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How do I guarantee execution of code only if and when optional main thread task and worker threads are finished?

Background:
I have an application I am developing that deals with a large number of addons for another application. One if its primary uses is to safely modify file records in files with fewer records so that they may be treated as one file (almost as if it is combing the files together into one set of records. To do this safely it keeps track of vital information about those files and changes made to them so that those changes can be undone if they don't work as expected.
When my application starts, it analyzes those files and keeps essential properties in a cache (to reduce load times). If a file is missing from the cache, the most important stuff is retrieved and then a background worker must process the file for more information. If a file that was previously modified has been updated with a new version of the file, the UI must confirm this with the user and its modification data removed. All of this information, including information on its modification is stored in the cache.
My Problem:
My problem is that neither of these processes are guaranteed to run (the confirmation window or the background file processor). If either of them run, then the cache must be updated by the main thread. I don't know enough about worker threads, and which thread runs the BackgroundWorker.RunWorkerCompleted event handler in order to effectively decide how to approach guaranteeing that the cache updater is run after either (or both) processes are completed.
To sum up: if either process is run, they both must finish and (potentially) wait for the other to be completed before running the cache update code. How can I do this?
ADJUNCT INFO (My current intervention that doesn't seem to work very well):
I have a line in the RunWorkerCompleted handler that waits until the form reference is null before continuing and exiting but maybe this was a mistake as it sometimes locks my program up.
SpinWait.SpinUntil(() => overwriteForm == null);
I haven't included any more code because I anticipate that this is more of a conceptual question than a code one. However, if necessary, I can supply code if it helps.

I think CountDownTask is what you need
using System;
using System.Threading;
public class Program
{
public class AtomicInteger
{
protected int value = 0;
public AtomicInteger(int value)
{
this.value = value;
}
public int DecrementAndGet()
{
int answer = Interlocked.Decrement(ref value);
return answer;
}
}
public interface Runnable
{
void Run();
}
public class CountDownTask
{
private AtomicInteger count;
private Runnable task;
private Object lk = new Object();
private volatile bool runnable;
private bool cancelled;
public CountDownTask(Int32 count, Runnable task)
{
this.count = new AtomicInteger(count);
this.task = task;
this.runnable = false;
this.cancelled = false;
}
public void CountDown()
{
if (count.DecrementAndGet() == 0)
{
lock (lk)
{
runnable = true;
Monitor.Pulse(lk);
}
}
}
public void Await()
{
lock (lk)
{
while (!runnable)
{
Monitor.Wait(lk);
}
if (cancelled)
{
Console.WriteLine("Sorry! I was cancelled");
}
else {
task.Run();
}
}
}
public void Cancel()
{
lock (lk)
{
runnable = true;
cancelled = true;
Monitor.Pulse(lk);
}
}
}
public class HelloWorldTask : Runnable
{
public void Run()
{
Console.WriteLine("Hello World, I'm last one");
}
}
public static void Main()
{
Thread.CurrentThread.Name = "Main";
Console.WriteLine("Current Thread: " + Thread.CurrentThread.Name);
CountDownTask countDownTask = new CountDownTask(3, new HelloWorldTask());
Thread worker1 = new Thread(() => {
Console.WriteLine("Worker 1 run");
countDownTask.CountDown();
});
Thread worker2 = new Thread(() => {
Console.WriteLine("Worker 2 run");
countDownTask.CountDown();
});
Thread lastThread = new Thread(() => countDownTask.Await());
lastThread.Start();
worker1.Start();
worker2.Start();
//countDownTask.Cancel();
Console.WriteLine("Main Thread Run");
countDownTask.CountDown();
Thread.Sleep(1000);
}
}
let me explain (but you can refer Java CountDownLatch)
1. To ensure a task must run after another tasks, we need create a Wait function to wait for they done, so I used
while(!runnable) {
Monitor.Wait(lk);
}
2. When there is a task done, we need count down, and if count down to zero (it means all of the tasks was done) we will need notify to blocked thread to wake up and process task
if(count.decrementAndGet() == 0) {
lock(lk) {
runnable = true;
Monitor.Pulse(lk);
}
}
Let read more about volatile, thanks

