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Subscribe multiple threads to the same event signal to wake up

I have a queue wrapper class which is storing items to a list, and multiple threads in the main program that are consuming these items until the list is empty. In this moment, the threads have to wait until the buffer will enqueue more items to the list.
I have an event fired every time that an item is enqueued, and I need to notify the threads that they have fresh items ready to consume.
public event EventHandler ItemEnqueued;
public void Enqueue(string item)
{
_itemsList.Add(item);
OnItemEnqueued();
}
void OnItemEnqueued()
{
ItemEnqueued?.Invoke(this, EventArgs.Empty);
}
How can I notify the threads of the main program that an item has been enqueued?
Many thanks!
EDIT to clarify
public class Queue
{
private readonly List<string> _itemsList = new List<string>();
public void Enqueue(String val)
{
_itemsList.Add(val);
OnItemEnqueued();
}
void OnItemEnqueued()
{
//here I have to tell the threads that a new item has been added
}
public string Dequeue()
{
//FIFO queue
if (_itemsList.Any())
{
var first = _itemsList.First();
_itemsList.RemoveAt(0);
return first;
}
return default(string);
}
public int Count()
{
return _itemsList.Count;
}
}
}
And I have to create two threads and make them "fight" for the enqueued items:
class Program
{
readonly object _syncLock = new object();
Queue _q = new Queue();
static void Main(string[] args)
{
Program p = new Program();
p.InitThreads();
}
public void InitThreads()
{
lock (_syncLock) //exclusively add items
{
for (var i = 0; i <= 99; i++)
{
_q.Enqueue(i.ToString());
}
}
Thread t1 = new Thread(() => {
while(_q.Count() > 0)
Console.WriteLine("T1 dequeued " + Consume());
});
Thread t2 = new Thread(() =>
{
while (_q.Count() > 0)
Console.WriteLine("T2 dequeued " + Consume());
});
t1.Start();
t2.Start();
//at some moment I have to tell the threads to wait until a new item has been enqueued
}
public string Consume()
{
lock (_syncLock) //safely get one item
{
return _q.Dequeue();
}
}
}

Not sure if this is what you're looking for, but here's a compliable code sample that demonstrates how to use AutoResetEvent to signal just one thread at once:
using System;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp1
{
static class Program
{
public static void Main()
{
var signal = new AutoResetEvent(initialState:false);
for (int i = 0; i < 4; ++i)
Task.Run(() => worker(signal));
while (true)
{
Console.WriteLine("Press <ENTER> to wake a thread.");
Console.ReadLine();
signal.Set();
}
}
static void worker(AutoResetEvent signal)
{
int threadId = Thread.CurrentThread.ManagedThreadId;
while (true)
{
Console.WriteLine($"Thread {threadId} is waiting for a signal.");
signal.WaitOne();
Console.WriteLine($"Thread {threadId} received a signal");
}
}
}
}
The key thing to note is that you create a single AutoResetEvent and pass it to all the threads, which then wait on it. Then the "main" thread signals the event when it wants one of the threads to respond.
I wouldn't use a ManualResetEvent for this kind of thing, because then all the threads would be signalled simultaneously, which it looks like you want to avoid.

Joining a thread started with StartNew()

