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Does lock section always guarantee thread safety?

I'm trying to understand thread-safe access to fields. For this, i implemented some test sample:
class Program
{
public static void Main()
{
Foo test = new Foo();
bool temp;
new Thread(() => { test.Loop = false; }).Start();
do
{
temp = test.Loop;
}
while (temp == true);
}
}
class Foo
{
public bool Loop = true;
}
As expected, sometimes it doesn't terminate. I know that this issue can be solved either with volatile keyword or with lock. I consider that i'm not author of class Foo, so i can't make field volatile. I tried using lock:
public static void Main()
{
Foo test = new Foo();
object locker = new Object();
bool temp;
new Thread(() => { test.Loop = false; }).Start();
do
{
lock (locker)
{
temp = test.Loop;
}
}
while (temp == true);
}
this seems to solve the issue. Just to be sure i moved the cycle inside the lock block:
lock(locker)
{
do
{
temp = test.Loop;
}
while (temp == true);
}
and... the program does not terminates anymore.
It is totally confusing me. Doesn't lock provides thread-safe access? If not, how to access non-volatile fields safely? I could use VolatileRead(), but it is not suitable for any case, like not primitive type or properties. I considered that Monitor.Enter does the job, Am i right? I don't understand how could it work.

This piece of code:
do
{
lock (locker)
{
temp = test.Loop;
}
}
while (temp == true);
works because of a side-effect of lock: it causes a 'memory-fence'. The actual locking is irrelevant here. Equivalent code:
do
{
Thread.MemoryBarrier();
temp = test.Loop;
}
while (temp == true);
And the issue you're trying to solve here is not exactly thread-safety, it is about caching of the variable (stale data).

It does not terminate anymore because you are accessing the variable outside of the lock as well.
In
new Thread(() => { test.Loop = false; }).Start();
you write to the variable outside the lock. This write is not guaranteed to be visible.
Two concurrent accesses to the same location of which at least one is a write is a data race. Don't do that.

Lock provides thread safety for 2 or more code blocks on different threads, that uses the lock.
Your Loop assignment inside the new thread declaration is not enclosed in lock.
That means there is no thread safety there.

In general, no, lock is not something that will magically make all code inside it thread-safe.
The simple rule is: If you have some data that's shared by multiple threads, but you always access it only inside a lock (using the same lock object), then that access is thread-safe.
Once you leave that “simple” code and start asking questions like “How could I use volatile/VolatileRed() safely here?” or “Why does this code that doesn't use lock properly seem to work?”, things get complicated quickly. And you should probably avoid that, unless you're prepared to spend a lot of time learning about the C# memory model. And even then, bugs that manifest only once in million runs or only on certain CPUs (ARM) are very easy to make.

Locking only works when all access to the field is controlled by a lock. In your example only the reading is locked, but since the writing is not, there is no thread-safety.
However it is also crucial that the locking takes place on a shared object, otherwise there is no way for another thread to know that someone is trying to access the field. So in your case when locking on an object which is only scoped inside the Main method, any other call on another thread, would not be able to block.
If you have no way to change Foo, the only way to obtain thread-safety is to have ALL calls actually lock on the same Foo instance. This would generally not be recommended though, since all methods on the object would be locked.
The volatile keyword is not a guarantuee of thread-safety in itself. It is meant to indicate that the value of a field can be changed from different threads, and so any thread reading that field, should not cache it, since the value could change.
To achieve thread-safety, Foo should probably look something along these lines:
class Program
{
public static void Main()
{
Foo test = new Foo();
test.Run();
new Thread(() => { test.Loop = false; }).Start();
do
{
temp = test.Loop;
}
while (temp == true);
}
}
class Foo
{
private volatile bool _loop = true;
private object _syncRoot = new object();
public bool Loop
{
// All access to the Loop value, is controlled by a lock on an instance-scoped object. I.e. when one thread accesses the value, all other threads are blocked.
get { lock(_syncRoot) return _loop; }
set { lock(_syncRoot) _loop = value; }
}
public void Run()
{
Task(() =>
{
while(_loop) // _loop is volatile, so value is not cached
{
// Do something
}
});
}
}

Exceptions inside the lock block

Say, if I have the following block on C# code:
public class SynchedClass
{
public void addData(object v)
{
lock(lockObject)
{
//Shall I worry about catching an exception here?
//Do the work
//arr.Add(v);
}
}
private List<object> arr = new List<object>();
private object lockObject = new object();
}
Shall I attempt to catch exceptions inside the lock block? (My main concern is that the exception may be raised inside the lock which will prevent the lock from being "unlocked".)