While dung ta van's "CountDownTask" answer isn't quite what I needed, it heavily inspired the solution below (see it for more info). Basically all I did was add some extra functionality and most importantly: made it so that each task "vote" on the outcome (true or false). Thanks dung ta van!
To be fair, dung ta van's solution DOES work to guarantee execution which as it turns out isn't quite what I needed. My solution adds the ability to make that execution conditional.
This was my solution which worked:
public enum PendingBool
{
Unknown = -1,
False,
True
}
public interface IRunnableTask
{
void Run();
}
public class AtomicInteger
{
int integer;
public int Value { get { return integer; } }
public AtomicInteger(int value) { integer = value; }
public int Decrement() { return Interlocked.Decrement(ref integer); }
public static implicit operator int(AtomicInteger ai) { return ai.integer; }
}
public class TaskElectionEventArgs
{
public bool VoteResult { get; private set; }
public TaskElectionEventArgs(bool vote) { VoteResult = vote; }
}
public delegate void VoteEventHandler(object sender, TaskElectionEventArgs e);
public class SingleVoteTask
{
private AtomicInteger votesLeft;
private IRunnableTask task;
private volatile bool runTask = false;
private object _lock = new object();
public event VoteEventHandler VoteCast;
public event VoteEventHandler TaskCompleted;
public bool IsWaiting { get { return votesLeft.Value > 0; } }
public PendingBool Result
{
get
{
if (votesLeft > 0)
return PendingBool.Unknown;
else if (runTask)
return PendingBool.True;
else
return PendingBool.False;
}
}
public SingleVoteTask(int numberOfVotes, IRunnableTask taskToRun)
{
votesLeft = new AtomicInteger(numberOfVotes);
task = taskToRun;
}
public void CastVote(bool vote)
{
votesLeft.Decrement();
runTask |= vote;
VoteCast?.Invoke(this, new TaskElectionEventArgs(vote));
if (votesLeft == 0)
lock (_lock)
{
Monitor.Pulse(_lock);
}
}
public void Await()
{
lock(_lock)
{
while (votesLeft > 0)
Monitor.Wait(_lock);
if (runTask)
task.Run();
TaskCompleted?.Invoke(this, new TaskElectionEventArgs(runTask));
}
}
}
Implementing the above solution was as simple as creating the SingleVoteTask in the UI thread and then having each thread affecting the outcome cast a vote.

Task counter C#, Interlocked

I would like to run tasks in parallel, with no more than 10 instances running at a given time.
This is the code I have so far:
private void Listen()
{
while (true)
{
var context = listener.GetContext();
var task = Task.Run(() => HandleContextAsync(context));
Interlocked.Increment(ref countTask);
if (countTask > 10)
{
//I save tasks in the collection
}
else
{
task.ContinueWith(delegate { Interlocked.Decrement(ref countTask); }); //I accomplish the task and reduce the counter
}
}
}

I would suggest that you use a Parallel loop; for example:
Parallel.For(1, 10, a =>
{
var context = listener.GetContext();
...
});
That will start a defined number of tasks without you needing to manage the process yourself.

If you want to continually execute code in parallel, with up to 10 instances at a time, this may be worth considering:
private void Listen()
{
var options = new ParallelOptions() { MaxDegreeOfParallelism = 10 };
Parallel.For(1, long.MaxValue - 1, options, (i) =>
{
var context = listener.GetContext();
HandleContextAsync(context);
});
}
Basically, it will run the code continually (well roughly long.MaxValue times). MaxDegreeOfParallelism ensures that it runs only 10 'instances' of the code at a time.

