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Catching events prior to subscription with FromEventPattern

I'm writing a listener for messages using the Rx framework.
The problem I'm facing is that the library I'm using uses a consumer that publishes events whenever a message has arrived.
I've managed to consume the incoming messages via Observable.FromEventPattern but I have a problem with the messages that are already in the server.
At the moment I have the following chain of commands
Create a consumer
Create an observable sequence with FromEventPattern and apply needed transformations
Tell the consumer to start
Subscribe to the sequence
The easiest solution would be to swap steps 3. and 4. but since they happen in different components of the system, it's very hard for me to do so.
Ideally I would like to execute step 3 when step 4 happens (like a OnSubscribe method).
Thanks for your help :)
PS: to add more details, the events are coming from a RabbitMQ queue and I am using the EventingBasicConsumer class found in the RabbitMQ.Client package.
Here you can find the library I am working on. Specifically, this is the class/method giving me problems.
Edit
Here is a stripped version of the problematic code
void Main()
{
var engine = new Engine();
var messages = engine.Start();
messages.Subscribe(m => m.Dump());
Console.ReadLine();
engine.Stop();
}
public class Engine
{
IConnection _connection;
IModel _channel;
public IObservable<Message> Start()
{
var connectionFactory = new ConnectionFactory();
_connection = connectionFactory.CreateConnection();
_channel = _connection.CreateModel();
EventingBasicConsumer consumer = new EventingBasicConsumer(_channel);
var observable = Observable.FromEventPattern<BasicDeliverEventArgs>(
a => consumer.Received += a,
a => consumer.Received -= a)
.Select(e => e.EventArgs);
_channel.BasicConsume("a_queue", false, consumer);
return observable.Select(Transform);
}
private Message Transform(BasicDeliverEventArgs args) => new Message();
public void Stop()
{
_channel.Dispose();
_connection.Dispose();
}
}
public class Message { }
The symptom I experience is that since I invoke BasicConsume before subscribing to the sequence, any message that is in the RabbitMQ queue is fetched but not passed down the pipeline.
Since I don't have "autoack" on, the messages are returned to the queue as soon as the program stops.

As some have noted in the comments, and as you note in the question, the issue is due to the way you're using the RabbitMQ client.
To get around some of these issues, what I actually did was create an ObservableConsumer class. This is an alternative to the EventingBasicConsumer which is in use currently. One reason I did this was to deal with the issue described in the question, but the other thing this does is allow you to re-use this consumer object beyond a single connection/channel instance. This has the benefit of allowing your downstream reactive code to remain wired in spite of transient connection/channel characteristics.
using System;
using System.Collections.Generic;
using System.Reactive.Concurrency;
using System.Reactive.Linq;
using System.Reactive.Subjects;
using RabbitMQ.Client;
namespace com.rabbitmq.consumers
{
public sealed class ObservableConsumer : IBasicConsumer
{
private readonly List<string> _consumerTags = new List<string>();
private readonly object _consumerTagsLock = new object();
private readonly Subject<Message> _subject = new Subject<Message>();
public ushort PrefetchCount { get; set; }
public IEnumerable<string> ConsumerTags { get { return new List<string>(_consumerTags); } }
/// <summary>
/// Registers this consumer on the given queue.
/// </summary>
/// <returns>The consumer tag assigned.</returns>
public string ConsumeFrom(IModel channel, string queueName)
{
Model = channel;
return Model.BasicConsume(queueName, false, this);
}
/// <summary>
/// Contains an observable of the incoming messages where messages are processed on a thread pool thread.
/// </summary>
public IObservable<Message> IncomingMessages
{
get { return _subject.ObserveOn(Scheduler.ThreadPool); }
}
///<summary>Retrieve the IModel instance this consumer is
///registered with.</summary>
public IModel Model { get; private set; }
///<summary>Returns true while the consumer is registered and
///expecting deliveries from the broker.</summary>
public bool IsRunning
{
get { return _consumerTags.Count > 0; }
}
/// <summary>
/// Run after a consumer is cancelled.
/// </summary>
/// <param name="consumerTag"></param>
private void OnConsumerCanceled(string consumerTag)
{
}
/// <summary>
/// Run after a consumer is added.
/// </summary>
/// <param name="consumerTag"></param>
private void OnConsumerAdded(string consumerTag)
{
}
public void HandleBasicConsumeOk(string consumerTag)
{
lock (_consumerTagsLock) {
if (!_consumerTags.Contains(consumerTag))
_consumerTags.Add(consumerTag);
}
}
public void HandleBasicCancelOk(string consumerTag)
{
lock (_consumerTagsLock) {
if (_consumerTags.Contains(consumerTag)) {
_consumerTags.Remove(consumerTag);
OnConsumerCanceled(consumerTag);
}
}
}
public void HandleBasicCancel(string consumerTag)
{
lock (_consumerTagsLock) {
if (_consumerTags.Contains(consumerTag)) {
_consumerTags.Remove(consumerTag);
OnConsumerCanceled(consumerTag);
}
}
}
public void HandleModelShutdown(IModel model, ShutdownEventArgs reason)
{
//Don't need to do anything.
}
public void HandleBasicDeliver(string consumerTag,
ulong deliveryTag,
bool redelivered,
string exchange,
string routingKey,
IBasicProperties properties,
byte[] body)
{
//Hack - prevents the broker from sending too many messages.
//if (PrefetchCount > 0 && _unackedMessages.Count > PrefetchCount) {
// Model.BasicReject(deliveryTag, true);
// return;
//}
var message = new Message(properties.HeaderFromBasicProperties()) { Content = body };
var deliveryData = new MessageDeliveryData()
{
ConsumerTag = consumerTag,
DeliveryTag = deliveryTag,
Redelivered = redelivered,
};
message.Tag = deliveryData;
if (AckMode != AcknowledgeMode.AckWhenReceived) {
message.Acknowledged += messageAcknowledged;
message.Failed += messageFailed;
}
_subject.OnNext(message);
}
void messageFailed(Message message, Exception ex, bool requeue)
{
try {
message.Acknowledged -= messageAcknowledged;
message.Failed -= messageFailed;
if (message.Tag is MessageDeliveryData) {
Model.BasicNack((message.Tag as MessageDeliveryData).DeliveryTag, false, requeue);
}
}
catch {}
}
void messageAcknowledged(Message message)
{
try {
message.Acknowledged -= messageAcknowledged;
message.Failed -= messageFailed;
if (message.Tag is MessageDeliveryData) {
var ackMultiple = AckMode == AcknowledgeMode.AckAfterAny;
Model.BasicAck((message.Tag as MessageDeliveryData).DeliveryTag, ackMultiple);
}
}
catch {}
}
}
}