When using the StartNew() method to kick off a process on a new thread, I need to figure out how to make another call into this object in that same thread (I assume this would be some sort of Join operation?).
The following example is dumbed down to illustrate the meat of what I am trying to do. I am well aware it is severely lacking in basic concurrency considerations. But I didn't want to cloud the code with all of that logic, so please forgive me on that.
The following console app shows what I am trying to accomplish. Assume on the StartNew() call a new thread with ID 9976 is created and the method invoked there. I would like the subsequent call to ProcessImmediate() in the file system watcher change event handler to be made on thread 9976 as well. As it stands, the call would share the same thread that is used for the file system watcher change event.
Can this be done, and if so, how?
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
var runner = new Runner();
runner.Run();
Console.ReadKey();
}
}
public class Runner
{
private Activity _activity = null;
private FileSystemWatcher _fileSystemWatcher;
public void Run()
{
_activity = new Activity();
// start activity on a new thread
Task.Factory.StartNew(() => _activity.Go());
_fileSystemWatcher = new FileSystemWatcher();
_fileSystemWatcher.Filter = "*.watcher";
_fileSystemWatcher.Path = "c:\temp";
_fileSystemWatcher.Changed += FileSystemWatcher_Changed;
_fileSystemWatcher.EnableRaisingEvents = true;
}
private void FileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
{
// WANT TO CALL THIS FOR ACTIVITY RUNNING ON PREVIOUSLY CALLED THREAD
_activity.ProcessImmediate();
}
}
public class Activity
{
public void Go()
{
while (!Stop)
{
// for purposes of this example, magically assume that ProcessImmediate has not been called when this is called
DoSomethingInteresting();
System.Threading.Thread.Sleep(2000);
}
}
protected virtual void DoSomethingInteresting() { }
public void ProcessImmediate()
{
// for purposes of this example, assume that Go is magically in its sleep state when ProcessImmediate is called
DoSomethingInteresting();
}
public bool Stop { get; set; }
}
}
* UPDATE *
Thanks for the excellent responses. I took Mike's suggestion and implemented it for my console app. Below is the full working code which also includes the use of a cancellation token. I post this in case someone else might find it useful.
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
var runner = new Runner();
runner.Run();
Console.ReadKey();
runner.Stop();
Console.ReadKey();
}
}
public class Runner
{
private Activity _activity = null;
private FileSystemWatcher _fileSystemWatcher;
private CancellationTokenSource _cts = new CancellationTokenSource();
public void Stop() { _cts.Cancel(); }
public void Run()
{
_activity = new Activity();
// start activity on a new thread
var task = new Task(() => _activity.Go(_cts.Token), _cts.Token, TaskCreationOptions.LongRunning);
task.Start();
_fileSystemWatcher = new FileSystemWatcher();
_fileSystemWatcher.Filter = "*.watcher";
_fileSystemWatcher.Path = "C:\\Temp\\FileSystemWatcherPath";
_fileSystemWatcher.Changed += FileSystemWatcher_Changed;
_fileSystemWatcher.EnableRaisingEvents = true;
}
private void FileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
{
// WANT TO CALL THIS FOR ACTIVITY RUNNING ON PREVIOUSLY CALLED THREAD
_activity.ProcessImmediate();
}
}
public class Activity : IDisposable
{
private AutoResetEvent _processing = new AutoResetEvent(false);
public void Go(CancellationToken ct)
{
Thread.CurrentThread.Name = "Go";
while (!ct.IsCancellationRequested)
{
// for purposes of this example, magically assume that ProcessImmediate has not been called when this is called
DoSomethingInteresting();
_processing.WaitOne(5000);
}
Console.WriteLine("Exiting");
}
protected virtual void DoSomethingInteresting()
{
Console.WriteLine(string.Format("Doing Something Interesting on thread {0}", Thread.CurrentThread.ManagedThreadId));
}
public void ProcessImmediate()
{
// for purposes of this example, assume that Go is magically in its sleep state when ProcessImmediate is called
_processing.Set();
}
public void Dispose()
{
if (_processing != null)
{
_processing.Dispose();
_processing = null;
}
}
}
}

First, you should use TaskCreationOptions.LongRunning if you are creating a task that will not complete quickly. Second, use an AutoResetEvent to signal the waiting thread to wake up. Note that below ProcessImmediate will return before DoSomethingInteresting has completed running on the other thread. Example:
using System.Threading;
public class Activity : IDisposable
{
private AutoResetEvent _processing = new AutoResetEvent(false);
public void Go()
{
while (!Stop)
{
// for purposes of this example, magically assume that ProcessImmediate has not been called when this is called
DoSomethingInteresting();
_processing.WaitOne(2000);
}
}
protected virtual void DoSomethingInteresting() { }
public void ProcessImmediate()
{
_processing.Set();
}
public bool Stop { get; set; }
public void Dispose()
{
if (_processing != null)
{
_processing.Dispose();
_processing = null;
}
}
}