I'm assuming that the result from GetContext is not created by you, so, its probably not useful to use a Parallel.For when you don't know how many times to run or don't have all the contexts to handle right away.
So, probably the best way to resolve this would be by implementing your own TaskScheduler. This way you can add more tasks to be resolved on demand with a fixed concurrency level.
Based on the example from Microsoft Docs website you can already achieve this.
I made an example program with some changes to the LimitedConcurrencyLevelTaskScheduler from the website.
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
namespace parallel
{
class Program
{
private static Random Rand = new Random();
static void Main(string[] args)
{
var ts = new LimitedConcurrencyLevelTaskScheduler(10);
var taskFactory = new TaskFactory(ts);
while (true)
{
var context = GetContext(ts);
if (context.Equals("Q", StringComparison.OrdinalIgnoreCase))
break;
taskFactory.StartNew(() => HandleContextAsync(context));
}
Console.WriteLine("Waiting...");
while (ts.CountRunning != 0)
{
Console.WriteLine("Now running {0}x tasks with {1}x queued.", ts.CountRunning, ts.CountQueued);
Thread.Yield();
Thread.Sleep(100);
}
}
private static void HandleContextAsync(string context)
{
// delays for 1-10 seconds to make the example easier to understand
Thread.Sleep(Rand.Next(1000, 10000));
Console.WriteLine("Context: {0}, from thread: {1}", context, Thread.CurrentThread.ManagedThreadId);
}
private static string GetContext(LimitedConcurrencyLevelTaskScheduler ts)
{
Console.WriteLine("Now running {0}x tasks with {1}x queued.", ts.CountRunning, ts.CountQueued);
return Console.ReadLine();
}
}
// Provides a task scheduler that ensures a maximum concurrency level while
// running on top of the thread pool.
public class LimitedConcurrencyLevelTaskScheduler : TaskScheduler
{
// Indicates whether the current thread is processing work items.
[ThreadStatic]
private static bool _currentThreadIsProcessingItems;
// The list of tasks to be executed
private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks)
public int CountRunning => _nowRunning;
public int CountQueued
{
get
{
lock (_tasks)
{
return _tasks.Count;
}
}
}
// The maximum concurrency level allowed by this scheduler.
private readonly int _maxDegreeOfParallelism;
// Indicates whether the scheduler is currently processing work items.
private volatile int _delegatesQueuedOrRunning = 0;
private volatile int _nowRunning;
// Creates a new instance with the specified degree of parallelism.
public LimitedConcurrencyLevelTaskScheduler(int maxDegreeOfParallelism)
{
if (maxDegreeOfParallelism < 1)
throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
_maxDegreeOfParallelism = maxDegreeOfParallelism;
}
// Queues a task to the scheduler.
protected sealed override void QueueTask(Task task)
{
// Add the task to the list of tasks to be processed. If there aren't enough
// delegates currently queued or running to process tasks, schedule another.
lock (_tasks)
{
_tasks.AddLast(task);
if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism)
{
Interlocked.Increment(ref _delegatesQueuedOrRunning);
NotifyThreadPoolOfPendingWork();
}
}
}
// Inform the ThreadPool that there's work to be executed for this scheduler.
private void NotifyThreadPoolOfPendingWork()
{
ThreadPool.UnsafeQueueUserWorkItem(_ =>
{
// Note that the current thread is now processing work items.
// This is necessary to enable inlining of tasks into this thread.
_currentThreadIsProcessingItems = true;
try
{
// Process all available items in the queue.
while (true)
{
Task item;
lock (_tasks)
{
// When there are no more items to be processed,
// note that we're done processing, and get out.
if (_tasks.Count == 0)
{
Interlocked.Decrement(ref _delegatesQueuedOrRunning);
break;
}
// Get the next item from the queue
item = _tasks.First.Value;
_tasks.RemoveFirst();
}
// Execute the task we pulled out of the queue
Interlocked.Increment(ref _nowRunning);
if (base.TryExecuteTask(item))
Interlocked.Decrement(ref _nowRunning);
}
}
// We're done processing items on the current thread
finally { _currentThreadIsProcessingItems = false; }
}, null);
}
// Attempts to execute the specified task on the current thread.
protected sealed override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
{
// If this thread isn't already processing a task, we don't support inlining
if (!_currentThreadIsProcessingItems) return false;
// If the task was previously queued, remove it from the queue
if (taskWasPreviouslyQueued)
// Try to run the task.
if (TryDequeue(task))
return base.TryExecuteTask(task);
else
return false;
else
return base.TryExecuteTask(task);
}
// Attempt to remove a previously scheduled task from the scheduler.
protected sealed override bool TryDequeue(Task task)
{
lock (_tasks) return _tasks.Remove(task);
}
// Gets the maximum concurrency level supported by this scheduler.
public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }
// Gets an enumerable of the tasks currently scheduled on this scheduler.
protected sealed override IEnumerable<Task> GetScheduledTasks()
{
bool lockTaken = false;
try
{
Monitor.TryEnter(_tasks, ref lockTaken);
if (lockTaken) return _tasks;
else throw new NotSupportedException();
}
finally
{
if (lockTaken) Monitor.Exit(_tasks);
}
}
}
}