I think there is no need to actually subscribe to rabbit queue (via BasicConsume) until you have subscribers to your observable. Right now you are starting rabbit subscription right away and push items to observable even if no one has subscribed to it.
Suppose we have this sample class:
class Events {
public event Action<string> MessageArrived;
Timer _timer;
public void Start()
{
Console.WriteLine("Timer starting");
int i = 0;
_timer = new Timer(_ => {
this.MessageArrived?.Invoke(i.ToString());
i++;
}, null, TimeSpan.Zero, TimeSpan.FromSeconds(1));
}
public void Stop() {
_timer?.Dispose();
Console.WriteLine("Timer stopped");
}
}
What you are doing now is basically:
var ev = new Events();
var ob = Observable.FromEvent<string>(x => ev.MessageArrived += x, x => ev.MessageArrived -= x);
ev.Start();
return ob;
What you need instead is observable which does exactly that, but only when someone subscribes:
return Observable.Create<string>(observer =>
{
var ev = new Events();
var ob = Observable.FromEvent<string>(x => ev.MessageArrived += x, x => ev.MessageArrived -= x);
// first subsribe
var sub = ob.Subscribe(observer);
// then start
ev.Start();
// when subscription is disposed - unsubscribe from rabbit
return new CompositeDisposable(sub, Disposable.Create(() => ev.Stop()));
});
Good, but now every subscription to observable will result in separate subscription to rabbit queues, which is not what we need. We can solve that with Publish().RefCount():
return Observable.Create<string>(observer => {
var ev = new Events();
var ob = Observable.FromEvent<string>(x => ev.MessageArrived += x, x => ev.MessageArrived -= x);
var sub = ob.Subscribe(observer);
ev.Start();
return new CompositeDisposable(sub, Disposable.Create(() => ev.Stop()));
}).Publish().RefCount();
Now what will happen is when first subscriber subscribes to observable (ref count goes from 0 to 1) - code from Observable.Create body is invoked and subscribes to rabbit queue. This subscription is then shared by all subsequent subscribers. When last unsubscribes (ref count goes to zero) - subscription is disposed, ev.Stop is called, and we unsubscribe from rabbit queue.
If so happens that you call Start() (which creates observable in your code) and never subscribe to it - nothing happens and no subscriptions to rabbit is made at all.

How can I split and pipe multiple NAudio stream

I have a C# project working with input audio Stream from Kinect 1, Kinect 2, Microphone or anything else.
waveIn.DataAvailable += (object sender, WaveInEventArgs e) => {
lock(buffer){
var pos = buffer.Position;
buffer.Write(e.Buffer, 0, e.BytesRecorded);
buffer.Position = pos;
}
};
The buffer variable is a Stream from component A that will be processed by a SpeechRecognition component B working on Streams.
I will add new components C, D, E, working on Streams to compute pitch, detect sound, do finger printing, or anything else ...
How can I duplicate that Stream for components C, D, E ?
Component A send an Event "I have a Stream do what you want" I don't want to reverse the logic by an Event "Give me your streams"
I'm looking for a "MultiStream" that could give me a Stream instance and will handle the job
Component A
var MultiStream buffer = new MultiStream()
...
SendMyEventWith(buffer)
Component B, C, D, E
public void HandleMyEvent(MultiStream buffer){
var stream = buffer.GetNewStream();
var engine = new EngineComponentB()
engine.SetStream(stream);
}
The MultiStream must be a Stream to wrap Write() method (because Stream do not have data available mechanics) ?
If a Stream is Dispose() by Component B the MultiStream should remove it from it's array ?
The MultiStream must throw an exception on Read() to require use of GetNewStream()
EDIT: Kinect 1 provide a Stream itself ... :-( should I use a Thread to pumpit into the MultiStream ?
Did anybody have that kind of MultiStream Class ?
Thanks

I'm not sure if this is the best way to do it or that it's better than the previous answer, and I'm not guaranteeing that this code is perfect, but I coded something that is literally what you asked for because it was fun - a MultiStream class.
You can find the code for the class here: http://pastie.org/10289142
Usage Example:
MultiStream ms = new MultiStream();
Stream copy1 = ms.CloneStream();
ms.Read( ... );
Stream copy2 = ms.CloneStream();
ms.Read( ... );
copy1 and copy2 will contain identical data after the example is ran, and they will continue to get updated as the MultiStream is written to. You can read, update position, and dispose of the cloned streams individually. If disposed the cloned streams will get removed from MultiStream, and disposing of Multistream will close all related and cloned streams (you can change this if it's not the behavior you want). Trying to write to the cloned streams will throw a not supported exception.