User mike has given a better solution, which will be appropriate when you like to call the same method immediately. If you want to call a different methods immediately I'll expand mike's answer to achieve that.
using System.Threading;
public class Activity : IDisposable
{
private AutoResetEvent _processing = new AutoResetEvent(false);
private ConcurrentQueue<Action> actionsToProcess = new ConcurrentQueue<Action>();
public void Go()
{
while (!Stop)
{
// for purposes of this example, magically assume that ProcessImmediate has not been called when this is called
DoSomethingInteresting();
_processing.WaitOne(2000);
while(!actionsToProcess.IsEmpty)
{
Action action;
if(actionsToProcess.TryDeque(out action))
action();
}
}
}
protected virtual void DoSomethingInteresting() { }
public void ProcessImmediate(Action action)
{
actionsToProcess.Enqueue(action);
_processing.Set();
}
public bool Stop { get; set; }
public void Dispose()
{
if (_processing != null)
{
_processing.Dispose();
_processing = null;
}
}
}

To execute different methods on the same thread you can use a message loop that dispatches incoming requests. A simple option would be to use the event loop scheduler of the Reactive Extensions and to "recursively" schedule your Go() function - if in the mean time a different operation is scheduled it would be processed before the next Go() operation.
Here is a sample:
class Loop
: IDisposable
{
IScheduler scheduler = new EventLoopScheduler();
MultipleAssignmentDisposable stopper = new MultipleAssignmentDisposable();
public Loop()
{
Next();
}
void Next()
{
if (!stopper.IsDisposed)
stopper.Disposable = scheduler.Schedule(Handler);
}
void Handler()
{
Thread.Sleep(1000);
Console.WriteLine("Handler: {0}", Thread.CurrentThread.ManagedThreadId);
Next();
}
public void Notify()
{
scheduler.Schedule(() =>
{
Console.WriteLine("Notify: {0}", Thread.CurrentThread.ManagedThreadId);
});
}
public void Dispose()
{
stopper.Dispose();
}
}
static void Main(string[] args)
{
using (var l = new Loop())
{
Console.WriteLine("Press 'q' to quit.");
while (Console.ReadKey().Key != ConsoleKey.Q)
l.Notify();
}
}

Managing high/low priority threads in .net

The scenario is as follows:
There are a couple of low priority threads that can be interrupted by high priority threads. Whenever a high priority thread asks the low priority threads to pause, they will go to Wait state (if they are not in wait state already). However when a high priority thread signals that the low priority threads can Resume, the low priority threads should not resume until all the high priority threads that asked the low priority threads to pause have consented.
To solve this I am keeping a track of Pause() calls from the high priority threads to the low priority thread in a counter variable. Whenever the high priority thread asks the low priority thread to Pause(), the value of the counter is incremented by 1. If after the increment the counter has a value of 1, it means the thread was not in Wait, so ask it to go in Wait state. Otherwise just increment counter value. On the contrary when a high priority thread calls Resume() we decrement the counter value and if after the decrement the value is 0, it means the low priority threads can Resume now.
Here is a simplified implementation of my problem. The comparison operation inside if statements with Interlocked.XXX is not correct i.e.
if (Interlocked.Increment(ref _remain) == 1)
, as the read/modify and comparison operations are not atomic.
What am I missing here? I don't want to use thread priority.
using System;
using System.Collections.Generic;
using System.Threading;
namespace TestConcurrency
{
// I borrowed this class from Joe Duffy's blog and modified it
public class LatchCounter
{
private long _remain;
private EventWaitHandle m_event;
private readonly object _lockObject;
public LatchCounter()
{
_remain = 0;
m_event = new ManualResetEvent(true);
_lockObject = new object();
}
public void Check()
{
if (Interlocked.Read(ref _remain) > 0)
{
m_event.WaitOne();
}
}
public void Increment()
{
lock(_lockObject)
{
if (Interlocked.Increment(ref _remain) == 1)
m_event.Reset();
}
}
public void Decrement()
{
lock(_lockObject)
{
// The last thread to signal also sets the event.
if (Interlocked.Decrement(ref _remain) == 0)
m_event.Set();
}
}
}
public class LowPriorityThreads
{
private List<Thread> _threads;
private LatchCounter _latch;
private int _threadCount = 1;
internal LowPriorityThreads(int threadCount)
{
_threadCount = threadCount;
_threads = new List<Thread>();
for (int i = 0; i < _threadCount; i++)
{
_threads.Add(new Thread(ThreadProc));
}
_latch = new CountdownLatch();
}
public void Start()
{
foreach (Thread t in _threads)
{
t.Start();
}
}
void ThreadProc()
{
while (true)
{
//Do something
Thread.Sleep(Rand.Next());
_latch.Check();
}
}
internal void Pause()
{
_latch.Increment();
}
internal void Resume()
{
_latch.Decrement();
}
}
public class HighPriorityThreads
{
private Thread _thread;
private LowPriorityThreads _lowPriorityThreads;
internal HighPriorityThreads(LowPriorityThreads lowPriorityThreads)
{
_lowPriorityThreads = lowPriorityThreads;
_thread = new Thread(RandomlyInterruptLowPriortyThreads);
}
public void Start()
{
_thread.Start();
}
void RandomlyInterruptLowPriortyThreads()
{
while (true)
{
Thread.Sleep(Rand.Next());
_lowPriorityThreads.Pause();
Thread.Sleep(Rand.Next());
_lowPriorityThreads.Resume();
}
}
}
class Program
{
static void Main(string[] args)
{
LowPriorityThreads lowPriorityThreads = new LowPriorityThreads(3);
HighPriorityThreads highPriorityThreadOne = new HighPriorityThreads(lowPriorityThreads);
HighPriorityThreads highPriorityThreadTwo = new HighPriorityThreads(lowPriorityThreads);
lowPriorityThreads.Start();
highPriorityThreadOne.Start();
highPriorityThreadTwo.Start();
}
}
class Rand
{
internal static int Next()
{
// Guid idea has been borrowed from somewhere on StackOverFlow coz I like it
return new System.Random(Guid.NewGuid().GetHashCode()).Next() % 30000;
}
}