Blocking collections + Multiple Worker threads per blocking collection + Wait For Work Completion

I have to do action in batch of 1000 message say Action A, B, C. I can do these actions in parallel.
I created groups for them. To increase parallelism, I created subgroups with in each group. Task with in a subgroup needs to be executed serially. But two subgroups can execute in parallel.
After a batch of 1000 finishes, I have to do some processing ie save in db. But I am unable to understand , how to wait for all the task to finish (I am not interested in waiting in middle just at the end of 1000 taks). Any suggestions are welcome.
public class OrderlyThreadPool<t> : IDisposable
{
BlockingCollection<t> _workingqueue = null;
Action<t> _handler = null;
public OrderlyThreadPool(int wrkerCount, Action<t> handler)
{
_workingqueue = new BlockingCollection<t>();
_handler = handler;
Worker worker = new Worker(wrkerCount, Process); //WorkerCount is always 1
worker.Start();
}
public void AddItem(t item)
{
_workingqueue.Add(item);
}
private void Process()
{
foreach (t item in _workingqueue.GetConsumingEnumerable())
{
_handler(item);
}
}
public void Dispose()
{
_workingqueue.CompleteAdding();
_workingqueue = null;
}
}
public class Worker
{
int _wrkerCount = 0;
Action _action = null;
public Worker(int workerCount, Action action)
{
_wrkerCount = workerCount;
_action = action;
}
public void Start()
{
// Create and start a separate Task for each consumer:
for (int i = 0; i < _wrkerCount; i++)
{
Task.Factory.StartNew(_action);
}
}
}
So basically I will create OrderlyThreadPool for each subgroup.
I am recv messages from say source, which blocks if no message is available. So my code, looks like
while(true)
{
var message = GetMsg();
foreach(OrderlyThreadPool<Msg> a in myList)
{
a.AddMsg(message);
}
if(msgCount > 1000)
{
Wait for all threads to finish work;
}
else
{
msgCount =msgCount+1;
}
}

You start your tasks but you don't keep a reference. Simply store these tasks, expose them through the Worker and OrderlyThreadPool and use Task.WhenAll to wait for all of them to complete:
public class Worker
{
//...
List<Task> _tasks = new List<Task>();
public Task Completion { get { return Task.WhenAll(_tasks); } }
public void Start()
{
// Create and start a separate Task for each consumer:
for (int i = 0; i < _wrkerCount; i++)
{
Tasks.Add(Task.Factory.StartNew(_action));
}
}
}
public class OrderlyThreadPool<t> : IDisposable
{
//...
public Task Completion { get { return _worker.Completion; }}
}
await Task.WhenAll(myList.Select(orderlyThreadPool => orderlyThreadPool.Completion));
However, you should probably consider using TPL Dataflow instead. It's an actor-based framework that encapsulates completion, batching, concurrency levels and so forth...

