Develop Reference

Is Thread.Sleep the right thing to do in a .NET Core 2.0 console app?

Usually I avoid using Thread.Sleep except for testing or debugging code.
In the following example, I'm trying to run a console app which will run different classes that will trigger timer ticks at certain times. The idea is to add many more classes that run independent services.
At the moment I'm using, Thread.Sleep(10000); just to keep the console open to allow the code to run.
Surely the Thread.Sleep is blocking some resouces of the CPU?
Is there a better way? (This would be for both Windows and Linux)
while(true)
{
Thread.Sleep(10000);
}
Program:
class Program
{
private static CultureInfo culture = new CultureInfo("en-gb");
static void Main(string[] args)
{
LongRunningClass longRunningClass = new LongRunningClass();
while(true)
{
Thread.Sleep(10000);
}
}
}
Long Running Task:
public class LongRunningClass
{
private Timer timer;
private List<TimeSpan> ScheduleTimes = new List<TimeSpan>()
{
new TimeSpan(4,0,0),
new TimeSpan(6,0,0),
new TimeSpan(21,0,0),
new TimeSpan(23,0,0),
};
public LongRunningClass()
{
this.timer = new Timer(1000);
this.timer.Elapsed += new ElapsedEventHandler(OnTick);
this.timer.Start();
}
protected virtual void OnTick(object sender, ElapsedEventArgs e)
{
this.timer.Stop();
RunLongRunningTask();
double nextTickInterval = 0;
TimeSpan timeOfDayNow = DateTime.Now.TimeOfDay;
foreach (TimeSpan scheduleTime in ScheduleTimes)
{
if (scheduleTime > timeOfDayNow)
{
nextTickInterval = (scheduleTime - timeOfDayNow).TotalMilliseconds;
break;
}
}
// If tick interval not set yet then restart for next day
if (nextTickInterval <= 0)
{
TimeSpan scheduleTime = ScheduleTimes[0].Add(new TimeSpan(1, 0, 0, 0));
nextTickInterval = (scheduleTime - timeOfDayNow).TotalMilliseconds;
}
this.timer.Interval = nextTickInterval;
this.timer.Start();
}
private void RunLongRunningTask()
{
// Long Running Task
}
}

If it's about keeping the console open.
Did you try?
while(true)
{
Console.Read();
}
or just:
Console.Read();
so, it wouldn't close unless you press a key.

You can keep the console open indefinitely until the user cancels by using an AutoResetEvent:
class Program
{
private static AutoResetEvent autoResetEvent;
private static CultureInfo culture = new CultureInfo("en-gb");
static void Main(string[] args)
{
LongRunningClass longRunningClass = new LongRunningClass();
WaitForCancel();
}
/// <summary>
/// When cancel keys Ctrl+C or Ctrl+Break are used, set the event.
/// </summary>
private static void WaitForCancel()
{
autoResetEvent = new AutoResetEvent(false);
Console.WriteLine("Press CTRL + C or CTRL + Break to exit...");
Console.CancelKeyPress += (sender, e) =>
{
e.Cancel = true;
autoResetEvent.Set();
};
autoResetEvent.WaitOne();
}
}
Obviously, this method relies on you knowing when your threads have finished processing.

Surely the Thread.Sleep is blocking some resouces of the CPU?
Its not the CPU you have to worry about, its the memory. You can read the complicated version here, but the simple version is that each thread uses 1MB of memory for the stack. If you are writing an application which needs a lot of threads then you should consider writing async Methods and using Task.Delay. This allows the thread to do something else while the method is waiting, which can reduce the total number of threads needed to process the same workload.
In this case however there isn't much point - while C# 7.1 does support async Main, its just syntactic sugar and doesn't free the thread up to perform other work. In any case I wouldn't loose sleep over using Thread.Sleep in a simple console app like this, or alternatively using something like Console.ReadLine like Neos07 suggests.

