I've been reading up on .NET Threading and was working on some code that uses a ManualResetEvent. I have found lots of code samples on the internet. However, when reading the documentation for WaitHandle, I saw the following:
WaitHandle implements the Dispose
pattern. See Implementing Finalize and
Dispose to Clean Up Unmanaged
Resources.
None of the samples seem to call .Close() on the ManualResetEvent objects they create, even the nice Recursion and Concurrency article from the pfxteam blog (Edit - this has a using block I has missed). Is this just example oversight, or not needed? I am curious because a WaitHandle "encapsulates operating system–specific objects," so there could easily be a resource leak.
I was recently forwarded an excerpt from C# 4.0 in a Nutshell: The Definitive Reference By Joseph Albahari, Ben Albahari. On page 834, in Chapter 21: Threading there is a section talking about this.
Disposing Wait Handles
Once you’ve finished with a wait
handle, you can call its Close method
to release the operating system
resource. Alternatively, you can
simply drop all references to the wait
handle and allow the garbage collector
to do the job for you sometime later
(wait handles implement the disposal
pattern whereby the finalizer calls
Close). This is one of the few
scenarios where relying on this backup
is (arguably) acceptable, because wait
handles have a light OS burden
(asynchronous delegates rely on
exactly this mechanism to release
their IAsyncResult’s wait handle).
Wait handles are released
automatically when an application
domain unloads.
In general, if an object implements IDisposable it is doing so for a reason and you should call Dispose (or Close, as the case may be). In the example you site, the ManualResetEvent is wrapped inside a using statement, which will "automatically" handle calling Dispose. In this case, Close is synonymous with Dispose (which is true in most IDisposable implementations that provide a Close method).
The code from the example:
using (var mre = new ManualResetEvent(false))
{
...
}
expands to
var mre = new ManualResetEvent(false);
try
{
...
}
finally
{
((IDispoable)mre).Dispose();
}
The Close is handled inside ManualResetEvent's Dispose, and that's called by the 'using' statement.
http://msdn.microsoft.com/en-us/library/yh598w02%28VS.100%29.aspx
You'll notice the code
using (var mre = new ManualResetEvent(false))
{
// Process the left child asynchronously
ThreadPool.QueueUserWorkItem(delegate
{
Process(tree.Left, action);
mre.Set();
});
// Process current node and right child synchronously
action(tree.Data);
Process(tree.Right, action);
// Wait for the left child
mre.WaitOne();
}
uses the 'using' keyword. This automatically calls the dispose method when finished even if the code throws an exception.
I've used ManualResetEvent a lot and don't think I've ever used it inside a single method--it's always an instance field of a class. Therefore using() often does not apply.
If you have a class instance field that is an instance of ManualResetEvent, make your class implement IDisposable and in your Dispose() method call ManualResetEvent.Close(). Then in all usages of your class, you need to use using() or make the containing class implement IDisposable and repeat, and repeat...
If you're using a ManualResetEvent with anonymous methods then it's obviously useful. But as Sam mentioned they can often be passed around into workers, and then set and closed.
So I would say it depends on the context of how you are using it - the MSDN WaitHandle.WaitAll() code sample has a good example of what I mean.
Here's an example based on the MSDN sample of how creating the WaitHandles with a using statement would exception:
System.ObjectDisposedException
"Safe handle has been closed"
const int threads = 25;
void ManualWaitHandle()
{
ManualResetEvent[] manualEvents = new ManualResetEvent[threads];
for (int i = 0; i < threads; i++)
{
using (ManualResetEvent manualResetEvent = new ManualResetEvent(false))
{
ThreadPool.QueueUserWorkItem(new WaitCallback(ManualWaitHandleThread), new FileState("filename", manualResetEvent));
manualEvents[i] = manualResetEvent;
}
}
WaitHandle.WaitAll(manualEvents);
}
void ManualWaitHandleThread(object state)
{
FileState filestate = (FileState) state;
Thread.Sleep(100);
filestate.ManualEvent.Set();
}
class FileState
{
public string Filename { get;set; }
public ManualResetEvent ManualEvent { get; set; }
public FileState(string fileName, ManualResetEvent manualEvent)
{
Filename = fileName;
ManualEvent = manualEvent;
}
}
Related
I have a simple logger with producer consumer pattern based on BlockingCollection (code is below).