Best way to update cmdlet progress from a separate thread

I have a powershell cmdlet written in C# (deriving from PSCmdlet) which will start a long-running task which should update its progress using WriteProgress() while it is running. Since powershell will not allow a separate thread to use WriteObject or WriteProgress I had to create a Queue<object> in the main thread and I add items to the queue from the task that I want to be written to the Pipeline/Progress. A while loop will dequeue objects as they come in and write to the pipline / progress bar.
This is working, but I wanted to see if there were any better practices for multi-threading with a powershell cmdlet that is written in C#/VB. For example with WPF I can always step onto the UI thread with UIComponent.Dispatcher.Invoke() if I need to update a progress bar or UI Component. Is there anything equivalent that I can use to 'step onto' the powershell thread to update the UI or write to the pipeline?

Here is an example of the queue system encapsulated in a class so it is easier to use and mimics Cmdllet.WriteObject's behavior. This way you can call WriteObject from within the separate thread and the object will be marshalled onto the powershell thread and written to the pipeline.
[Cmdlet("Test", "Adapter")]
public class TestCmdlet : PSCmdlet
{
protected override void ProcessRecord()
{
PowerShellAdapter adapter = new PowerShellAdapter(this, 100);
Task.Factory.StartNew(() => {
for (int x = 0; x < 100; x++) {
adapter.WriteObject(x);
Thread.Sleep(100);
}
adapter.Finished = true;
});
adapter.Listen();
}
}
public class PowerShellAdapter
{
private Cmdlet Cmdlet { get; set; }
private Queue<object> Queue { get; set; }
private object LockToken { get; set; }
public bool Finished { get; set; }
public int Total { get; set; }
public int Count { get; set; }
public PowerShellAdapter(Cmdlet cmdlet, int total)
{
this.Cmdlet = cmdlet;
this.LockToken = new object();
this.Queue = new Queue<object>();
this.Finished = false;
this.Total = total;
}
public void Listen()
{
ProgressRecord progress = new ProgressRecord(1, "Counting to 100", " ");
while (!Finished || Queue.Count > 0)
{
while (Queue.Count > 0)
{
progress.PercentComplete = ++Count*100 / Total;
progress.StatusDescription = Count + "/" + Total;
Cmdlet.WriteObject(Queue.Dequeue());
Cmdlet.WriteProgress(progress);
}
Thread.Sleep(100);
}
}
public void WriteObject(object obj)
{
lock (LockToken)
Queue.Enqueue(obj);
}
}

The answer provided by Despertar will work, but it can be improved on slightly.
Polling in a loop with Thread.Sleep should be replaced with use of an AutoResetEvent. This will cause the main thread to only "wake up" when there is actually data available, and can allow the cmdlet to complete faster than 100ms. The Thread.Sleep will always cause the cmdlet to take at least 100ms, even if it could run must faster. This might not be a problem if you have a simple cmdlet, but if you insert it into a complex pipeline this 100ms can easily multiply and cause things to run very slowly. Additionally, a lock should be taken when accessing the Queue on the main thread inside the Listen method.
The moral of the story: if you do cross-thread synchronization Thread.Sleep is not the right tool.
using System.Threading;
public class PowerShellAdapter
{
private Cmdlet Cmdlet { get; set; }
private Queue<object> Queue { get; set; }
AutoResetEvent sync;
private object LockToken { get; set; }
// volatile, since it will be written/read from different threads.
volatile bool finished;
public bool Finished
{
get { return finished; }
set
{
this.finished = value;
// allow the main thread to exit the outer loop.
sync.Set();
}
}
public int Total { get; set; }
public int Count { get; set; }
public PowerShellAdapter(Cmdlet cmdlet, int total)
{
this.Cmdlet = cmdlet;
this.LockToken = new object();
this.Queue = new Queue<object>();
this.finished = false;
this.Total = total;
this.sync = new AutoResetEvent(false);
}
public void Listen()
{
ProgressRecord progress = new ProgressRecord(1, "Counting to 100", " ");
while (!Finished)
{
while (true) { // loop until we drain the queue
object item;
lock (LockToken) {
if (Queue.Count == 0)
break; // exit while
item = Queue.Dequeue();
}
progress.PercentComplete = ++Count * 100 / Total;
progress.StatusDescription = Count + "/" + Total;
Cmdlet.WriteObject(item);
Cmdlet.WriteProgress(progress);
}
sync.WaitOne();// wait for more data to become available
}
}
public void WriteObject(object obj)
{
lock (LockToken)
{
Queue.Enqueue(obj);
}
sync.Set(); // alert that data is available
}
}
Note, I haven't actually tested this code, but it illustrates the idea.