Lock will be released when exception escapes from the lock block.
That is because lock(){...} is translate by compiler roughly into:
Monitor.Enter(obj);
try{
// contents of the lock block
}finally{
Monitor.Exit(obj);
}

There is more to consider than just releasing the mutex.
An exception occuring within a lock will release the lock, but what state is the program in now? A thread waiting on the lock will now wake up, and will perhaps now be dealing with invalid state. This is a difficult problem with no ideal solution.
The best thing is to try to keep your locks as small as possible and to call methods that don't throw. (That's ignoring the elephant in the room that is the evil ThreadAbortException...)
For a good discussion of these issues, see Eric Lippert's article: Locks and exceptions do not mix.

A lock statement of the form "lock (x) ..." where x is an expression of a reference-type, is precisely equivalent to (C# 4.0):
bool entered = false;
try {
System.Threading.Monitor.Enter(x, ref entered);
...
}
finally { if (entered) System.Threading.Monitor.Exit(x); }

Object synchronization method was called from an unsynchronized block of code. Exception on Mutex.Release()

I have found different articles about this exception but none of them was my case.
Here is the source code:
class Program
{
private static Mutex mutex;
private static bool mutexIsLocked = false;
static void Main(string[] args)
{
ICrmService crmService =
new ArmenianSoftware.Crm.Common.CrmServiceWrapper(GetCrmService("Armsoft", "crmserver"));
//Lock mutex for concurrent access to workflow
mutex = new Mutex(true, "ArmenianSoftware.Crm.Common.FilterCtiCallLogActivity");
mutexIsLocked = true;
//Create object for updating filtered cti call log
ArmenianSoftware.Crm.Common.FilterCtiCallLog filterCtiCallLog =
new ArmenianSoftware.Crm.Common.FilterCtiCallLog(crmService);
//Bind events
filterCtiCallLog.CtiCallsRetrieved += new EventHandler<ArmenianSoftware.Crm.Common.CtiCallsRetrievedEventArgs>(filterCtiCallLog_CtiCallsRetrieved);
//Execute filter
try
{
filterCtiCallLog.CreateFilteredCtiCallLogSync();
}
catch (Exception ex)
{
throw ex;
}
finally
{
if (mutexIsLocked)
{
mutexIsLocked = false;
mutex.ReleaseMutex();
}
}
}
static void filterCtiCallLog_CtiCallsRetrieved(object sender,
ArmenianSoftware.Crm.Common.CtiCallsRetrievedEventArgs e)
{
tryasasas
{
if (mutexIsLocked)
{
mutexIsLocked = false;
mutex.ReleaseMutex();
}
}
catch (Exception ex)
{
throw ex;
}
}
}
filterCtiCallLog.CreateFilteredCtiCallLogSync(); function executes requests to server, and raises some events, one of which is CtiCallsRetrieve event. And I need to release the mutex when this event is fired. But on calling the mutex.Release() function exception is thrown. CreateFilteredCtiCallLogSync works synchronously. What is the problem?

Keeping a bool around that indicates that the mutex is owned is a grave mistake. You are not making the bool thread-safe. You got into this pickle because you are using the wrong synchronization object. A mutex has thread-affinity, the owner of a mutex is a thread. The thread that acquired it must also be the one that calls ReleaseMutex(). Which is why your code bombs.
You in all likelihood need an event here, use AutoResetEvent. Create it in the main thread, call Set() in the worker, WaitOne() in the main thread to wait for the worker to complete its job. And dispose it afterwards. Also note that using a thread to perform a job and having your main thread wait for its completion is not productive. You might as well have the main thread do the job.
If you are actually doing this to protect access to an object that's not thread-safe (it isn't clear) then use the lock statement.