public class Logger
{
public Logger()
{
_messages = new BlockingCollection<LogMessage>(int.MaxValue);
_worker = new Thread(Work) {IsBackground = true};
_worker.Start();
}
~Logger()
{
_messages.CompleteAdding();
_worker.Join(); // Wait for the consumer's thread to finish.
//Some logic on closing log file
}
/// <summary>
/// This is message consumer thread
/// </summary>
private void Work()
{
while (!_messages.IsCompleted)
{
//Try to get data from queue
LogMessage message;
try
{
message = _messages.Take();
}
catch (ObjectDisposedException) { break; } //The BlockingCollection(Of T) has been disposed.
catch(InvalidOperationException){ continue; } //the BlockingCollection(Of T) is empty and the collection has been marked as complete for adding.
//... some simple logic to write 'message'
}
}
}
The problem is that application is not ending instantly with that. It takes 20-40 seconds to end an application and if I pause it with debugger in a middle, I see that:
1. GC.Finalize thread is set on _worker.Join();
2. _worker thread is on _messages.Take().
I would await that _messages.Take() is ended short after _messages.CompleteAdding(); But looks like it is not.
What's wrong with this finalization and how to better finalize worker thread in this situation?
P.S. I could simply drop _worker.Join() but then Work() can write something to closed file. I mean, this is concurrent non determined situation then.
Update
As a proof of concept I've renamed ~Logger() to Close() and call it at some point. It closes logger instantly. So _messages.Take() is ending right after _messages.CompleteAdding() as expected in this case.
The only explanation of the 20-40 seconds delay in ~Logger I see in high priority of the GC thread. Could there be another explanation?
In C#, Finalizers (aka destructors) are non-deterministic, which means you cannot predict when they will be called or in what order. For example in your code, it's entirely possible for the finalizer of _worker to be before after the finalizer for Logger. For this reason, you should never access managed objects (such as FileStreams etc) inside a finalizer, because the finalizers of other managed resources could have already completed, making their references invalid.
Also the finalizer will not be called until after the GC determines that a collection is necessary (due to the need for additional memory). In your case, the GC probably takes 20-40 seconds before it makes the required collection(s).
What you want to do is get rid of the finalizer and use the IDisposable interface instead (optionally with a Close() method that might provide better readability).
Then you would just call logger.Close() when it is no longer required.
void IDisposable.Dispose()
{
Close();
}
void Close()
{
_messages.CompleteAdding();
_worker.Join(); // Wait for the consumer's thread to finish.
//Some logic on closing log file
}
In general, only use a finalizer when you have unmanaged resources to clean up (for example, if you are using P/Invoke WinAPI function calls etc). If you are using only .Net classes, etc. you probably do not have any reason to use one. IDisposable is almost always the better choice, because it provides deterministic cleanup.
For more information on finalizers vs destructors, take a look here:
What is the difference between using IDisposable vs a destructor in C#?
Another change I would make in your code is using TryTake instead of Take. This gets rid of the need for the try/catch because it will not throw an exception when the collection is empty and CompleteAdding is called. It will simply return false.
private void Work()
{
//Try to get data from queue
LogMessage message;
while (_messages.TryTake(out message, Timeout.Infinite))
//... some simple logic to write 'message'
}
The two exceptions you catch in your code can still occur for other reasons such as accessing it after it is disposed or modifying the BlockingCollection's underlying collection (see MSDN for more info). But neither of those should occur in your code, because you don't hold a reference to the underlying collection, and you don't dispose of the BlockingCollection before the Work function is complete.
If you still wanted to catch those exceptions, just in case, you can place a try/catch block outside of the while loop (because you would NOT want to continue the loop after either exception occurs).