Thread.Sleep does still work, but you can consider using Task.Delay as an alternative here, like
await Task.Delay(1000);

C# console program wait forever for event

I have a simple C# console application that attaches to an event. I need the program to keep running continuously so it can respond to the event. What is the right way to keep it running?
Here is my application:
using System;
using NAudio.CoreAudioApi;
namespace MaxVolume
{
class Program
{
private const float DesiredLevel = -15;
private static MMDevice _device;
static void Main(string[] args)
{
MMDeviceEnumerator mmde = new MMDeviceEnumerator();
_device = mmde.GetDefaultAudioEndpoint(DataFlow.Render, Role.Multimedia);
_device.AudioEndpointVolume.MasterVolumeLevel = DesiredLevel;
_device.AudioEndpointVolume.OnVolumeNotification += SetVolume;
}
static void SetVolume(AudioVolumeNotificationData data)
{
if (Math.Abs(data.MasterVolume - DesiredLevel) > 0.1)
{
_device.AudioEndpointVolume.MasterVolumeLevel = DesiredLevel;
}
}
}
}

You can call Console.ReadLine() (if you want to terminate on keystroke), or simply Thread.Sleep(Timeout.Infinite).

In case of async main method, one could also use await Task.Delay(-1);

You can just create a while-loop like so:
while(!eventFired) {}
and then have a field:
private bool eventFired = false;
and then finally when the event is fired make:
eventFired = true;
(if you want it to exit after the event was fired once, that is. if it should run forever see the other answers with sleep and readkey)
NOTE: this is using up a CPU and shouldn't be used in any production, but in a quick-and-dirty test setting it might be worth it.

How can i make a thread "report back" to main thread?

Im making a app that monitors stuff on my computer, and i want to make it somewhat more difficult then just implementing a while loop.
So my question is how do i do it if i would like to fetch cpu load in a seperate thread, that updates a static variable in class
namespace threads
{
class Program
{
static int cpuload = 0;
static void Main(string[] args)
{
while (true)
{
Thread th = new Thread(new ThreadStart(CheckCPULoad));
th.Start();
Thread.Sleep(1000); // sleep the main thread
th.Abort();
Console.WriteLine("load: {0}%", cpuload);
}
}
static void CheckCPULoad()
{
// things are updated every 3 secs, dummy data
Thread.Sleep(3000);
Random rnd = new Random();
cpuload++;// = rnd.Next(0, 100); // dummy data
}
}
}
As it is "load: 0%" is printed every time. what do i need to fix to make it show
load: 0%
load: 0%
load: 0%
?
thanks

In order to "report back" to the main thread, the main thread has to be "listening". Which means, still running in a while loop and checking some kind of a queue for new items that represent the reports.
What you basically need is a queue where the worker thread will put its reports, and the main thread will periodically check this queue for reports from the worker.
You have two main approaches:
A blocking queue. Means that when
there are no items the caller thread
blocks until items arrive. This is
good if the main thread has nothing
to do except wait for items in the
queue.
A non-blocking queue. Means that it
returns immediately to the caller
regardless of the items count. This
is good if you want your main thread
to be busy doing stuff and sometimes
checking the queue for reports.
If your application is a UI application you automatically get the first approach, as this is how the UI works. To add "an item" you can use Control.BeginInvoke (in winforms) or Dispatcher.BeginInvoke (in wpf).

If i get you right, this should solve your purpose. Notice the while loop inside the CheckCPULoad() method.
class Program
{
static int cpuload = 0;
static void Main(string[] args)
{
Thread th = new Thread(new ThreadStart(CheckCPULoad));
th.Start();
while (true)
{
Thread.Sleep(1000);
Console.WriteLine("load: {0}%", cpuload);
}
th.Abort(); // Don't ever reach this line with while (true)
}
static void CheckCPULoad()
{
while (true)
{
Thread.Sleep(3000);
cpuload++;
}
}
}

