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How do I run a method both parallel and sequentially in C#?

I have a C# console app. In this app, I have a method that I will call DoWorkAsync. For the context of this question, this method looks like this:
private async Task<string> DoWorkAsync()
{
System.Threading.Thread.Sleep(5000);
var random = new Random();
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var length = random.Next(10, 101);
await Task.CompletedTask;
return new string(Enumerable.Repeat(chars, length)
.Select(s => s[random.Next(s.Length)]).ToArray());
}
I call DoWorkAsync from another method that determines a) how many times this will get ran and b) if each call will be ran in parallel or sequentially. That method looks like this:
private async Task<Task<string>[]> DoWork(int iterations, bool runInParallel)
{
var tasks = new List<Task<string>>();
for (var i=0; i<iterations; i++)
{
if (runInParallel)
{
var task = Task.Run(() => DoWorkAsync());
tasks.Add(task);
}
else
{
await DoWorkAsync();
}
}
return tasks.ToArray();
}
After all of the tasks are completed, I want to display the results. To do this, I have code that looks like this:
var random = new Random();
var tasks = await DoWork(random.Next(10, 101);
Task.WaitAll(tasks);
foreach (var task in tasks)
{
Console.WriteLine(task.Result);
}
This code works as expected if the code runs in parallel (i.e. runInParallel is true). However, when runInParallel is false (i.e. I want to run the Tasks sequentially) the Task array doesn't get populated. So, the caller doesn't have any results to work with. I don't know how to fix it though. I'm not sure how to add the method call as a Task that will run sequentially. I understand that the idea behind Tasks is to run in parallel. However, I have this need to toggle between parallel and sequential.
Thank you!

the Task array doesn't get populated.
So populate it:
else
{
var task = DoWorkAsync();
tasks.Add(task);
await task;
}
P.S.
Also your DoWorkAsync looks kinda wrong to me, why Thread.Sleep and not await Task.Delay (it is more correct way to simulate asynchronous execution, also you won't need await Task.CompletedTask this way). And if you expect DoWorkAsync to be CPU bound just make it like:
private Task<string> DoWorkAsync()
{
return Task.Run(() =>
{
// your cpu bound work
return "string";
});
}
After that you can do something like this (for both async/cpu bound work):
private async Task<string[]> DoWork(int iterations, bool runInParallel)
{
if(runInParallel)
{
var tasks = Enumerable.Range(0, iterations)
.Select(i => DoWorkAsync());
return await Task.WhenAll(tasks);
}
else
{
var result = new string[iterations];
for (var i = 0; i < iterations; i++)
{
result[i] = await DoWorkAsync();
}
return result;
}
}

Why is DoWorkAsync an async method?
It isn't currently doing anything asynchronous.
It seems that you are trying to utilise multiple threads to improve the performance of expensive CPU-bound work, so you would be better to make use of Parallel.For, which is designed for this purpose:
private string DoWork()
{
System.Threading.Thread.Sleep(5000);
var random = new Random();
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var length = random.Next(10, 101);
return new string(Enumerable.Repeat(chars, length)
.Select(s => s[random.Next(s.Length)]).ToArray());
}
private string[] DoWork(int iterations, bool runInParallel)
{
var results = new string[iterations];
if (runInParallel)
{
Parallel.For(0, iterations - 1, i => results[i] = DoWork());
}
else
{
for (int i = 0; i < iterations; i++) results[i] = DoWork();
}
return results;
}
Then:
var random = new Random();
var serial = DoWork(random.Next(10, 101));
var parallel = DoWork(random.Next(10, 101), true);

I think you'd be better off doing the following:
Create a function that creates a (cold) list of tasks (or an array Task<string>[] for instance). No need to run them. Let's call this GetTasks()
var jobs = GetTasks();
Then, if you want to run them "sequentially", just do
var results = new List<string>();
foreach (var job in jobs)
{
var result = await job;
results.Add(result);
}
return results;
If you want to run them in parallel :
foreach (var job in jobs)
{
job.Start();
}
await results = Task.WhenAll(jobs);
Another note,
All this in itself should be a Task<string[]>, the Task<Task<... smells like a problem.