Finally, why do you specify int.MaxValue as the collection's capacity? You shouldn't do this unless you expect to routinely add close to that many messages to the collection.
So altogether, I would re-write your code as follows:
public class Logger : IDisposable
{
private BlockingCollection<LogMessage> _messages = null;
private Thread _worker = null;
private bool _started = false;
public void Start()
{
if (_started) return;
//Some logic to open log file
OpenLogFile();
_messages = new BlockingCollection<LogMessage>(); //int.MaxValue is the default upper-bound
_worker = new Thread(Work) { IsBackground = true };
_worker.Start();
_started = true;
}
public void Stop()
{
if (!_started) return;
// prohibit adding new messages to the queue,
// and cause TryTake to return false when the queue becomes empty.
_messages.CompleteAdding();
// Wait for the consumer's thread to finish.
_worker.Join();
//Dispose managed resources
_worker.Dispose();
_messages.Dispose();
//Some logic to close log file
CloseLogFile();
_started = false;
}
/// <summary>
/// Implements IDiposable
/// In this case, it is simply an alias for Stop()
/// </summary>
void IDisposable.Dispose()
{
Stop();
}
/// <summary>
/// This is message consumer thread
/// </summary>
private void Work()
{
LogMessage message;
//Try to get data from queue
while(_messages.TryTake(out message, Timeout.Infinite))
WriteLogMessage(message); //... some simple logic to write 'message'
}
}
As you can see, I added Start() and Stop() methods to enable/disable queue processing. If you want, you can call Start() from your constructor, but in general, you probably don't want expensive operations (such as thread creation) in a constructor. I used Start/Stop instead of Open/Close, because it seemed to make more sense for a logger, but that's just a personal preference, and either pair would work fine. As I mentioned before, you don't even have to use a Stop or Close method. Simply adding Dispose() is enough, but some classes (like Streams etc) use Close or Stop as an alias for Dispose just to make the code more readable.
I have a class that uses the Thread class:
class A
{
public Thread thread
{ get; set; }
}
Should I implement IDisposable and set Thread property to null?
class A : IDisposable
{
public Thread Thread
{ get; set; }
protected bool Disposed
{ get; set; }
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!this.Disposed)
{
if (disposing)
{
if (Thread != null)
Thread = null;
}
Disposed = true;
}
}
}
Or not?
Why?
You implement IDisposable only when your class is handling an unmanaged object, resources or other IDisposable objects. A Thread is not an unmanaged object and will get garbage collected when nothing is referencing it or when the process handling it is terminated. Since Thread is not implementing IDisposable, your class referencing it does not need to implement it either.
Optionally, for IDisposable within the scope of a method, they can be wrapped in a using statement and the Dispose() method is automatically called when the scope is exited.
It depends what your thread is doing. If your thread is performing a long running task that may run indefinitely, then I would consider that thread as a resource (which will not be garbage collected). For example consider if the thread is designed to poll some state indefinitely, or consume items from a queue (like a thread-pool thread consumes tasks or a TCP server consumes new connections) etc. In this case, I would say the natural effect of disposing your class would be to free up this thread resource. Setting it to null is not really useful in this case. Rather Dispose should probably involve flagging a synchronization event (or maybe a CancellationToken) to notify the thread that it should finish up its infinite task, and then the disposing thread should wait some time for the thread to finish (join). As always with joins, be careful of a deadlock scenario and consider some alternative action if the thread refuses to terminate. For obvious reasons I would not do this join in the finalizer.