I don't know about your requirements hence I won't discuss them here.
As far as the implementation goes, I would introduce a "dispatcher" class that will handle inter-threads interaction and also acts a a factory for "runnable" objects.
The implementation, of course is very rough and open for criticism.
class Program
{
static void Main(string[] args)
{
ThreadDispatcher td=new ThreadDispatcher();
Runner r1 = td.CreateHpThread(d=>OnHpThreadRun(d,1));
Runner r2 = td.CreateHpThread(d => OnHpThreadRun(d, 2));
Runner l1 = td.CreateLpThread(d => Console.WriteLine("Running low priority thread 1"));
Runner l2 = td.CreateLpThread(d => Console.WriteLine("Running low priority thread 2"));
Runner l3 = td.CreateLpThread(d => Console.WriteLine("Running low priority thread 3"));
l1.Start();
l2.Start();
l3.Start();
r1.Start();
r2.Start();
Console.ReadLine();
l1.Stop();
l2.Stop();
l3.Stop();
r1.Stop();
r2.Stop();
}
private static void OnHpThreadRun(ThreadDispatcher d,int number)
{
Random r=new Random();
Thread.Sleep(r.Next(100,2000));
d.CheckedIn();
Console.WriteLine(string.Format("*** Starting High Priority Thread {0} ***",number));
Thread.Sleep(r.Next(100, 2000));
Console.WriteLine(string.Format("+++ Finishing High Priority Thread {0} +++", number));
Thread.Sleep(300);
d.CheckedOut();
}
}
public abstract class Runner
{
private Thread _thread;
protected readonly Action<ThreadDispatcher> _action;
private readonly ThreadDispatcher _dispathcer;
private long _running;
readonly ManualResetEvent _stopEvent=new ManualResetEvent(false);
protected Runner(Action<ThreadDispatcher> action,ThreadDispatcher dispathcer)
{
_action = action;
_dispathcer = dispathcer;
}
public void Start()
{
_thread = new Thread(OnThreadStart);
_running = 1;
_thread.Start();
}
public void Stop()
{
_stopEvent.Reset();
Interlocked.Exchange(ref _running, 0);
_stopEvent.WaitOne(2000);
_thread = null;
Console.WriteLine("The thread has been stopped.");
}
protected virtual void OnThreadStart()
{
while (Interlocked.Read(ref _running)!=0)
{
OnStartWork();
_action.Invoke(_dispathcer);
OnFinishWork();
}
OnFinishWork();
_stopEvent.Set();
}
protected abstract void OnStartWork();
protected abstract void OnFinishWork();
}
public class ThreadDispatcher
{
private readonly ManualResetEvent _signal=new ManualResetEvent(true);
private int _hpCheckedInThreads;
private readonly object _lockObject = new object();
public void CheckedIn()
{
lock(_lockObject)
{
_hpCheckedInThreads++;
_signal.Reset();
}
}
public void CheckedOut()
{
lock(_lockObject)
{
if(_hpCheckedInThreads>0)
_hpCheckedInThreads--;
if (_hpCheckedInThreads == 0)
_signal.Set();
}
}
private class HighPriorityThread:Runner
{
public HighPriorityThread(Action<ThreadDispatcher> action, ThreadDispatcher dispatcher) : base(action,dispatcher)
{
}
protected override void OnStartWork()
{
}
protected override void OnFinishWork()
{
}
}
private class LowPriorityRunner:Runner
{
private readonly ThreadDispatcher _dispatcher;
public LowPriorityRunner(Action<ThreadDispatcher> action, ThreadDispatcher dispatcher)
: base(action, dispatcher)
{
_dispatcher = dispatcher;
}
protected override void OnStartWork()
{
Console.WriteLine("LP Thread is waiting for a signal.");
_dispatcher._signal.WaitOne();
Console.WriteLine("LP Thread got the signal.");
}
protected override void OnFinishWork()
{
}
}
public Runner CreateLpThread(Action<ThreadDispatcher> action)
{
return new LowPriorityRunner(action, this);
}
public Runner CreateHpThread(Action<ThreadDispatcher> action)
{
return new HighPriorityThread(action, this);
}
}
}