TaskFactory, Starting a new Task when one ends

I have found many methods of using the TaskFactory but I could not find anything about starting more tasks and watching when one ends and starting another one.
I always want to have 10 tasks working.
I want something like this
int nTotalTasks=10;
int nCurrentTask=0;
Task<bool>[] tasks=new Task<bool>[nThreadsNum];
for (int i=0; i<1000; i++)
{
string param1="test";
string param2="test";
if (nCurrentTask<10) // if there are less than 10 tasks then start another one
tasks[nCurrentThread++] = Task.Factory.StartNew<bool>(() =>
{
MyClass cls = new MyClass();
bool bRet = cls.Method1(param1, param2, i); // takes up to 2 minutes to finish
return bRet;
});
// How can I stop the for loop until a new task is finished and start a new one?
}

Check out the Task.WaitAny method:
Waits for any of the provided Task objects to complete execution.
Example from the documentation:
var t1 = Task.Factory.StartNew(() => DoOperation1());
var t2 = Task.Factory.StartNew(() => DoOperation2());
Task.WaitAny(t1, t2)

I would use a combination of Microsoft's Reactive Framework (NuGet "Rx-Main") and TPL for this. It becomes very simple.
Here's the code:
int nTotalTasks=10;
string param1="test";
string param2="test";
IDisposable subscription =
Observable
.Range(0, 1000)
.Select(i => Observable.FromAsync(() => Task.Factory.StartNew<bool>(() =>
{
MyClass cls = new MyClass();
bool bRet = cls.Method1(param1, param2, i); // takes up to 2 minutes to finish
return bRet;
})))
.Merge(nTotalTasks)
.ToArray()
.Subscribe((bool[] results) =>
{
/* Do something with the results. */
});
The key part here is the .Merge(nTotalTasks) which limits the number of concurrent tasks.
If you need to stop the processing part way thru just call subscription.Dispose() and everything gets cleaned up for you.
If you want to process each result as they are produced you can change the code from the .Merge(...) like this:
.Merge(nTotalTasks)
.Subscribe((bool result) =>
{
/* Do something with each result. */
});

This should be all you need, not complete, but all you need to do is wait on the first to complete and then run the second.
Task.WaitAny(task to wait on);
Task.Factory.StartNew()

Have you seen the BlockingCollection class? It allows you to have multiple threads running in parallel and you can wait from results from one task to execute another. See more information here.