The code you're using there starts the CheckCPULoad thread, waits 1 second and then aborts it. However, the first thing the CheckCPULoad thread does is to sleep for 3 seconds. So you never actually reach the cpuload++ instruction. I suspect this would be closer to what you intended:
namespace threads
{
class Program
{
static int cpuload = 0;
static void Main(string[] args)
{
Thread th = new Thread(new ThreadStart(CheckCPULoad));
th.Start();
while (true)
{
Thread.Sleep(1000);
Console.WriteLine("load: {0}%", cpuload);
}
th.Abort(); // Don't ever reach this line with while (true)
}
static void CheckCPULoad()
{
while (true)
{
Thread.Sleep(3000);
cpuload++;
}
}
}
}

Use a timer and events instead. This way you avoid your sleeping/busy waiting. Also consider using Interlocked.Increment as suggested if several threads can modify the static variable at the same time.
using System;
using System.Threading;
using System.Timers;
using Timer = System.Timers.Timer;
namespace CpuLoad
{
internal class Program
{
private static int cpuload;
private static readonly AutoResetEvent autoEvent = new AutoResetEvent(false);
private static void Main(string[] args)
{
var timer = new Timer(3000);
timer.Elapsed += CheckCPULoad;
timer.Start();
while (true)
{
autoEvent.WaitOne();
autoEvent.Reset();
Console.WriteLine(cpuload);
}
}
private static void CheckCPULoad(object sender, ElapsedEventArgs e)
{
cpuload++;
autoEvent.Set();
}
}
}

Instead of cpuload++ try using
Interlocked.Increment(ref cpuload);
Check - http://msdn.microsoft.com/en-us/library/dd78zt0c.aspx

The thread sleeps for 3 secs. You abort it after 1. Go figure :)

With callback you can do that
here is an exsample:
public class Example2
{
// Declaration - Take 1 parameter, return nothing
public delegate void LogHandler(string message);
// Instantiation - Create a function which takes delegate as one parameter
// Verify if it is null before you use it
public void Process(LogHandler logHandler)
{
if (logHandler != null)
{
logHandler("Process() begin");
}
if (logHandler != null)
{
logHandler("Process() end");
}
}
}
public class Example2DelegateConsumer
{
// Create a method with the same signature as the delegate
static void Logger(string s)
{
Console.WriteLine(s);
}
public static void Main(string[] args)
{
Example2 ex2 = new Example2();
// Invocation in the client
Example2.LogHandler myLogger = new Example2.LogHandler(Logger);
ex2.Process(myLogger);
}
}

In addition to my original (plagiarised) answer below, this sort of situation where you're observing a set of values over time is a great fit for Reactive Extensions for .NET (http://blogs.msdn.com/b/rxteam/). You can get the desired effect with Rx thus:
static void Main()
{
var cpuLoadSequence = Observable.GenerateWithTime(
0, // initial value
i => true, // continue forever
i => i + 1, // increment value
i => i, // result = value
i => TimeSpan.FromSeconds(3)); // delay 3 seconds
using (cpuLoadSequence.Subscribe(x => Console.WriteLine("load: {0}%", x)))
{
Console.WriteLine("Press ENTER to stop.");
Console.ReadLine();
}
}

C# killing a thread

In my app, I have a thread that runs continuously. By using Thread.Sleep(), the function executes every 10 minutes.
I need to be able to kill this thread when a user clicks a button. I know Thread.Abort() is not reliable. I can use a variable to stop the thread, but since it is sleeping it could be another 10 minutes before the thread kills itself.
Any ideas?

Why don't you use a timer to schedule the task every ten minutes instead. That will run your code on a thread pool thread and thus you will not have to manage this yourself.
For more details see the System.Threading.Timer class.

Instead of Thread.Sleep use a System.Threading.ManualResetEvent. The WaitOne method has a timeout just like Thread.Sleep, your thread will sleep for that interval unless the event is triggered first, and the return value tells you whether the interval elapsed or the event was set.