You could take a look at the Start-Job cmdlet together with Get-Job, Wait-Job and Receive-Job.
Start-Job will effectively start a new thread and output a JobId which you can query with Receive-Job to get the output. You could then loop through all currently running jobs and update your progress bar.
Take a look at http://blogs.technet.com/b/heyscriptingguy/archive/2012/08/10/use-background-jobs-to-run-a-powershell-server-uptime-report.aspx

How to get current running task in a blockingqueue of C#?

I want to get the current executing Task that I am adding in the blockingqueue, how it can be done?
EDIT: I am using this priority scheduler, and adding multiple tasks with different priorities:
public class PriorityScheduler : TaskScheduler
{
public static PriorityScheduler Highest = new PriorityScheduler(ThreadPriority.Highest);
public static PriorityScheduler AboveNormal = new PriorityScheduler(ThreadPriority.AboveNormal);
public static PriorityScheduler Normal = new PriorityScheduler(ThreadPriority.Normal);
public static PriorityScheduler BelowNormal = new PriorityScheduler(ThreadPriority.BelowNormal);
public static PriorityScheduler Lowest = new PriorityScheduler(ThreadPriority.Lowest);
public static BlockingCollection<Task> _tasks = new BlockingCollection<Task>();
private Thread[] _threads;
private ThreadPriority _priority;
private readonly int _maximumConcurrencyLevel = Math.Max(1, Environment.ProcessorCount);
public PriorityScheduler(ThreadPriority priority)
{
_priority = priority;
}
public override int MaximumConcurrencyLevel
{
get { return _maximumConcurrencyLevel; }
}
protected override IEnumerable<Task> GetScheduledTasks()
{
return _tasks;
}
protected override void QueueTask(Task task)
{
_tasks.Add(task);
if (_threads == null)
{
_threads = new Thread[_maximumConcurrencyLevel];
for (int i = 0; i < _threads.Length; i++)
{
int local = i;
_threads[i] = new Thread(() =>
{
foreach (Task t in _tasks.GetConsumingEnumerable())
base.TryExecuteTask(t);
});
_threads[i].Name = string.Format("PriorityScheduler: ", i);
_threads[i].Priority = _priority;
_threads[i].IsBackground = true;
_threads[i].Start();
}
}
}
protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
{
return false; // we might not want to execute task that should schedule as high or low priority inline
}
}
I want to stop and resume tasks depending upon there priorities, e.g. if a new task with higher priority arrives, the lower stops and lets the task execute and then resumes itself...

If you are referring to BlockingCollection<T>, you can't (directly). When you call Take() (or get the next item via GetConsumingEnumerable()), the item (task?) is actually removed from the underlying connection.
You would need to have your consumer store and expose the "current task" if you want this to be available.
Note that the Parallel Extension Extras project provides a great QueuedTaskScheduler which may accomplish your goals here. It allows you to create prioritized TaskScheduler instances, and handles all of the scheduling for you.