The answer depends on whether the tasks to be scheduled are CPU or I/O bound.
For CPU-intensive work I would use Parallel.For() API setting the number of thread/tasks through MaxDegreeOfParallelism property of ParallelOptions
For I/O bound work the number of concurrently executing tasks can be significantly larger than the number of available CPUs, so the strategy is to rely on async methods as much as possible, which reduces the total number of threads waiting for completion.
How can I stop the for loop until a new task is finished and start a
new one?
The loop can be throttled by using await:
static void Main(string[] args)
{
var task = DoWorkAsync();
task.Wait();
// handle results
// task.Result;
Console.WriteLine("Done.");
}
async static Task<bool> DoWorkAsync()
{
const int NUMBER_OF_SLOTS = 10;
string param1="test";
string param2="test";
var results = new bool[NUMBER_OF_SLOTS];
AsyncWorkScheduler ws = new AsyncWorkScheduler(NUMBER_OF_SLOTS);
for (int i = 0; i < 1000; ++i)
{
await ws.ScheduleAsync((slotNumber) => DoWorkAsync(i, slotNumber, param1, param2, results));
}
ws.Complete();
await ws.Completion;
}
async static Task DoWorkAsync(int index, int slotNumber, string param1, string param2, bool[] results)
{
results[slotNumber] = results[slotNumber} && await Task.Factory.StartNew<bool>(() =>
{
MyClass cls = new MyClass();
bool bRet = cls.Method1(param1, param2, i); // takes up to 2 minutes to finish
return bRet;
}));
}
A helper class AsyncWorkScheduler uses TPL.DataFlow components as well as Task.WhenAll():
class AsyncWorkScheduler
{
public AsyncWorkScheduler(int numberOfSlots)
{
m_slots = new Task[numberOfSlots];
m_availableSlots = new BufferBlock<int>();
m_errors = new List<Exception>();
m_tcs = new TaskCompletionSource<bool>();
m_completionPending = 0;
// Initial state: all slots are available
for(int i = 0; i < m_slots.Length; ++i)
{
m_slots[i] = Task.FromResult(false);
m_availableSlots.Post(i);
}
}
public async Task ScheduleAsync(Func<int, Task> action)
{
if (Volatile.Read(ref m_completionPending) != 0)
{
throw new InvalidOperationException("Unable to schedule new items.");
}
// Acquire a slot
int slotNumber = await m_availableSlots.ReceiveAsync().ConfigureAwait(false);
// Schedule a new task for a given slot
var task = action(slotNumber);
// Store a continuation on the task to handle completion events
m_slots[slotNumber] = task.ContinueWith(t => HandleCompletedTask(t, slotNumber), TaskContinuationOptions.ExecuteSynchronously);
}
public async void Complete()
{
if (Interlocked.CompareExchange(ref m_completionPending, 1, 0) != 0)
{
return;
}
// Signal the queue's completion
m_availableSlots.Complete();
await Task.WhenAll(m_slots).ConfigureAwait(false);
// Set completion
if (m_errors.Count != 0)
{
m_tcs.TrySetException(m_errors);
}
else
{
m_tcs.TrySetResult(true);
}
}
public Task Completion
{
get
{
return m_tcs.Task;
}
}
void SetFailed(Exception error)
{
lock(m_errors)
{
m_errors.Add(error);
}
}
void HandleCompletedTask(Task task, int slotNumber)
{
if (task.IsFaulted || task.IsCanceled)
{
SetFailed(task.Exception);
return;
}
if (Volatile.Read(ref m_completionPending) == 1)
{
return;
}
// Release a slot
m_availableSlots.Post(slotNumber);
}
int m_completionPending;
List<Exception> m_errors;
BufferBlock<int> m_availableSlots;
TaskCompletionSource<bool> m_tcs;
Task[] m_slots;
}

Why TPL Dataflow block.LinkTo does not give any output?

I am quite new to the topic TPL Dataflow. In the book Concurrency in C# I tested the following example. I can't figure out why there's no output which should be 2*2-2=2;
static void Main(string[] args)
{
//Task tt = test();
Task tt = test1();
Console.ReadLine();
}
static async Task test1()
{
try
{
var multiplyBlock = new TransformBlock<int, int>(item =>
{
if (item == 1)
throw new InvalidOperationException("Blech.");
return item * 2;
});
var subtractBlock = new TransformBlock<int, int>(item => item - 2);
multiplyBlock.LinkTo(subtractBlock,
new DataflowLinkOptions { PropagateCompletion = true });
multiplyBlock.Post(2);
await subtractBlock.Completion;
int temp = subtractBlock.Receive();
Console.WriteLine(temp);
}
catch (AggregateException e)
{
// The exception is caught here.
foreach (var v in e.InnerExceptions)
{
Console.WriteLine(v.Message);
}
}
}
Update1: I tried another example. Still I did not use Block.Complete() but I thought when the first block's completed, the result is passed into the second block automatically.
private static async Task test3()
{
TransformManyBlock<int, int> tmb = new TransformManyBlock<int, int>((i) => { return new int[] {i, i + 1}; });
ActionBlock<int> ab = new ActionBlock<int>((i) => Console.WriteLine(i));
tmb.LinkTo(ab);
for (int i = 0; i < 4; i++)
{
tmb.Post(i);
}
//tmb.Complete();
await ab.Completion;
Console.WriteLine("Finished post");
}

This part of the code:
await subtractBlock.Completion;
int temp = subtractBlock.Receive();
is first (asynchronously) waiting for the subtraction block to complete, and then attempting to retrieve an output from the block.
There are two problems: the source block is never completed, and the code is attempting to retrieve output from a completed block. Once a block has completed, it will not produce any more data.
(I assume you're referring to the example in recipe 4.2, which will post 1, causing the exception, which completes the block in a faulted state).
So, you can fix this test by completing the source block (and the completion will propagate along the link to the subtractBlock automatically), and by reading the output before (asynchronously) waiting for subtractBlock to complete:
multiplyBlock.Complete();
int temp = subtractBlock.Receive();
await subtractBlock.Completion;