Somehow I don't think streams really fit what you're trying to do. You're setting up a situation where a long run of the program is going to continually expand the data requirements for no apparent reason.
I'd suggest a pub/sub model that publishes the received audio data to subscribers, preferably using a multi-threaded approach to minimize the impact of a bad subscriber. Some ideas can be found here.
I've done this before with a processor class that implements IObserver<byte[]> and uses a Queue<byte[]> to store the sample blocks until the process thread is ready for them. Here's are the base classes:
public abstract class BufferedObserver<T> : IObserver<T>, IDisposable
{
private object _lck = new object();
private IDisposable _subscription = null;
public bool Subscribed { get { return _subscription != null; } }
private bool _completed = false;
public bool Completed { get { return _completed; } }
protected readonly Queue<T> _queue = new Queue<T>();
protected bool DataAvailable { get { lock(_lck) { return _queue.Any(); } } }
protected int AvailableCount { get { lock (_lck) { return _queue.Count; } } }
protected BufferedObserver()
{
}
protected BufferedObserver(IObservable<T> observable)
{
SubscribeTo(observable);
}
public virtual void Dispose()
{
if (_subscription != null)
{
_subscription.Dispose();
_subscription = null;
}
}
public void SubscribeTo(IObservable<T> observable)
{
if (_subscription != null)
_subscription.Dispose();
_subscription = observable.Subscribe(this);
_completed = false;
}
public virtual void OnCompleted()
{
_completed = true;
}
public virtual void OnError(Exception error)
{ }
public virtual void OnNext(T value)
{
lock (_lck)
_queue.Enqueue(value);
}
protected bool GetNext(ref T buffer)
{
lock (_lck)
{
if (!_queue.Any())
return false;
buffer = _queue.Dequeue();
return true;
}
}
protected T NextOrDefault()
{
T buffer = default(T);
GetNext(ref buffer);
return buffer;
}
}
public abstract class Processor<T> : BufferedObserver<T>
{
private object _lck = new object();
private Thread _thread = null;
private object _cancel_lck = new object();
private bool _cancel_requested = false;
private bool CancelRequested
{
get { lock(_cancel_lck) return _cancel_requested; }
set { lock(_cancel_lck) _cancel_requested = value; }
}
public bool Running { get { return _thread == null ? false : _thread.IsAlive; } }
public bool Finished { get { return _thread == null ? false : !_thread.IsAlive; } }
protected Processor(IObservable<T> observable)
: base(observable)
{ }
public override void Dispose()
{
if (_thread != null && _thread.IsAlive)
{
//CancelRequested = true;
_thread.Join(5000);
}
base.Dispose();
}
public bool Start()
{
if (_thread != null)
return false;
_thread = new Thread(threadfunc);
_thread.Start();
return true;
}
private void threadfunc()
{
while (!CancelRequested && (!Completed || _queue.Any()))
{
if (DataAvailable)
{
T data = NextOrDefault();
if (data != null && !data.Equals(default(T)))
ProcessData(data);
}
else
Thread.Sleep(10);
}
}
// implement this in a sub-class to process the blocks
protected abstract void ProcessData(T data);
}
This way you're only keeping the data as long as you need it, and you can attach as many process threads as you need to the same observable data source.
And for the sake of completeness, here's a generic class that implements IObservable<T> so you can see how it all fits together. This one even has comments:
/// <summary>Generic IObservable implementation</summary>
/// <typeparam name="T">Type of messages being observed</typeparam>
public class Observable<T> : IObservable<T>
{
/// <summary>Subscription class to manage unsubscription of observers.</summary>
private class Subscription : IDisposable
{
/// <summary>Observer list that this subscription relates to</summary>
public readonly ConcurrentBag<IObserver<T>> _observers;
/// <summary>Observer to manage</summary>
public readonly IObserver<T> _observer;
/// <summary>Initialize subscription</summary>
/// <param name="observers">List of subscribed observers to unsubscribe from</param>
/// <param name="observer">Observer to manage</param>
public Subscription(ConcurrentBag<IObserver<T>> observers, IObserver<T> observer)
{
_observers = observers;
_observer = observer;
}
/// <summary>On disposal remove the subscriber from the subscription list</summary>
public void Dispose()
{
IObserver<T> observer;
if (_observers != null && _observers.Contains(_observer))
_observers.TryTake(out observer);
}
}
// list of subscribed observers
private readonly ConcurrentBag<IObserver<T>> _observers = new ConcurrentBag<IObserver<T>>();
/// <summary>Subscribe an observer to this observable</summary>
/// <param name="observer">Observer instance to subscribe</param>
/// <returns>A subscription object that unsubscribes on destruction</returns>
/// <remarks>Always returns a subscription. Ensure that previous subscriptions are disposed
/// before re-subscribing.</remarks>
public IDisposable Subscribe(IObserver<T> observer)
{
// only add observer if it doesn't already exist:
if (!_observers.Contains(observer))
_observers.Add(observer);
// ...but always return a new subscription.
return new Subscription(_observers, observer);
}
// delegate type for threaded invocation of IObserver.OnNext method
private delegate void delNext(T value);
/// <summary>Send <paramref name="data"/> to the OnNext methods of each subscriber</summary>
/// <param name="data">Data object to send to subscribers</param>
/// <remarks>Uses delegate.BeginInvoke to send out notifications asynchronously.</remarks>
public void Notify(T data)
{
foreach (var observer in _observers)
{
delNext handler = observer.OnNext;
handler.BeginInvoke(data, null, null);
}
}
// delegate type for asynchronous invocation of IObserver.OnComplete method
private delegate void delComplete();
/// <summary>Notify all subscribers that the observable has completed</summary>
/// <remarks>Uses delegate.BeginInvoke to send out notifications asynchronously.</remarks>
public void NotifyComplete()
{
foreach (var observer in _observers)
{
delComplete handler = observer.OnCompleted;
handler.BeginInvoke(null, null);
}
}
}
Now you can create an Observable<byte[]> to use as your transmitter for Process<byte[]> instances that are interested. Pull data blocks out of the input stream, audio reader, etc. and pass them to the Notify method. Just make sure that you clone the arrays beforehand...

IObserver and IObservable in C# for Observer vs Delegates, Events

All I am trying to do is implementing the observer pattern.
So, I came up with this solution:
We have a PoliceHeadQuarters whose primary job is to send notifications to all those who are subscribed to it. Consider that the DSP, Inspector and SubInspector classes are subscribed to PoliceHeadQuarters.
Using Events and Delegates I wrote
public class HeadQuarters
{
public delegate void NewDelegate(object sender, EventArgs e);
public event EventHandler NewEvent;
public void RaiseANotification()
{
var handler = this.NewEvent;
if (handler != null)
{
handler(this, new EventArgs());
}
}
}
public class SubInspector
{
public void Listen(object sender, EventArgs e)
{
MessageBox.Show(string.Format("Event Notification received by sender = {0} with eventArguments = {1}", sender, e.ToString()));
}
}
public class Inspector
{
public void Listen(object sender, EventArgs e)
{
MessageBox.Show(string.Format("Event Notification received by sender = {0} with eventArguments = {1}", sender, e.ToString()));
}
}
and this is how I invoked it
var headQuarters = new HeadQuarters();
var SubInspector = new SubInspector();
var Inspector = new Inspector();
headQuarters.NewEvent += Inspector.Listen;
headQuarters.NewEvent += SubInspector.Listen;
headQuarters.RaiseANotification();
so, both Inspector and SubInspector classes get notification whenever there the function RaiseANotification() is invoked.
It seems that the DotNet Framework 4, 4.5 supports a new way called IObserver and IObservable.
Can anyone give me a super simple example using IObservable and IObserver pattern for the above scenario? I googled only to find the available examples in the internet too bloated and difficult to understand.
My hinch: (probably i think it's wrong)
class DSP : IObserver //since it observes the headquarters ?
class PoliceHeadQuarters: IObservable // since here's where we send the notifications ?
Thanks in advance.
EDIT: Somebody also said that the MSDN documentation is also incorrect for IObservable #
IObservable vs Plain Events or Why Should I use IObservable?.

