.Value.Remove(observable);
}
}
public void Dispose()
{
for (var i = this.observables.Value.Count - 1; i >= 0; i--)
{
this.Remove(this.observables.Value[i]);
}
}
private static void HandleEvent(object sender, EventArgs args)
{
Console.WriteLine("Something happened to " + sender);
}
}
}

Applying the Observer Pattern with delegates and events in c# is named "Event Pattern" according to MSDN which is a slight variation.
In this Article you will find well structured examples of how to apply the pattern in c# both the classic way and using delegates and events.
Exploring the Observer Design Pattern
public class Stock
{
//declare a delegate for the event
public delegate void AskPriceChangedHandler(object sender,
AskPriceChangedEventArgs e);
//declare the event using the delegate
public event AskPriceChangedHandler AskPriceChanged;
//instance variable for ask price
object _askPrice;
//property for ask price
public object AskPrice
{
set
{
//set the instance variable
_askPrice = value;
//fire the event
OnAskPriceChanged();
}
}//AskPrice property
//method to fire event delegate with proper name
protected void OnAskPriceChanged()
{
AskPriceChanged(this, new AskPriceChangedEventArgs(_askPrice));
}//AskPriceChanged
}//Stock class
//specialized event class for the askpricechanged event
public class AskPriceChangedEventArgs : EventArgs
{
//instance variable to store the ask price
private object _askPrice;
//constructor that sets askprice
public AskPriceChangedEventArgs(object askPrice) { _askPrice = askPrice; }
//public property for the ask price
public object AskPrice { get { return _askPrice; } }
}//AskPriceChangedEventArgs

/**********************Simple Example ***********************/
class Program
{
static void Main(string[] args)
{
Parent p = new Parent();
}
}
////////////////////////////////////////////
public delegate void DelegateName(string data);
class Child
{
public event DelegateName delegateName;
public void call()
{
delegateName("Narottam");
}
}
///////////////////////////////////////////
class Parent
{
public Parent()
{
Child c = new Child();
c.delegateName += new DelegateName(print);
//or like this
//c.delegateName += print;
c.call();
}
public void print(string name)
{
Console.WriteLine("yes we got the name : " + name);
}
}

I did't want to change my source code to add additional observer , so I have written following simple example:
//EVENT DRIVEN OBSERVER PATTERN
public class Publisher
{
public Publisher()
{
var observable = new Observable();
observable.PublishData("Hello World!");
}
}
//Server will send data to this class's PublishData method
public class Observable
{
public event Receive OnReceive;
public void PublishData(string data)
{
//Add all the observer below
//1st observer
IObserver iObserver = new Observer1();
this.OnReceive += iObserver.ReceiveData;
//2nd observer
IObserver iObserver2 = new Observer2();
this.OnReceive += iObserver2.ReceiveData;
//publish data
var handler = OnReceive;
if (handler != null)
{
handler(data);
}
}
}
public interface IObserver
{
void ReceiveData(string data);
}
//Observer example
public class Observer1 : IObserver
{
public void ReceiveData(string data)
{
//sample observers does nothing with data :)
}
}
public class Observer2 : IObserver
{
public void ReceiveData(string data)
{
//sample observers does nothing with data :)
}
}

Something like this:
// interface implementation publisher
public delegate void eiSubjectEventHandler(eiSubject subject);
public interface eiSubject
{
event eiSubjectEventHandler OnUpdate;
void GenereteEventUpdate();
}
// class implementation publisher
class ecSubject : eiSubject
{
private event eiSubjectEventHandler _OnUpdate = null;
public event eiSubjectEventHandler OnUpdate
{
add
{
lock (this)
{
_OnUpdate -= value;
_OnUpdate += value;
}
}
remove { lock (this) { _OnUpdate -= value; } }
}
public void GenereteEventUpdate()
{
eiSubjectEventHandler handler = _OnUpdate;
if (handler != null)
{
handler(this);
}
}
}
// interface implementation subscriber
public interface eiObserver
{
void DoOnUpdate(eiSubject subject);
}
// class implementation subscriber
class ecObserver : eiObserver
{
public virtual void DoOnUpdate(eiSubject subject)
{
}
}
.
observer pattern C# with event
.
link to the repository