Processing on shared variable by parallel tasks

I am beginning with task library. I wrote simple code like
private void button1_Click(object sender, EventArgs e)
{
simpleMost();
}
string mesg;
void simpleMost()
{
mesg = "";
button1.Enabled = false;
Task t1 = new Task(delegate { makeResult(1); });
Task t2 = new Task(delegate { makeResult(2); });
Task t3 = new Task(delegate { makeResult(3); });
t1.Start();
t2.Start();
t3.Start();
t1.Wait();
t2.Wait();
t3.Wait();
richTextBox1.Text = mesg;
button1.Enabled = true;
}
void makeResult(int a)
{
mesg += "\nTask" + a;
}
I click my button1 again and again and get following outputs
1.
Task3
Task1
Task2
2.
Task1
Task2
Task3
3.
Task1
Task2
Task3
4.
Task2
Task3
5.
Task1
Task3
6.
Task1
Task2
In 4,5,6 cases why does not makeResult work correctly. When I checked the Task states after wait statements they are found all completed but one of them misses the correct execution of function passed to it

You have several tasks, each of which is modifying the same shared resource in a non-atomic way.
mesg += "\nTask" + a;
is effectively:
mesg = mesg + "\nTask" + a;
... so if one task reads mesg, then another task writes a new value to it, then the first task will concatenate the strings using the old value, and write that back to the variable.
The simplest way to avoid this would be to use locking, so that only one thread can execute the modification statement at a time. It's not as elegant as it might be, but it should remove the race condition you currently have.

I think sometimes 2 tasks trying to access to "mesg" at the same time. Try it with a lock.
object syncObject = new object();
void makeResult(int a)
{
lock (syncObject)
{
mesg += "\nTask" + a;
}
}

Create multiple threads and wait for all of them to complete

How can I create multiple threads and wait for all of them to complete?

It depends which version of the .NET Framework you are using. .NET 4.0 made thread management a whole lot easier using Tasks:
class Program
{
static void Main(string[] args)
{
Task task1 = Task.Factory.StartNew(() => doStuff());
Task task2 = Task.Factory.StartNew(() => doStuff());
Task task3 = Task.Factory.StartNew(() => doStuff());
Task.WaitAll(task1, task2, task3);
Console.WriteLine("All threads complete");
}
static void doStuff()
{
//do stuff here
}
}
In previous versions of .NET you could use the BackgroundWorker object, use ThreadPool.QueueUserWorkItem(), or create your threads manually and use Thread.Join() to wait for them to complete:
static void Main(string[] args)
{
Thread t1 = new Thread(doStuff);
t1.Start();
Thread t2 = new Thread(doStuff);
t2.Start();
Thread t3 = new Thread(doStuff);
t3.Start();
t1.Join();
t2.Join();
t3.Join();
Console.WriteLine("All threads complete");
}

I think you need WaitHandler.WaitAll. Here is an example:
public static void Main(string[] args)
{
int numOfThreads = 10;
WaitHandle[] waitHandles = new WaitHandle[numOfThreads];
for (int i = 0; i < numOfThreads; i++)
{
var j = i;
// Or you can use AutoResetEvent/ManualResetEvent
var handle = new EventWaitHandle(false, EventResetMode.ManualReset);
var thread = new Thread(() =>
{
Thread.Sleep(j * 1000);
Console.WriteLine("Thread{0} exits", j);
handle.Set();
});
waitHandles[j] = handle;
thread.Start();
}
WaitHandle.WaitAll(waitHandles);
Console.WriteLine("Main thread exits");
Console.Read();
}
FCL has a few more convenient functions.
(1) Task.WaitAll, as well as its overloads, when you want to do some tasks in parallel (and with no return values).
var tasks = new[]
{
Task.Factory.StartNew(() => DoSomething1()),
Task.Factory.StartNew(() => DoSomething2()),
Task.Factory.StartNew(() => DoSomething3())
};
Task.WaitAll(tasks);
(2) Task.WhenAll when you want to do some tasks with return values. It performs the operations and puts the results in an array. It's thread-safe, and you don't need to using a thread-safe container and implement the add operation yourself.
var tasks = new[]
{
Task.Factory.StartNew(() => GetSomething1()),
Task.Factory.StartNew(() => GetSomething2()),
Task.Factory.StartNew(() => GetSomething3())
};
var things = Task.WhenAll(tasks);