Here's a modification of MSDN example to fit your framework:
public struct Message
{
string text;
public Message(string newText)
{
this.text = newText;
}
public string Text
{
get
{
return this.text;
}
}
}
public class Headquarters : IObservable<Message>
{
public Headquarters()
{
observers = new List<IObserver<Message>>();
}
private List<IObserver<Message>> observers;
public IDisposable Subscribe(IObserver<Message> observer)
{
if (!observers.Contains(observer))
observers.Add(observer);
return new Unsubscriber(observers, observer);
}
private class Unsubscriber : IDisposable
{
private List<IObserver<Message>> _observers;
private IObserver<Message> _observer;
public Unsubscriber(List<IObserver<Message>> observers, IObserver<Message> observer)
{
this._observers = observers;
this._observer = observer;
}
public void Dispose()
{
if (_observer != null && _observers.Contains(_observer))
_observers.Remove(_observer);
}
}
public void SendMessage(Nullable<Message> loc)
{
foreach (var observer in observers)
{
if (!loc.HasValue)
observer.OnError(new MessageUnknownException());
else
observer.OnNext(loc.Value);
}
}
public void EndTransmission()
{
foreach (var observer in observers.ToArray())
if (observers.Contains(observer))
observer.OnCompleted();
observers.Clear();
}
}
public class MessageUnknownException : Exception
{
internal MessageUnknownException()
{
}
}
public class Inspector : IObserver<Message>
{
private IDisposable unsubscriber;
private string instName;
public Inspector(string name)
{
this.instName = name;
}
public string Name
{
get
{
return this.instName;
}
}
public virtual void Subscribe(IObservable<Message> provider)
{
if (provider != null)
unsubscriber = provider.Subscribe(this);
}
public virtual void OnCompleted()
{
Console.WriteLine("The headquarters has completed transmitting data to {0}.", this.Name);
this.Unsubscribe();
}
public virtual void OnError(Exception e)
{
Console.WriteLine("{0}: Cannot get message from headquarters.", this.Name);
}
public virtual void OnNext(Message value)
{
Console.WriteLine("{1}: Message I got from headquarters: {0}", value.Text, this.Name);
}
public virtual void Unsubscribe()
{
unsubscriber.Dispose();
}
}
public class Program
{
public static void Main(string[] args)
{
Inspector inspector1 = new Inspector("Greg Lestrade");
Inspector inspector2 = new Inspector("Sherlock Holmes");
Headquarters headquarters = new Headquarters();
inspector1.Subscribe(headquarters);
inspector2.Subscribe(headquarters);
headquarters.SendMessage(new Message("Catch Moriarty!"));
headquarters.EndTransmission();
Console.ReadKey();
}
}

Another suggestion - you probably want to consider leveraging the reactive extensions library for any code using IObservable. The nuget package is Rx-Main and the homepage for it is here: http://msdn.microsoft.com/en-us/data/gg577609.aspx
Update: ReactiveX has been translated to many platforms and languages and is now managed as an open source project. Here is the landing page.
This will save you a lot of boilerplate code. Here's a super simple example:
var hq = new Subject<string>();
var inspectorSubscription = hq.Subscribe(
m => Console.WriteLine("Inspector received: " + m));
var subInspectorSubscription = hq.Subscribe(
m => Console.WriteLine("Sub Inspector received: " + m));
hq.OnNext("Catch Moriarty!");
It will output:
Inspector received: Catch Moriarty!
Sub Inspector received: Catch Moriarty!
Reactive Extensions is a big subject, and a very powerful library - worth investigating. I recommend the hands-on lab from the link above.
You would probably want to embed those subscriptions within your Inspector, SubInspector immplementatinos to more closely reflect your code. But hopefully this gives you an insight into what you can do with Rx.

How can I duplicate the F# discriminated union type in C#?