So here's a sample that users timers to do the work as suggested by Brian. Use start/stop as needed. To cleanup the (Program) object once you are done with it make sure you call Dispose.
Just note that when you call Stop it will prevent the timer from firing again, however you still may have a worker thread in the middle of executing the timer_Elapsed handler, i.e. stopping the timer doesn't stop any currently executing worker thread.
using System;
using System.Timers;
namespace TimerApp
{
class Program : IDisposable
{
private Timer timer;
public Program()
{
this.timer = new Timer();
this.timer.Elapsed += new ElapsedEventHandler(timer_Elapsed);
this.timer.AutoReset = true;
this.timer.Interval = TimeSpan.FromMinutes(10).TotalMilliseconds;
}
void timer_Elapsed(object sender, ElapsedEventArgs e)
{
// TODO...your periodic processing, executed in a worker thread.
}
static void Main(string[] args)
{
// TODO...your app logic.
}
public void Start()
{
this.timer.Start();
}
public void Stop()
{
this.timer.Stop();
}
public void Dispose()
{
this.timer.Dispose();
}
}
}

Building on Ben's answer, here's the pattern to help you out...
using System.Threading;
public class MyWorker {
private ManualResetEvent mResetEvent = new ManualResetEvent(false);
private volatile bool mIsAlive;
private const int mTimeout = 6000000;
public void Start()
{
if (mIsAlive == false)
{
mIsAlive = true;
Thread thread = new Thread(new ThreadStart(RunThread));
thread.Start();
}
}
public void Stop()
{
mIsAlive = false;
mResetEvent.Set();
}
public void RunThread()
{
while(mIsAlive)
{
//Reset the event -we may be restarting the thread.
mResetEvent.Reset();
DoWork();
//The thread will block on this until either the timeout
//expires or the reset event is signaled.
if (mResetEvent.WaitOne(mTimeout))
{
mIsAlive = false; // Exit the loop.
}
}
}
public void DoWork()
{
//...
} }

One possibility is to not have it sleep for ten minutes. Have it sleep for 10 seconds then only do its work on every sixtieth wakeup. Then you only have a latency of ten seconds before it stops.
Aside: This is not necessarily the best solution but it's probably the quickest to implement. As with all possibilities you should do a cost/benefit analysis when selecting which solution is right for you.
If ten seconds is still too much, you can drop it further although keep in mind that dropping it too far will result in a possible performance impact.
You're right that you shouldn't kill threads from outside, it's usually a recipe for disaster if you happen to do it while they have a lock on some resource that's not freed on kill. Threads should always be responsible for their own resources, including their lifetimes.

Synchronizing a timer to prevent overlap

I'm writing a Windows service that runs a variable length activity at intervals (a database scan and update). I need this task to run frequently, but the code to handle isn't safe to run multiple times concurrently.
How can I most simply set up a timer to run the task every 30 seconds while never overlapping executions? (I'm assuming System.Threading.Timer is the correct timer for this job, but could be mistaken).

You could do it with a Timer, but you would need to have some form of locking on your database scan and update. A simple lock to synchronize may be enough to prevent multiple runs from occurring.
That being said, it might be better to start a timer AFTER your operation is complete, and just use it one time, then stop it. Restart it after your next operation. This would give you 30 seconds (or N seconds) between events, with no chance of overlaps, and no locking.
Example :
System.Threading.Timer timer = null;
timer = new System.Threading.Timer((g) =>
{
Console.WriteLine(1); //do whatever
timer.Change(5000, Timeout.Infinite);
}, null, 0, Timeout.Infinite);
Work immediately .....Finish...wait 5 sec....Work immediately .....Finish...wait 5 sec....

I'd use Monitor.TryEnter in your elapsed code:
if (Monitor.TryEnter(lockobj))
{
try
{
// we got the lock, do your work
}
finally
{
Monitor.Exit(lockobj);
}
}
else
{
// another elapsed has the lock
}