Waiting for a 3rd party thread to finish

I need to call a 3rd party code that optionally starts a new thread, performs some processing, and then calls a different method on my object. What I need is wait for the 3rd party processing to be finished, then return from the original method. In other words, I have a class like this (C#):
class MyClass: IThirdPartyInterface {
void MyMethod() {
//some preprocessing
//call a 3rd party static method
ThirdParty.DoSomething(this);
}
void FinishedProcessing() {
//some postprocessing
//???
}
}
I want to modify MyMethod so that it return only after the thread that started in DoSomething has finished its execution and called the FinishedProcessing method. Since the thread is started by the third party code, I don't have access to it, so I cannot use Thread.Join here. So, what do I do instead?

You need to use an System.Threading.AutoResetEvent, it would be like this:
class MyClass: IThirdPartyInterface {
AutoResetEvent _event = new AutoResetEvent(false);
void MyMethod() {
ThirdParty.DoSomething(this);
_event.WaitOne();
}
void FinishedProcessing() {
_event.Set();
}
}
If the thread continues running after your FinishedProcessing method is called by the 3rdparty class, it would be a little diferent:
class MyClass: IThirdPartyInterface {
AutoResetEvent _event = new AutoResetEvent(false);
Thread _thread;
void MyMethod() {
ThirdParty.DoSomething(this);
_event.WaitOne();
_thread.Join();
}
void FinishedProcessing() {
_thread = Thread.CurrentThread;
_event.Set();
}
}

Make your MyMethod() async and then run thirdparty method inside your custom await method, moething like this:
private async void MyMethod()
{
var result = await WaitAsynchronouslyAsync();
}
public async Task<string> WaitAsynchronouslyAsync()
{
await ThirdParty.DoSomething(this);
return "Finished";
}

If ThirdParty.DoSomething does not support async pattern
you can use additional proxy with finalizer.
But it could affect application performance like a "while(myBoolFlag){}".
class Program
{
static void Main(string[] args)
{
var list = new List<ManualResetEvent>();
for (var i = 0; i < 10000; i++)
{
var m = new ManualResetEvent(false);
list.Add(m);
new Thread(Start).Start(m);
if (i > 0 && (i % 10) == 0)
for (int j = i - 10; j < i; j++)
{
list[j].WaitOne(1000);// wait signal
GC.Collect(); //force finalizer
A.Print();
}
}
}
private static void Start(object obj)
{
new A(obj as ManualResetEvent, null);
}
}
public class A : IThirdPartyInterface
{
public static long time1;
public static long count1;
private DateTime start = DateTime.Now;
private ManualResetEvent _stop;
private IThirdPartyInterface _origin;
public A(ManualResetEvent stop, IThirdPartyInterface origin)
{
_stop = stop;
_origin = origin;
}
~A()
{
Interlocked.Increment(ref count1);
Interlocked.Add(ref time1, (long)(DateTime.Now - start).TotalMilliseconds);
_stop.Set(); //send signal
}
public static void Print()
{
Console.Write("\r" + A.time1 + "\\" + A.count1 + " ");
if (A.count1 != 0)
Console.Write((A.time1 / A.count1).ToString());
}
}