I've created a new class called Actor which processes messages passed to it.
The problem I am running into is figuring out what is the most elegant way to pass related but different messages to the Actor. My first idea is to use inheritance but it seems so bloated but it is strongly types which is a definite requirement.
Have any ideas?
Example
private abstract class QueueMessage { }
private class ClearMessage : QueueMessage
{
public static readonly ClearMessage Instance = new ClearMessage();
private ClearMessage() { }
}
private class TryDequeueMessage : QueueMessage
{
public static readonly TryDequeueMessage Instance = new TryDequeueMessage();
private TryDequeueMessage() { }
}
private class EnqueueMessage : QueueMessage
{
public TValue Item { get; private set; }
private EnqueueMessage(TValue item)
{
Item = item;
}
}
Actor Class
/// <summary>Represents a callback method to be executed by an Actor.</summary>
/// <typeparam name="TReply">The type of reply.</typeparam>
/// <param name="reply">The reply made by the actor.</param>
public delegate void ActorReplyCallback<TReply>(TReply reply);
/// <summary>Represents an Actor which receives and processes messages in concurrent applications.</summary>
/// <typeparam name="TMessage">The type of message this actor accepts.</typeparam>
/// <typeparam name="TReply">The type of reply made by this actor.</typeparam>
public abstract class Actor<TMessage, TReply> : IDisposable
{
/// <summary>The default total number of threads to process messages.</summary>
private const Int32 DefaultThreadCount = 1;
/// <summary>Used to serialize access to the message queue.</summary>
private readonly Locker Locker;
/// <summary>Stores the messages until they can be processed.</summary>
private readonly System.Collections.Generic.Queue<Message> MessageQueue;
/// <summary>Signals the actor thread to process a new message.</summary>
private readonly ManualResetEvent PostEvent;
/// <summary>This tells the actor thread to stop reading from the queue.</summary>
private readonly ManualResetEvent DisposeEvent;
/// <summary>Processes the messages posted to the actor.</summary>
private readonly List<Thread> ActorThreads;
/// <summary>Initializes a new instance of the Genex.Concurrency<TRequest, TResponse> class.</summary>
public Actor() : this(DefaultThreadCount) { }
/// <summary>Initializes a new instance of the Genex.Concurrency<TRequest, TResponse> class.</summary>
/// <param name="thread_count"></param>
public Actor(Int32 thread_count)
{
if (thread_count < 1) throw new ArgumentOutOfRangeException("thread_count", thread_count, "Must be 1 or greater.");
Locker = new Locker();
MessageQueue = new System.Collections.Generic.Queue<Message>();
EnqueueEvent = new ManualResetEvent(true);
PostEvent = new ManualResetEvent(false);
DisposeEvent = new ManualResetEvent(true);
ActorThreads = new List<Thread>();
for (Int32 i = 0; i < thread_count; i++)
{
var thread = new Thread(ProcessMessages);
thread.IsBackground = true;
thread.Start();
ActorThreads.Add(thread);
}
}
/// <summary>Posts a message and waits for the reply.</summary>
/// <param name="value">The message to post to the actor.</param>
/// <returns>The reply from the actor.</returns>
public TReply PostWithReply(TMessage message)
{
using (var wrapper = new Message(message))
{
lock (Locker) MessageQueue.Enqueue(wrapper);
PostEvent.Set();
wrapper.Channel.CompleteEvent.WaitOne();
return wrapper.Channel.Value;
}
}
/// <summary>Posts a message to the actor and executes the callback when the reply is received.</summary>
/// <param name="value">The message to post to the actor.</param>
/// <param name="callback">The callback that will be invoked once the replay is received.</param>
public void PostWithAsyncReply(TMessage value, ActorReplyCallback<TReply> callback)
{
if (callback == null) throw new ArgumentNullException("callback");
ThreadPool.QueueUserWorkItem(state => callback(PostWithReply(value)));
}
/// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
public void Dispose()
{
if (DisposeEvent.WaitOne(10))
{
DisposeEvent.Reset();
PostEvent.Set();
foreach (var thread in ActorThreads)
{
thread.Join();
}
((IDisposable)PostEvent).Dispose();
((IDisposable)DisposeEvent).Dispose();
}
}
/// <summary>Processes a message posted to the actor.</summary>
/// <param name="message">The message to be processed.</param>
protected abstract void ProcessMessage(Message message);
/// <summary>Dequeues the messages passes them to ProcessMessage.</summary>
private void ProcessMessages()
{
while (PostEvent.WaitOne() && DisposeEvent.WaitOne(10))
{
var message = (Message)null;
while (true)
{
lock (Locker)
{
message = MessageQueue.Count > 0 ?
MessageQueue.Dequeue() :
null;
if (message == null)
{
PostEvent.Reset();
break;
}
}
try
{
ProcessMessage(message);
}
catch
{
}
}
}
}
/// <summary>Represents a message that is passed to an actor.</summary>
protected class Message : IDisposable
{
/// <summary>The actual value of this message.</summary>
public TMessage Value { get; private set; }
/// <summary>The channel used to give a reply to this message.</summary>
public Channel Channel { get; private set; }
/// <summary>Initializes a new instance of Genex.Concurrency.Message class.</summary>
/// <param name="value">The actual value of the message.</param>
public Message(TMessage value)
{
Value = value;
Channel = new Channel();
}
/// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
public void Dispose()
{
Channel.Dispose();
}
}
/// <summary>Represents a channel used by an actor to reply to a message.</summary>
protected class Channel : IDisposable
{
/// <summary>The value of the reply.</summary>
public TReply Value { get; private set; }
/// <summary>Signifies that the message has been replied to.</summary>
public ManualResetEvent CompleteEvent { get; private set; }
/// <summary>Initializes a new instance of Genex.Concurrency.Channel class.</summary>
public Channel()
{
CompleteEvent = new ManualResetEvent(false);
}
/// <summary>Reply to the message received.</summary>
/// <param name="value">The value of the reply.</param>
public void Reply(TReply value)
{
Value = value;
CompleteEvent.Set();
}
/// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
public void Dispose()
{
((IDisposable)CompleteEvent).Dispose();
}
}
}

Steve Gilham summarized how the compiler actually handles discriminated unions. For your own code, you could consider a simplified version of that. Given the following F#:
type QueueMessage<T> = ClearMessage | TryDequeueMessage | EnqueueMessage of T
Here's one way to emulate it in C#:
public enum MessageType { ClearMessage, TryDequeueMessage, EnqueueMessage }
public abstract class QueueMessage<T>
{
// prevents unwanted subclassing
private QueueMessage() { }
public abstract MessageType MessageType { get; }
/// <summary>
/// Only applies to EnqueueMessages
/// </summary>
public abstract T Item { get; }
public static QueueMessage<T> MakeClearMessage() { return new ClearMessage(); }
public static QueueMessage<T> MakeTryDequeueMessage() { return new TryDequeueMessage(); }
public static QueueMessage<T> MakeEnqueueMessage(T item) { return new EnqueueMessage(item); }
private sealed class ClearMessage : QueueMessage<T>
{
public ClearMessage() { }
public override MessageType MessageType
{
get { return MessageType.ClearMessage; }
}
/// <summary>
/// Not implemented by this subclass
/// </summary>
public override T Item
{
get { throw new NotImplementedException(); }
}
}
private sealed class TryDequeueMessage : QueueMessage<T>
{
public TryDequeueMessage() { }
public override MessageType MessageType
{
get { return MessageType.TryDequeueMessage; }
}
/// <summary>
/// Not implemented by this subclass
/// </summary>
public override T Item
{
get { throw new NotImplementedException(); }
}
}
private sealed class EnqueueMessage : QueueMessage<T>
{
private T item;
public EnqueueMessage(T item) { this.item = item; }
public override MessageType MessageType
{
get { return MessageType.EnqueueMessage; }
}
/// <summary>
/// Gets the item to be enqueued
/// </summary>
public override T Item { get { return item; } }
}
}
Now, in code that is given a QueueMessage, you can switch on the MessageType property in lieu of pattern matching, and make sure that you access the Item property only on EnqueueMessages.
EDIT
Here's another alternative, based on Juliet's code. I've tried to streamline things so that it's got a more usable interface from C#, though. This is preferable to the previous version in that you can't get a MethodNotImplemented exception.
public abstract class QueueMessage<T>
{
// prevents unwanted subclassing
private QueueMessage() { }
public abstract TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase);
public static QueueMessage<T> MakeClearMessage() { return new ClearMessage(); }
public static QueueMessage<T> MakeTryDequeueMessage() { return new TryDequeueMessage(); }
public static QueueMessage<T> MakeEnqueueMessage(T item) { return new EnqueueMessage(item); }
private sealed class ClearMessage : QueueMessage<T>
{
public ClearMessage() { }
public override TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase)
{
return clearCase();
}
}
private sealed class TryDequeueMessage : QueueMessage<T>
{
public TryDequeueMessage() { }
public override TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase)
{
return tryDequeueCase();
}
}
private sealed class EnqueueMessage : QueueMessage<T>
{
private T item;
public EnqueueMessage(T item) { this.item = item; }
public override TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase)
{
return enqueueCase(item);
}
}
}
You'd use this code like this:
public class MessageUserTest
{
public void Use()
{
// your code to get a message here...
QueueMessage<string> msg = null;
// emulate pattern matching, but without constructor names
int i =
msg.Match(
clearCase: () => -1,
tryDequeueCase: () => -2,
enqueueCase: s => s.Length);
}
}