I prefer System.Threading.Timer for things like this, because I don't have to go through the event handling mechanism:
Timer UpdateTimer = new Timer(UpdateCallback, null, 30000, 30000);
object updateLock = new object();
void UpdateCallback(object state)
{
if (Monitor.TryEnter(updateLock))
{
try
{
// do stuff here
}
finally
{
Monitor.Exit(updateLock);
}
}
else
{
// previous timer tick took too long.
// so do nothing this time through.
}
}
You can eliminate the need for the lock by making the timer a one-shot and re-starting it after every update:
// Initialize timer as a one-shot
Timer UpdateTimer = new Timer(UpdateCallback, null, 30000, Timeout.Infinite);
void UpdateCallback(object state)
{
// do stuff here
// re-enable the timer
UpdateTimer.Change(30000, Timeout.Infinite);
}

instead of locking (which could cause all of your timed scans to wait and eventually stack up). You could start the scan/update in a thread and then just do a check to see if the thread is still alive.
Thread updateDBThread = new Thread(MyUpdateMethod);
...
private void timer_Elapsed(object sender, ElapsedEventArgs e)
{
if(!updateDBThread.IsAlive)
updateDBThread.Start();
}

Starting from .NET 6 there is a new timer available, the PeriodicTimer. This is a lightweight async-enabled timer, that becomes the perfect tool when overlapping executions should be strictly forbidden. You use this timer by writing an asynchronous method with a loop, and invoking it to start the loop:
private Task _operation;
private CancellationTokenSource _operationCancellation = new();
//...
_operation = StartTimer();
//...
private async Task StartTimer()
{
PeriodicTimer timer = new(TimeSpan.FromSeconds(30));
while (true)
{
await timer.WaitForNextTickAsync(_operationCancellation.Token);
try
{
DoSomething();
}
catch (Exception ex)
{
_logger.LogError(ex);
}
}
}
Instead of using a CancellationTokenSource, you can also stop the loop by disposing the PeriodicTimer. In this case the await timer.WaitForNextTickAsync() will return false.
It is possible that the DoSomething will be invoked subsequently with smaller interval than 30 seconds, but it's impossible that it will be invoked in overlapping fashion, unless you start accidentally two asynchronous loops.
This timer does not support disabling and reenabling it. If you need this functionality you could look at the third-party Nito.AsyncEx.PauseTokenSource component.
In case you are targeting a .NET version earlier than .NET 6, you could look at this question for an alternative: Run async method regularly with specified interval.

You could use the AutoResetEvent as follows:
// Somewhere else in the code
using System;
using System.Threading;
// In the class or whever appropriate
static AutoResetEvent autoEvent = new AutoResetEvent(false);
void MyWorkerThread()
{
while(1)
{
// Wait for work method to signal.
if(autoEvent.WaitOne(30000, false))
{
// Signalled time to quit
return;
}
else
{
// grab a lock
// do the work
// Whatever...
}
}
}
A slightly "smarter" solution is as follow in pseudo-code:
using System;
using System.Diagnostics;
using System.Threading;
// In the class or whever appropriate
static AutoResetEvent autoEvent = new AutoResetEvent(false);
void MyWorkerThread()
{
Stopwatch stopWatch = new Stopwatch();
TimeSpan Second30 = new TimeSpan(0,0,30);
TimeSpan SecondsZero = new TimeSpan(0);
TimeSpan waitTime = Second30 - SecondsZero;
TimeSpan interval;
while(1)
{
// Wait for work method to signal.
if(autoEvent.WaitOne(waitTime, false))
{
// Signalled time to quit
return;
}
else
{
stopWatch.Start();
// grab a lock
// do the work
// Whatever...
stopwatch.stop();
interval = stopwatch.Elapsed;
if (interval < Seconds30)
{
waitTime = Seconds30 - interval;
}
else
{
waitTime = SecondsZero;
}
}
}
}
Either of these has the advantage that you can shutdown the thread, just by signaling the event.
Edit
I should add, that this code makes the assumption that you only have one of these MyWorkerThreads() running, otherwise they would run concurrently.

I've used a mutex when I've wanted single execution:
private void OnMsgTimer(object sender, ElapsedEventArgs args)
{
// mutex creates a single instance in this application
bool wasMutexCreatedNew = false;
using(Mutex onlyOne = new Mutex(true, GetMutexName(), out wasMutexCreatedNew))
{
if (wasMutexCreatedNew)
{
try
{
//<your code here>
}
finally
{
onlyOne.ReleaseMutex();
}
}
}
}
Sorry I'm so late...You will need to provide the mutex name as part of the GetMutexName() method call.