Frequent lag spikes in multi-threaded code

I'm attempting to make my simple C# graphics library multi-threaded. However, after the introduction of this code:
/* foreach (IAffector affector in affectorLookup.Values)
affector.Update(timestep); */
taskManager.Value = timestep; taskManager.Start();
foreach (IAffector affector in affectorLookup.Values)
taskManager.AddToQueue(affector.Update);
taskManager.StopWhenDone();
taskManager.Wait();
the simulation starts experiencing sharp lag-spikes, which seem to originate in TaskHandler.Run (I can't tell for sure, because adding the previous code makes my code profiler ignore anything outside TaskHandler.Run).
The task manager:
public class TaskManager
{
public delegate void MethodDel(float timestep);
private Queue<MethodDel> queue;
private List<TaskHandler> handlers;
private float value;
public float Value
{
get
{
return value;
}
set
{
this.value = value;
}
}
public TaskManager()
{
this.queue = new Queue<MethodDel>();
this.handlers = new List<TaskHandler>(System.Environment.ProcessorCount);
for (int t = 0; t < this.handlers.Capacity; ++t)
this.handlers.Add(new TaskHandler(this));
this.value = 0;
}
public void Start()
{
foreach (var handler in handlers)
handler.Start();
}
public void Stop()
{
lock (queue)
queue.Clear();
foreach (var handler in handlers)
handler.StopWhenDone();
}
public void StopWhenDone()
{
foreach (var handler in handlers)
handler.StopWhenDone();
}
public void AddToQueue(MethodDel method)
{
lock (queue)
queue.Enqueue(method);
}
public bool GetFromQueue(out MethodDel method)
{
lock (queue)
{
if (queue.Count == 0) { method = null; return false; }
method = queue.Dequeue();
return true;
}
}
public int GetQueueCount()
{
return queue.Count;
}
internal void Wait()
{
// Have to wait for them one at a time because the main thread is STA.
WaitHandle[] waitHandles = new WaitHandle[1];
// for (int t = 0; t < handlers.Count; ++t)
// waitHandles[t] = handlers[t].WaitHandle;
// WaitHandle.WaitAll(waitHandles);
for (int t = 0; t < handlers.Count; ++t)
{ waitHandles[0] = handlers[t].WaitHandle; WaitHandle.WaitAll(waitHandles); }
}
}
And the task handler:
public class TaskHandler
{
private TaskManager manager;
private Thread thread;
private bool stopWhenDone;
private ManualResetEvent waitHandle;
public ManualResetEvent WaitHandle
{
get
{
return waitHandle;
}
}
public TaskHandler(TaskManager manager)
{
this.manager = manager;
}
public void Start()
{
waitHandle = new ManualResetEvent(false);
stopWhenDone = false;
thread = new Thread(Run);
thread.IsBackground = true;
thread.SetApartmentState(ApartmentState.MTA);
thread.Start();
}
public void StopWhenDone()
{
this.stopWhenDone = true;
}
// Possible source of slowdown
private void Run()
{
TaskManager.MethodDel curMethod;
while (!stopWhenDone || manager.GetQueueCount() > 0)
{
if (manager.GetFromQueue(out curMethod))
{
curMethod(manager.Value);
}
}
waitHandle.Set();
}
}

Starting a thread is a heavy operation. Not sure if it's as heavy as you are experiencing, but that could be it. Also, having all your processing run parallel can be putting a big strain on your system with possibly little benefit...

I'm going to venture that the spikes have something to do with waitHandle.Set();
I like the overall design, but I have not used WaitHandle before, so I am unsure how this interacts with your design.