In your example code, you implement PostWithAsyncReply in terms of PostWithReply. That isn't ideal, because it means that when you call PostWithAsyncReply and the actor takes a while to handle it, there are actually two threads tied up: the one executing the actor and the one waiting for it to finish. It would be better to have the one thread executing the actor and then calling the callback in the asynchronous case. (Obviously in the synchronous case there's no avoiding the tying up of two threads).
Update:
More on the above: you construct an actor with an argument telling it how many threads to run. For simplicity suppose every actor runs with one thread (actually quite a good situation because actors can then have internal state with no locking on it, as only one thread accesses it directly).
Actor A calls actor B, waiting for a response. In order to handle the request, actor B needs to call actor C. So now A and B's only threads are waiting, and C's is the only one actually giving the CPU any work to do. So much for multi-threading! But this is what you get if you wait for answers all the time.
Okay, you could increase the number of threads you start in each actor. But you'd be starting them so they could sit around doing nothing. A stack uses up a lot of memory, and context switching can be expensive.
So it's better to send messages asynchronously, with a callback mechanism so you can pick up the finished result. The problem with your implementation is that you grab another thread from the thread pool, purely to sit around and wait. So you basically apply the workaround of increasing the number of threads. You allocate a thread to the task of never running.
It would be better to implement PostWithReply in terms of PostWithAsyncReply, i.e. the opposite way round. The asynchronous version is low-level. Building on my delegate-based example (because it involves less typing of code!):
private bool InsertCoinImpl(int value)
{
// only accept dimes/10p/whatever it is in euros
return (value == 10);
}
public void InsertCoin(int value, Action<bool> accepted)
{
Submit(() => accepted(InsertCoinImpl(value)));
}
So the private implementation returns a bool. The public asynchronous method accepts an action that will receive the return value; both the private implementation and the callback action are executed on the same thread.
Hopefully the need to wait synchronously is going to be the minority case. But when you need it, it could be supplied by a helper method, totally general purpose and not tied to any specific actor or message type:
public static T Wait<T>(Action<Action<T>> activity)
{
T result = default(T);
var finished = new EventWaitHandle(false, EventResetMode.AutoReset);
activity(r =>
{
result = r;
finished.Set();
});
finished.WaitOne();
return result;
}
So now in some other actor we can say:
bool accepted = Helpers.Wait<bool>(r => chocMachine.InsertCoin(5, r));
The type argument to Wait may be unnecessary, haven't tried compiling any of this. But Wait basically magics-up a callback for you, so you can pass it to some asynchronous method, and on the outside you just get back whatever was passed to the callback as your return value. Note that the lambda you pass to Wait still actually executes on the same thread that called Wait.
We now return you to our regular program...
As for the actual problem you asked about, you send a message to an actor to get it to do something. Delegates are helpful here. They let you effectively get the compiler to generate you a class with some data, a constructor that you don't even have to call explicitly and also a method. If you're having to write a bunch of little classes, switch to delegates.
abstract class Actor
{
Queue<Action> _messages = new Queue<Action>();
protected void Submit(Action action)
{
// take out a lock of course
_messages.Enqueue(action);
}
// also a "run" that reads and executes the
// message delegates on background threads
}
Now a specific derived actor follows this pattern:
class ChocolateMachineActor : Actor
{
private void InsertCoinImpl(int value)
{
// whatever...
}
public void InsertCoin(int value)
{
Submit(() => InsertCoinImpl(value));
}
}
So to send a message to the actor, you just call the public methods. The private Impl method does the real work. No need to write a bunch of message classes by hand.
Obviously I've left out the stuff about replying, but that can all be done with more parameters. (See update above).