Another reason why this exception may occur:
if (Monitor.TryEnter(_lock))
{
try
{
... await MyMethodAsync(); ...
}
finally
{
Monitor.Exit(_lock);
}
}
I get this exception on Monitor.Exit when after 'await' another thread continues execution.
Edit:
Use SemaphoreSlim, because it doesn't require releasing thread to be the same.

You will also run into this exception if you do the following:
mutex.WaitOne();
… Some Work...
await someTask;
mutex.ReleaseMutex();
That's because the code after the await can be executed on a different thread from the line just before. Basically, it seems that if you asynch code now (in early 2020), Mutexes simply don't work. Use events or something.

I have found the problem. First several things about the filterCtiCallLog class. I have designed it so to work both asynchronous and synchronous. For first I have written code for asynchronous execution. I needed a way to trigger events from child worker thread to parent, to report the working state. For this I have used AsyncOperation class and it's post method. Here is the code part for triggering CtiCallsRetrieved event.
public class FilterCtiCallLog
{
private int RequestCount = 0;
private AsyncOperation createCallsAsync = null;
private SendOrPostCallback ctiCallsRetrievedPost;
public void CreateFilteredCtiCallLogSync()
{
createCallsAsync = AsyncOperationManager.CreateOperation(null);
ctiCallsRetrievedPost = new SendOrPostCallback(CtiCallsRetrievedPost);
CreateFilteredCtiCallLog();
}
private void CreateFilteredCtiCallLog()
{
int count=0;
//do the job
//............
//...........
//Raise the event
createCallsAsync.Post(CtiCallsRetrievedPost, new CtiCallsRetrievedEventArgs(count));
//...........
//...........
}
public event EventHandler<CtiCallsRetrievedEventArgs> CtiCallsRetrieved;
private void CtiCallsRetrievedPost(object state)
{
CtiCallsRetrievedEventArgs args = state as CtiCallsRetrievedEventArgs;
if (CtiCallsRetrieved != null)
CtiCallsRetrieved(this, args);
}
}
As you can see the code is executing synchronously. The problem here is in AsyncOperation.Post() method. I presumed that if it is called in the main thread it will act as simply triggering the event, not posting it to parent thread. However it wasn't the case. I don't know how it is working, but I have changed the code, to check if the CreateFilteredCtiCallLog is called sync or async. And if it is async call I used AsyncOperation.Post method, if not, I have simply triggered the EventHandler if it is not null. Here is the corrected code
public class FilterCtiCallLog
{
private int RequestCount = 0;
private AsyncOperation createCallsAsync = null;
private SendOrPostCallback ctiCallsRetrievedPost;
public void CreateFilteredCtiCallLogSync()
{
createCallsAsync = AsyncOperationManager.CreateOperation(null);
ctiCallsRetrievedPost = new SendOrPostCallback(CtiCallsRetrievedPost);
CreateFilteredCtiCallLog(false);
}
private void CreateFilteredCtiCallLog(bool isAsync)
{
int count=0;
//do the job
//............
//...........
//Raise the event
RaiseEvent(CtiCallsRetrievedPost, new CtiCallsRetrievedEventArgs(count),isAsync);
//...........
//...........
}
public event EventHandler<CtiCallsRetrievedEventArgs> CtiCallsRetrieved;
private void RaiseEvent(SendOrPostCallback callback, object state, bool isAsync)
{
if (isAsync)
createCallsAsync.Post(callback, state);
else
callback(state);
}
private void CtiCallsRetrievedPost(object state)
{
CtiCallsRetrievedEventArgs args = state as CtiCallsRetrievedEventArgs;
if (CtiCallsRetrieved != null)
CtiCallsRetrieved(this, args);
}
}
Thanks everybody for the answers!

I have seen this happen when you lock code using a Monitor, then call an async code and you get this, when using a lock(object) you get a compiler error, however between monitor.enter(object) and Monitor.Exist(object) the compiler does not complain... unfortunately.