As an example of what I'm meaning, consider the scenario where your class A is actually class MyTcpListener, designed to listen and wait for new TCP connections on a given port indefinitely. Then consider what you expect following (somewhat unlikely) code to do:
using (MyTcpListener listener = new MyTcpListener(port:1234))
{
// Do something here
}
// Create another one. This would fail if the previous Dispose
// did not unbind from the port.
using (MyTcpListener listener = new MyTcpListener(port:1234))
{
// Do something else here
}
Assuming I know the constructor of MyTcpListener creates a listener thread, I would expect that after the Dispose call has returned that the MyTcpListener would no longer be bound to the TCP port - i.e. that the TCP listener thread would have fully terminated. It goes without saying that if you didn't provide some mechanism to stop the listener that there would be a resource leak. The stopping mechanism could be a call to some method "Stop", but I personally think the "Dispose" pattern fits this scenario more cleanly since forgetting to stop something does not generally imply a resource leak.
Your code may call for different assumptions, so I would suggest judging it on the scenario. If your thread is short-running, e.g. it has some known finite task to complete and then it will terminate on its own, then I would say that disposing is less critical or perhaps useless.
I create normal threads in asp.net application. After the thread is done what should I do ? leave it (it will get back to thread pool) or abort it.
Thread thread = new Thread(new ThreadStart(work));
Leave it. There is no sense in creating a pointless exception.
Recall that the IDisposable interface exists specifically for the scenario where some shared resource needs to be released. (It has been applied in other contexts as well, of course; but that is the situation it was originally meant for.)
Now consider that the managed Thread class does not implement IDisposable and you might guess (correctly) that it does not require any specific cleanup beyond normal handling by the GC.
using threadpools in C#
[STAThread]
public static void Main(string[] args)
{
foreach(var fileNamePath in DirectoryFiles)
{
ThreadPool.QueueUserWorkItem(ThreadPoolCallback, fileNamePath);
}
}
public void ThreadPoolCallback(object threadContext)
{
//do something
}
The threadPool in .NET handles everything else.
Let's say I have this class Logger that is logging strings in a low-priority worker thread, which isn't a background thread. Strings are queued in Logger.WriteLine and munched in Logger.Worker. No queued strings are allowed to be lost. Roughly like this (implementation, locking, synchronizing, etc. omitted for clarity):
public class Logger
{
private Thread workerThread;
private Queue<String> logTexts;
private AutoResetEvent logEvent;
private AutoResetEvent stopEvent;
// Locks the queue, adds the text to it and sets the log event.
public void WriteLine(String text);
// Sets the stop event without waiting for the thread to stop.
public void AsyncStop();
// Waits for any of the log event or stop event to be signalled.
// If log event is set, it locks the queue, grabs the texts and logs them.
// If stop event is set, it exits the function and the thread.
private void Worker();
}
Since the worker thread is a foreground thread, I have to be able to deterministically stop it if the process should be able to finish.
Question: Is the general recommendation in this scenario to let Logger implement IDisposable and stop the worker thread in Dispose()? Something like this:
public class Logger : IDisposable
{
...
public void Dispose()
{
AsyncStop();
this.workerThread.Join();
}
}
Or are there better ways of handling it?
That would certainly work - a Thread qualifies as a resource, etc. The main benefit of IDisposable comes from the using statement, so it really depends on whether the typical use for the owner of the object is to use the object for a duration of time in a single method - i.e.
void Foo() {
...
using(var obj = YourObject()) {
... some loop?
}
...
}
If that makes sense (perhaps a work pump), then fine; IDisposable would be helpful for the case when an exception is thrown. If that isn't the typical use then other than highlighting that it needs some kind of cleanup, it isn't quite so helpful.
That's usually the best, as long as you have a deterministic way to dispose the logger (using block on the main part of the app, try/finally, shutdown handler, etc).
It may be a good idea to have the thread hold a WeakReference to the managing object, and periodically check to ensure that it still exists. In theory, you could use a finalizer to nudge your thread (note that the finalizer, unlike the Dispose, should not do a Thread.Join), but it may be a good idea to allow for the possibility of the finalizer failing.
You should be aware that if user doesn't call Dispose manually (via using or otherwise) application will never exit, as Thread object will hold strong reference to your Logger. Answer provided by supercat is much better general solution to this problem.
I've noticed that in some case, Visual Studio recommends to do this
await using var disposable = new Disposable();
// Do something
Instead of this
using var disposable = new Disposable();
// Do something
What is the difference between using and await using?