Union types and pattern matching map pretty directly to the visitor pattern, I've posted about this a few times before:
What task is best done in a functional programming style?
https://stackoverflow.com/questions/1883246/none-pure-functional-code-smells/1884256#1884256
So if you want to pass messages with lots of different types, you're stuck implementing the visitor pattern.
(Warning, untested code ahead, but should give you an idea of how its done)
Let's say we have something like this:
type msg =
| Add of int
| Sub of int
| Query of ReplyChannel<int>
let rec counts = function
| [] -> (0, 0, 0)
| Add(_)::xs -> let (a, b, c) = counts xs in (a + 1, b, c)
| Sub(_)::xs -> let (a, b, c) = counts xs in (a, b + 1, c)
| Query(_)::xs -> let (a, b, c) = counts xs in (a, b, c + 1)
You end up with this bulky C# code:
interface IMsgVisitor<T>
{
T Visit(Add msg);
T Visit(Sub msg);
T Visit(Query msg);
}
abstract class Msg
{
public abstract T Accept<T>(IMsgVistor<T> visitor)
}
class Add : Msg
{
public readonly int Value;
public Add(int value) { this.Value = value; }
public override T Accept<T>(IMsgVisitor<T> visitor) { return visitor.Visit(this); }
}
class Sub : Msg
{
public readonly int Value;
public Add(int value) { this.Value = value; }
public override T Accept<T>(IMsgVisitor<T> visitor) { return visitor.Visit(this); }
}
class Query : Msg
{
public readonly ReplyChannel<int> Value;
public Add(ReplyChannel<int> value) { this.Value = value; }
public override T Accept<T>(IMsgVisitor<T> visitor) { return visitor.Visit(this); }
}
Now whenever you want to do something with the message, you need to implement a visitor:
class MsgTypeCounter : IMsgVisitor<MsgTypeCounter>
{
public readonly Tuple<int, int, int> State;
public MsgTypeCounter(Tuple<int, int, int> state) { this.State = state; }
public MsgTypeCounter Visit(Add msg)
{
Console.WriteLine("got Add of " + msg.Value);
return new MsgTypeCounter(Tuple.Create(1 + State.Item1, State.Item2, State.Item3));
}
public MsgTypeCounter Visit(Sub msg)
{
Console.WriteLine("got Sub of " + msg.Value);
return new MsgTypeCounter(Tuple.Create(State.Item1, 1 + State.Item2, State.Item3));
}
public MsgTypeCounter Visit(Query msg)
{
Console.WriteLine("got Query of " + msg.Value);
return new MsgTypeCounter(Tuple.Create(State.Item1, 1 + State.Item2, State.Item3));
}
}
Then finally you can use it like this:
var msgs = new Msg[] { new Add(1), new Add(3), new Sub(4), new ReplyChannel(null) };
var counts = msgs.Aggregate(new MsgTypeVisitor(Tuple.Create(0, 0, 0)),
(acc, x) => x.Accept(acc)).State;
Yes, its as obtuse as it seems, but that's how you pass multiple messages a class in a type-safe manner, and that's also why we don't implement unions in C# ;)

A long shot, but anyway..
I am assuming that discriminated-union is F# for ADT (Abstract Data Type). Which means the type could be one of several things.
In case there are two, you could try and put it in a simple generic class with two type parameters:
public struct DiscriminatedUnion<T1,T2>
{
public DiscriminatedUnion(T1 t1) { value = t1; }
public DiscriminatedUnion(T2 t1) { value = t2; }
public static implicit operator T1(DiscriminatedUnion<T1,T2> du) {return (T1)du.value; }
public static implicit operator T2(DiscriminatedUnion<T1,T2> du) {return (T2)du.value; }
object value;
}
To make it work for 3 or more, we need to replicate this class a number of times.
Any one has a solution for function overloading depending on the runtime type?

If you have this
type internal Either<'a, 'b> =
| Left of 'a
| Right of 'b
in F#, then the C# equivalent of the CLR generated for class Either<'a, 'b> has inner types like
internal class _Left : Either<a, b>
{
internal readonly a left1;
internal _Left(a left1);
}
each with a tag, a getter and a factory method
internal const int tag_Left = 0;
internal static Either<a, b> Left(a Left1);
internal a Left1 { get; }
plus a discriminator
internal int Tag { get; }
and a raft of methods to implement interfaces IStructuralEquatable, IComparable, IStructuralComparable

There is a compile-time checked discriminated union type at Discriminated union in C#
private class ClearMessage
{
public static readonly ClearMessage Instance = new ClearMessage();
private ClearMessage() { }
}
private class TryDequeueMessage
{
public static readonly TryDequeueMessage Instance = new TryDequeueMessage();
private TryDequeueMessage() { }
}
private class EnqueueMessage
{
public TValue Item { get; private set; }
private EnqueueMessage(TValue item) { Item = item; }
}
Using the discriminated union could be done as follows:
// New file
// Create an alias
using Message = Union<ClearMessage, TryDequeueMessage, EnqueMessage>;
int ProcessMessage(Message msg)
{
return Message.Match(
clear => 1,
dequeue => 2,
enqueue => 3);
}

C# Multi Thread Design Example

I am relatively new to C#/.Net. I'm developing a desktop application that requires multi threading. I came up with the following pattern below as a base. I was wondering if anyone could point out how to make it better in terms of coding, being thread safe, and being efficient.
Hopefully this makes some sense.
public abstract class ThreadManagerBase
{
// static class variables
private static ThreadManagerBase instance = null;
private static BackgroundWorker thread = null;
private static ProgressBarUIForm progress = null;
/// <summary>
/// Create a new instance of this class. The internals are left to the derived class to figure out.
/// Only one instance of this can run at any time. There should only be the main thread and this thread.
/// </summary>
public abstract static ThreadManagerBase NewInstance();
/// <summary>
/// Clears the instance.
/// </summary>
public static void ClearInstance()
{
instance = null;
}
/// <summary>
/// Initializes the background worker with some presets.
/// Displays progress bar.
/// </summary>
private abstract static void InitializeThread()
{
thread = new BackgroundWorker();
thread.WorkerReportsProgress = true;
thread.WorkerSupportsCancellation = true;
thread.DoWork += new DoWorkEventHandler(thread_DoWork);
thread.RunWorkerCompleted += new RunWorkerCompletedEventHandler(thread_RunWorkerCompleted);
thread.ProgressChanged += new ProgressChangedEventHandler(thread_ProgressChanged);
thread.RunWorkerAsync();
progress = new ProgressBarUIForm();
progress.EnableCancelButton = true;
progress.UserCanceled += new EventHandlerCancelClicked(progress_UserCanceled);
progress.ShowDialog();
thread.Dispose();
thread = null;
}
private static void progress_UserCanceled(bool userCanceled)
{
thread.CancelAsync();
}
private static void thread_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
progress.SetProgressLevel = e.ProgressPercentage;
progress.SetProgressMessage = e.UserState.ToString();
}
private static void thread_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
progress.Close();
progress = null;
}
private static void thread_DoWork(object sender, DoWorkEventArgs e)
{
ProcessWork();
}
private abstract static void ProcessWork()
{
// do actuall stuff here.
// the derived classes will take care of the plumbing.
}
}

Have you looked into the Microsoft Parallel Extensions to .NET Framework 3.5? It's a pretty good library that takes a lot of the work out of threading.
There are also a lot of articles on MSDN about threading patterns that you should research too. Threading can get really complicated really fast. It's nice to have someone else to have though of all the important stuff that can go wrong, and simplify it down to a library or a pattern. Of course, there's danger in that too if you don't understand the gotchas of any particular solution. So, make sure you research well whatever solution you choose.