Using a flag to attempt to monitor a kernel synchro object state will just not work - the point of using those synchro calls is that they work correctly without any explicit checking. Setting flags will just cause intermittent problems because the flag may be changed inappropriately due to interrupts between checking the flag and acting on it.
A mutex can only be released by the threat that acquired it. If you callback is called by a different thread, (one internal to CreateFilteredCtiCallLogSync() or a kernel thread pool), the release will fail.
It's not clear exactly what you are attempting to do. Presumably, you want to serialize access to CreateFilteredCtiCallLogSync() and the callback flags that the instance is available for re-use? If so, you could use a semaphore instead - init. it to one unit, wait for it at the start and release it in the callback.
Is there some issue where sometimes the callback is not called, and hence the try/finally/release? If so this way out seems a bit dodgy if the callback is asychronous and may be called by another thread after the setup thread has left the function.

I only had this one once or twice, and in every case it came about by trying to release a mutex I didn't own.
Are you sure the events are raised on the same thread the mutex was acquired on?
Although you mention that filterCtiCallLog.CreateFilteredCtiCallLogSync() is a blocking call, perhaps it spawns of worker threads that raise the event?

Maybe not the most meaningful error message, I've seen this happen in some third party code as below,
object obj = new object();
lock (obj)
{
//do something
Monitor.Exit(obj);//obj released
}//exception happens here, when trying to release obj

I have read the thread and got some ideas. But did not know what exactly need to do to solve the issue. I face the same error when uploading the image to the s3 at nopCommerce solution.And the below code is working for me.
using var mutex = new Mutex(false, thumbFileName);
mutex.WaitOne();
try
{
if (pictureBinary != null)
{
try
{
using var image = SKBitmap.Decode(pictureBinary);
var format = GetImageFormatByMimeType(picture.MimeType);
pictureBinary = ImageResize(image, format, targetSize);
}
catch
{
}
}
if (s3Enabled)
//await S3UploadImageOnThumbsAsync(thumbFileName, pictureBinary, picture.MimeType, picture, targetSize);
// The above code was causing the issue. Because it is wait for the thread.
//So I replace the code below line and the error disappear. This also kind of same implementation by nopCommerce.
//The thread need to wait.
S3UploadImageOnThumbsAsync(thumbFileName, pictureBinary, picture.MimeType, picture, targetSize).Wait();
else
File.WriteAllBytes(thumbFilePath, pictureBinary);
}
finally
{
mutex.ReleaseMutex();
}

C# Multithreading

Okay. I want to have two threads running. Current code:
public void foo()
{
lock(this)
{
while (stopThreads == false)
{
foreach (var acc in myList)
{
// process some stuff
}
}
}
}
public void bar()
{
lock(this)
{
while (stopThreads == false)
{
foreach (var acc in myList)
{
// process some stuff
}
}
}
}
Both are accessing the same List, the problem is that the first thread "foo" is not releasing the lock i guess; because "bar" only starts when "foo" is done. Thanks

Yes, that's how lock is designed to work.
The lock keyword marks a statement block as a critical section by obtaining the mutual-exclusion lock for a given object, executing a statement, and then releasing the lock.
Mutual-exclusion means that there can be at most one thread that holds the lock at any time.
Locking on this is a bad idea and is discouraged. You should create a private object and lock on that instead. To solve your problem you could lock on two different objects.
private object lockObject1 = new object();
private object lockObject2 = new object();
public void foo()
{
lock (lockObject1)
{
// ...
}
}
public void bar()
{
lock (lockObject2)
{
// ...
}
}
Alternatively you could reuse the same lock but move it inside the loop so that each loop has a chance to proceed:
while (stopThreads == false)
{
foreach (var acc in myList)
{
lock (lockObject)
{
// process some stuff
}
}
}
However I would suggest that you spend some time to understand what is going on rather than reordering the lines of code until it appears to work on your machine. Writing correct multithreaded code is difficult.
For stopping a thread I would recommend this article:
Shutting Down Worker Threads Gracefully

Since you are not really asking a question, I suggest you should read a tutorial on how threading works. A .Net specific guide can be found here. It features the topics "Getting Started", "Basic Synchronization", "Using Threads", "Advanced Threading" and "Parallel Programming".
Also, you are locking on "this". The Msdn says:
In general, avoid locking on a public
type, or instances beyond your code's
control. The common constructs lock
(this), lock (typeof (MyType)), and
lock ("myLock") violate this
guideline:
lock (this) is a problem if the
instance can be accessed publicly.
lock (typeof (MyType)) is a problem if
MyType is publicly accessible.
lock(“myLock”) is a problem because
any other code in the process using
the same string, will share the same
lock.
Best practice is to define a private
object to lock on, or a private static
object variable to protect data common
to all instances.