I've made a very simple extension method to wait for all threads of a collection:
using System.Collections.Generic;
using System.Threading;
namespace Extensions {
public static class ThreadExtension {
public static void WaitAll (this IEnumerable<Thread> threads) {
if (threads != null) {
foreach (Thread thread in threads) {
thread.Join();
}
}
}
}
}
Then you simply call:
List<Thread> threads = new List<Thread>();
// Add your threads to this collection
threads.WaitAll();

In .NET 4.0, you can use the Task Parallel Library.
In earlier versions, you can create a list of Thread objects in a loop, calling Start on each one, and then make another loop and call Join on each one.

If you don't want to use the Task class (for instance, in .NET 3.5), you can just start all your threads, and then add them to the list and join them in a foreach loop.
Example:
List<Thread> threads = new List<Thread>();
// Start threads
for (int i = 0; i < 10; i++) {
int tmp = i; // Copy value for closure
Thread t = new Thread(() => Console.WriteLine(tmp));
t.Start();
threads.Add(t);
}
// Join threads (wait threads)
foreach (Thread thread in threads) {
thread.Join();
}

I don't know if there is a better way, but the following describes how I did it with a counter and background worker thread.
private object _lock = new object();
private int _runningThreads = 0;
private int Counter{
get{
lock(_lock)
return _runningThreads;
}
set{
lock(_lock)
_runningThreads = value;
}
}
Now whenever you create a worker thread, increment the counter:
var t = new BackgroundWorker();
// Add RunWorkerCompleted handler
// Start thread
Counter++;
In work completed, decrement the counter:
private void RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
Counter--;
}
Now you can check for the counter anytime to see if any thread is running:
if(Couonter>0){
// Some thread is yet to finish.
}

Most proposed answers don't take into account a time-out interval, which is very important to prevent a possible deadlock. Next is my sample code. (Note that I'm primarily a Win32 developer, and that's how I'd do it there.)
//'arrRunningThreads' = List<Thread>
//Wait for all threads
const int knmsMaxWait = 3 * 1000; //3 sec timeout
int nmsBeginTicks = Environment.TickCount;
foreach(Thread thrd in arrRunningThreads)
{
//See time left
int nmsElapsed = Environment.TickCount - nmsBeginTicks;
int nmsRemain = knmsMaxWait - nmsElapsed;
if(nmsRemain < 0)
nmsRemain = 0;
//Then wait for thread to exit
if(!thrd.Join(nmsRemain))
{
//It didn't exit in time, terminate it
thrd.Abort();
//Issue a debugger warning
Debug.Assert(false, "Terminated thread");
}
}

In my case, I could not instantiate my objects on the the thread pool with Task.Run() or Task.Factory.StartNew(). They would not synchronize my long running delegates correctly.
I needed the delegates to run asynchronously, pausing my main thread for their collective completion. The Thread.Join() would not work since I wanted to wait for collective completion in the middle of the parent thread, not at the end.
With the Task.Run() or Task.Factory.StartNew(), either all the child threads blocked each other or the parent thread would not be blocked, ... I couldn't figure out how to go with async delegates because of the re-serialization of the await syntax.
Here is my solution using Threads instead of Tasks:
using (EventWaitHandle wh = new EventWaitHandle(false, EventResetMode.ManualReset))
{
int outdex = mediaServerMinConnections - 1;
for (int i = 0; i < mediaServerMinConnections; i++)
{
new Thread(() =>
{
sshPool.Enqueue(new SshHandler());
if (Interlocked.Decrement(ref outdex) < 1)
wh.Set();
}).Start();
}
wh.WaitOne();
}