I don't see a good reason to create this abstraction over BackgroundWorker.
If you insist, just a warning: I'm not sure if it changed in later releases, but in NET 2.0, it wasn't possible to really cancel the DoWork handler (unless it checked once in a while if it was asked to stop). Read here for a solution.

I have done something similar to this. There is a good reason if you do have multiple tasks that you want to perform, but you dont want to have BackgroundWorker code replicated through the entire project. I dont have the progressbar tied to the actual base class, I just have that in the main form. Here is the solution I came up with:
The following is the base class:
public abstract class Operation
{
#region public Event Handlers
///
/// The event that updates the progress of the operation
///
public event OperationProgressChangedEventHandler OperationProgressChanged;
///
/// The event that notifies that the operation is complete (and results)
///
public event OperationCompletedEventHandler OperationCompleted;
#endregion
#region Members
// Whether or not we can cancel the operation
private bool mWorkerSupportsCancellation = false;
// The task worker that handles running the operation
private BackgroundWorker mOperationWorker;
// The operation parameters
private object[] mOperationParameters;
#endregion
///
/// Base class for all operations
///
public Operation(params object[] workerParameters)
{
mOperationParameters = workerParameters;
// Setup the worker
SetupOperationWorker();
}
#region Setup Functions
///
/// Setup the background worker to run our Operations
///
private void SetupOperationWorker()
{
mOperationWorker = new BackgroundWorker();
mOperationWorker.WorkerSupportsCancellation = mWorkerSupportsCancellation;
mOperationWorker.WorkerReportsProgress = true;
mOperationWorker.WorkerSupportsCancellation = true;
mOperationWorker.DoWork += new DoWorkEventHandler(OperationWorkerDoWork);
mOperationWorker.ProgressChanged += new ProgressChangedEventHandler(OperationWorkerProgressChanged);
mOperationWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(OperationWorkerRunWorkerCompleted);
}
#endregion
#region Properties
///
/// Whether or not to allow the user to cancel the operation
///
public bool CanCancel
{
set
{
mWorkerSupportsCancellation = value;
}
}
#endregion
#region Operation Start/Stop Details
///
/// Start the operation with the given parameters
///
/// The parameters for the worker
public void StartOperation()
{
// Run the worker
mOperationWorker.RunWorkerAsync(mOperationParameters);
}
///
/// Stop the operation
///
public void StopOperation()
{
// Signal the cancel first, then call cancel to stop the test
if (IsRunning())
{
// Sets the backgroundworker CancelPending to true, so we can break
// in the sub classes operation
mOperationWorker.CancelAsync();
// This allows us to trigger an event or "Set" if WaitOne'ing
Cancel();
// Wait for it to actually stop before returning
while (IsRunning())
{
Application.DoEvents();
}
}
}
///
/// Whether or not the operation is currently running
///
///
public bool IsRunning()
{
return mOperationWorker.IsBusy;
}
#endregion
#region BackgroundWorker Events
///
/// Fires when the operation has completed
///
///
///
private void OperationWorkerRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
// Allow the sub class to clean up anything that might need to be updated
Clean();
// Notify whoever is register that the operation is complete
if (OperationCompleted != null)
{
OperationCompleted(e);
}
}
///
/// Fires when the progress needs to be updated for a given test (we might not care)
///
///
///
private void OperationWorkerProgressChanged(object sender, ProgressChangedEventArgs e)
{
// Notify whoever is register of what the current percentage is
if (OperationProgressChanged != null)
{
OperationProgressChanged(e);
}
}
///
/// Fires when we start the operation (this does the work)
///
///
///
private void OperationWorkerDoWork(object sender, DoWorkEventArgs e)
{
// Run the operation
Run(sender, e);
}
#endregion
#region Abstract methods
///
/// Abstract, implemented in the sub class to do the work
///
///
///
protected abstract void Run(object sender, DoWorkEventArgs e);
///
/// Called at the end of the test to clean up anything (ex: Disconnected events, etc)
///
protected abstract void Clean();
///
/// If we are waiting on something in the operation, this will allow us to
/// stop waiting (ex: WaitOne).
///
protected abstract void Cancel();
#endregion
}

The following is an example test class for the example I posted:
class TestOperation : Operation
{
AutoResetEvent mMsgRec;
public TestOperation(params object[] workerParameters)
: base(workerParameters)
{
CanCancel = true;
mMsgRec = new AutoResetEvent(false);
//mSomeEvent += DoSomething();
}
protected override void Cancel()
{
mMsgRec.Set();
}
protected override void Clean()
{
//mSomeEvent -= DoSomething();
}
protected override void Run(object sender, DoWorkEventArgs e)
{
BackgroundWorker bg = (BackgroundWorker)sender;
for (int i = 0; !bg.CancellationPending && (i &lt 90); i++)
{
bg.ReportProgress(i);
Thread.Sleep(100);
}
for (int i = 90; !bg.CancellationPending && (i &lt 100); i++)
{
mMsgRec.WaitOne(2000, false);
bg.ReportProgress(i);
}
if (bg.CancellationPending)
{
e.Cancel = true;
}
else
{
e.Result = "Complete"; // Or desired result
}
}
}
And here is what the main form would look like (very basic example):
public partial class Form1 : Form
{
TestOperation t;
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
t = new TestOperation();
t.CanCancel = true;
t.OperationProgressChanged += new OperationProgressChangedEventHandler(t_OperationProgressChanged);
t.OperationCompleted += new OperationCompletedEventHandler(t_OperationCompleted);
t.StartOperation();
}
void t_OperationCompleted(RunWorkerCompletedEventArgs e)
{
progressBar1.Value = 0;
}
void t_OperationProgressChanged(ProgressChangedEventArgs e)
{
progressBar1.Value = e.ProgressPercentage;
}
private void button2_Click(object sender, EventArgs e)
{
t.StopOperation();
}
}

I'm currently investigation Threadmare over at http://sklobovsky.nstemp.com/community/threadmare/threadmare.htm for a C# project. It looks very, very useful. It's in Delphi, but the principles apply to any language that can handle events.

You don't need a BackgroundWorker unless you want to be spoonfed, normal threads are perfectly acceptable, as long as you follow the rules.