The problem you have is that you work with a very coarse lock. Both Foo and Bad basically do not work concurrently because whoever starts first stops the other one for the COMPLETE WORK CYCLE.
It should, though, ONLY lock WHILE IT TAKES THINGS OUT OF THE LIST. Foreach does not work here - per definition. You ahve to put up a second list and have each thread REMOVE THE TOP ITEM (while lockin), then work on it.
Basically:
Foreach does not work, as both threads will run through the compelte list
Second, locks must be granular in that they only lock while needed.
In your case, you lock in foo will only be released when foo is finished.

Multi-Threading - waiting for all threads to be signalled

I have scenarios where I need a main thread to wait until every one of a set of possible more than 64 threads have completed their work, and for that I wrote the following helper utility, (to avoid the 64 waithandle limit on WaitHandle.WaitAll())
public static void WaitAll(WaitHandle[] handles)
{
if (handles == null)
throw new ArgumentNullException("handles",
"WaitHandle[] handles was null");
foreach (WaitHandle wh in handles) wh.WaitOne();
}
With this utility method, however, each waithandle is only examined after every preceding one in the array has been signalled... so it is in effect synchronous, and will not work if the waithandles are autoResetEvent wait handles (which clear as soon as a waiting thread has been released)
To fix this issue I am considering changing this code to the following, but would like others to check and see if it looks like it will work, or if anyone sees any issues with it, or can suggest a better way ...
Thanks in advance:
public static void WaitAllParallel(WaitHandle[] handles)
{
if (handles == null)
throw new ArgumentNullException("handles",
"WaitHandle[] handles was null");
int actThreadCount = handles.Length;
object locker = new object();
foreach (WaitHandle wh in handles)
{
WaitHandle qwH = wh;
ThreadPool.QueueUserWorkItem(
delegate
{
try { qwH.WaitOne(); }
finally { lock(locker) --actThreadCount; }
});
}
while (actThreadCount > 0) Thread.Sleep(80);
}

If you know how many threads you have, you can use an interlocked decrement. This is how I usually do it:
{
eventDone = new AutoResetEvent();
totalCount = 128;
for(0...128) {ThreadPool.QueueUserWorkItem(ThreadWorker, ...);}
}
void ThreadWorker(object state)
try
{
... work and more work
}
finally
{
int runningCount = Interlocked.Decrement(ref totalCount);
if (0 == runningCount)
{
// This is the last thread, notify the waiters
eventDone.Set();
}
}
Actually, most times I don't even signal but instead invoke a callback continues the processing from where the waiter would continue. Less blocked threads, more scalability.
I know is different and may not apply to your case (eg. for sure will not work if some of thoe handles are not threads, but I/O or events), but it may worth thinking about this.

I'm not sure what exactly you're trying to do, but would a CountdownEvent (.NET 4.0) conceptually solve your problem?

I'm not a C# or .NET programmer, but you could use a semaphore that is posted when one of your worker threads exits. The monitoring thread would simply wait on the semaphore n times where n is the number of worker threads. Semaphores are traditionally used to count resources in use but they can be used to count jobs completed by waiting on the same semaphore for n times.

When working with lots of simultaneous threads, I prefer to add each thread's ManagedThreadId into a Dictionary when I start the thread, and then have each thread invoke a callback routine that removes the dying thread's id from the Dictionary. The Dictionary's Count property tells you how many threads are active. Use the value side of the key/value pair to hold info that your UI thread can use to report status. Wrap the Dictionary with a lock to keep things safe.

ThreadPool.QueueUserWorkItem(o =>
{
try
{
using (var h = (o as WaitHandle))
{
if (!h.WaitOne(100000))
{
// Alert main thread of the timeout
}
}
}
finally
{
Interlocked.Decrement(ref actThreadCount);
}
}, wh);