How should I decide which one to use?
Classic sync using
Classic using calls the Dispose() method of an object implementing the IDisposable interface.
using var disposable = new Disposable();
// Do Something...
Is equivalent to
IDisposable disposable = new Disposable();
try
{
// Do Something...
}
finally
{
disposable.Dispose();
}
New async await using
The new await using calls and await the DisposeAsync() method of an object implementing the IAsyncDisposable interface.
await using var disposable = new AsyncDisposable();
// Do Something...
Is equivalent to
IAsyncDisposable disposable = new AsyncDisposable();
try
{
// Do Something...
}
finally
{
await disposable.DisposeAsync();
}
The IAsyncDisposable Interface was added in .NET Core 3.0 and .NET Standard 2.1.
In .NET, classes that own unmanaged resources usually implement the IDisposable interface to provide a mechanism for releasing unmanaged resources synchronously. However, in some cases they need to provide an asynchronous mechanism for releasing unmanaged resources in addition to (or instead of) the synchronous one. Providing such a mechanism enables the consumer to perform resource-intensive dispose operations without blocking the main thread of a GUI application for a long time.
The IAsyncDisposable.DisposeAsync method of this interface returns a ValueTask that represents the asynchronous dispose operation. Classes that own unmanaged resources implement this method, and the consumer of these classes calls this method on an object when it is no longer needed.
Justin Lessard's answer explains the difference between using and await using, so I'll focus on which one to use. There are two cases: either the two methods Dispose/DisposeAsync are complementary, or they are doing something different.
If the methods are complementary, which is the common case, you can call either one of them and the result will be the same: the unmanaged resources will be released. There is no reason to call both of them, sequentially. If you do, the second call will be a no-op: the resources are already released, so there will be nothing more to do. Choosing which one to call is easy: if you are in a synchronous context, call the Dispose() (use the using). If you are in an asynchronous context, call the await DisposeAsync() (use the await using)¹.
If the methods are doing something different, you should read the documentation and decide which behavior is more suitable for the scenario at hand. Let's talk for example for the System.Threading.Timer class, that implements both interfaces (IDisposable and IAsyncDisposable). The Dispose method releases the unmanaged resources as expected, but the DisposeAsync is doing something more than that: it also awaits the completion of any callbacks that are currently running. Let's make an experiment to demonstrate this difference:
var stopwatch = Stopwatch.StartNew();
using (new Timer(_ => Thread.Sleep(1000), null, 0, Timeout.Infinite))
{
Thread.Sleep(100);
}
Console.WriteLine($"Duration: {stopwatch.ElapsedMilliseconds:#,0} msec");
We create a timer that fires after 0 msec, so practically immediately, then we wait for 100 msec to be sure that the callback has been invoked (it is invoked on a ThreadPool thread), and then we dispose the timer synchronously. Here is the output of this experiment:
Duration: 102 msec
Now let's switch from using to await using. Here is the output of the second experiment:
Duration: 1,005 msec
The implicit call to DisposeAsync returned a ValueTask that completed only after the completion of the timer's callback.
So in the case of a Timer, choosing between using and await using is not just a choice that depends on the context. You might prefer to use the synchronous using in an async context, because you don't care about the callback (you know that it's not harmful to let it become fire-and-forget). Or you might be in a synchronous context but you might prefer the behavior of await using, because fire-and-forget is unacceptable. In that case you'll have to abandon the convenience of using, and invoke instead the DisposeAsync explicitly in a finally block:
var timer = new Timer(_ => Thread.Sleep(1000), null, 0, Timeout.Infinite);
try { Thread.Sleep(100); }
finally { timer.DisposeAsync().AsTask().Wait(); }
¹ Be aware, especially if you are writing a library, that the await using captures the synchronization context by default. In case this is undesirable, which usually is for library code, you'll have to configure it with ConfigureAwait(false). This has some implications that are discussed here: How do I get the "await using" syntax correct?