Develop Reference

stopped in the debugger with a C# thread hung on a lock statement, how do I figure out which thread currently owns the lock? [duplicate]

one of the threads in my application blocked at the following lock statement and resulted in a deadlock
void ExecuteCommand()
{
lock(this._lockinstance)
{
// do some operation
}
}
Is it possible to easily identify which thread is currently holding the lock?.. My application has more than 50 threads, which makes it difficult to go through each callstack using visual studio to locate the thread that holds the lock

Some sample code to try out:
class Test {
private object locker = new object();
public void Run() {
lock (locker) { // <== breakpoint here
Console.WriteLine(System.Threading.Thread.CurrentThread.ManagedThreadId);
}
}
}
Set a breakpoint on the indicated line. When it breaks, use Debug + Windows + Memory + Memory 1. Right click the window and choose "4-byte Integer". In the Address box, type &locker. The 2nd word is the thread ID of the thread that owns the lock. Step past the lock statement to see it change.
Beware that the number is the managed thread ID, not the operating system thread ID that you see in the Debug + Windows + Threads window. That kinda sucks, you probably should add some logging to your program that dumps the value of ManagedThreadId so you have a way to match the value to a thread. Update: fixed in later VS versions, the Debug > Windows > Threads debugger window now shows the ManagedThreadId.

Recently I was trying to determine what function was holding a lock and found the following very useful and had not seen in demonstrated anywhere before. I've placed it as an answer here in case others find it useful too.
Many of the other solutions posted earlier require writing a new class and then converting of all lock(blah) to BetterLock(blah) which is a lot of work for debugging and which you may not want in the production/shipped version of your code. Others required having the debugger attached which changes the code's timing and could obscure the issue.
Instead, try the following...
Original code:
object obj = new object();
lock(obj)
{
// Do stuff
}
Modified code for debugging:
object _obj = new object();
object obj
{
get
{
System.Diagnostics.StackFrame frame = new System.Diagnostics.StackFrame(1);
System.Diagnostics.Trace.WriteLine(String.Format("Lock acquired by: {0} on thread {1}", frame.GetMethod().Name, System.Threading.Thread.CurrentThread.ManagedThreadId));
return _obj;
}
}
// Note that the code within lock(obj) and the lock itself remain unchanged.
lock(obj)
{
// Do stuff
}
By exposing obj as a property, at least temporarily, with very minimal code changes you can determine what function acquired the lock last and on what thread - just look at the Trace output for the last entry. Of course you can output any other information you might find useful in the getter as well.
No, this will not let you determine when a lock was released, but if it was getting released in a timely fashion, then you didn't actually have a lock contention issue in the first place.

You can implement a Monitor wrapper that saves stack traces & thread names on enter.
Old way:
private object myLock = new object();
...
lock(myLock)
{
DoSomething();
}
...
With code below:
private SmartLock myLock = new SmartLock();
...
myLock.Lock( () =>
{
DoSomething();
}
);
...
Source:
public class SmartLock
{
private object LockObject = new object();
private string HoldingTrace = "";
private static int WARN_TIMEOUT_MS = 5000; //5 secs
public void Lock(Action action)
{
try
{
Enter();
action.Invoke();
}
catch (Exception ex)
{
Globals.Error("SmartLock Lock action", ex);
}
finally
{
Exit();
}
}
private void Enter()
{
try
{
bool locked = false;
int timeoutMS = 0;
while (!locked)
{
//keep trying to get the lock, and warn if not accessible after timeout
locked = Monitor.TryEnter(LockObject, WARN_TIMEOUT_MS);
if (!locked)
{
timeoutMS += WARN_TIMEOUT_MS;
Globals.Warn("Lock held: " + (timeoutMS / 1000) + " secs by " + HoldingTrace + " requested by " + GetStackTrace());
}
}
//save a stack trace for the code that is holding the lock
HoldingTrace = GetStackTrace();
}
catch (Exception ex)
{
Globals.Error("SmartLock Enter", ex);
}
}
private string GetStackTrace()
{
StackTrace trace = new StackTrace();
string threadID = Thread.CurrentThread.Name ?? "";
return "[" + threadID + "]" + trace.ToString().Replace('\n', '|').Replace("\r", "");
}
private void Exit()
{
try
{
Monitor.Exit(LockObject);
HoldingTrace = "";
}
catch (Exception ex)
{
Globals.Error("SmartLock Exit", ex);
}
}
}

Yes, there is a 'Threads' view that you can use in VS. Break anywhere in your application (or click the 'Break All' button) then you can select each thread and view who has the lock (if anyone).
To add it, go to Debug > Windows > Threads (Ctrl+D,T)

Old posts are old.
But i thought i might give a solution i find to be fairly useful for trying to track down dead locks and other locking problems.
I use a disposable class for my lock - I like Monitor but any locking mechanism could be used.
public class MonitorLock : IDisposable
{
public static MonitorLock CreateLock(object value)
{
return new MonitorLock(value);
}
private readonly object _l;
protected MonitorLock(object l)
{
_l = l;
Console.WriteLine("Lock {0} attempt by {1}", _l, Thread.CurrentThread.ManagedThreadId);
Monitor.Enter(_l);
Console.WriteLine("Lock {0} held by {1}" , _l, Thread.CurrentThread.ManagedThreadId);
}
public void Dispose()
{
Monitor.Exit(_l);
Console.WriteLine("Lock {0} released by {1}", _l, Thread.CurrentThread.ManagedThreadId);
}
}
I use a lock object with a name so I can be clear as to which lock I'm trying to aquire.
public class LockObject
{
public string Name { get; set; }
public LockObject(string name)
{
Name = name;
}
public override string ToString()
{
return Name;
}
}
Finally create a lock object, and then in a using block hold the object.
//create an object to lock on
private readonly object _requestLock = new LockObject("_requestLock");
using (MonitorLock.CreateLock(_requestLock))
{
//do some work
}
Output should be something along the lines of
Lock _requestLock attempt by 92
Lock _requestLock held by 92
Lock _requestLock attempt by 19
Lock _requestLock released by 92
Lock _requestLock held by 19
Lock _requestLock released by 19
Hope that someone finds this useful :)

The Managed Stack Explorer from http://mse.codeplex.com/ or http://www.microsoft.com/downloadS/details.aspx?FamilyID=80cf81f7-d710-47e3-8b95-5a6555a230c2&displaylang=en is excellent in such cases.
It hooks into running managed code (appropriate permissions needed) including live code, and grabs a list of running threads. You can double-click on any of them or (more useful in cases like this) select the lot and hit enter for a quick relatively non-invasive (obviously it's going to consume resources, but it goes in and out as quickly as it can) dump of the current stacks of different threads. Great for finding a deadlock, infinite loop, near-infinite loop (for those times when your application accidentally depends upon astronomers being pessimistic about how long the earth will last to have a hope of completing) and other such cases.

I'm not sure in which version this feature was added, but the Visual Studio 2022 debugger now shows in its Call Stack window the ID of the thread that owns the lock on which another thread is waiting to acquire, e.g.,
I found this over here.

Multithreading design-pattern

While keeping in mind that:
I am using a blocking queue that waits for ever until something is added to it
I might get a FileSystemWatcher event twice
The updated code:
{
FileProcessingManager processingManager = new FileProcessingManager();
processingManager.RegisterProcessor(new ExcelFileProcessor());
processingManager.RegisterProcessor(new PdfFileProcessor());
processingManager.Completed += new ProcessingCompletedHandler(ProcessingCompletedHandler);
processingManager.Completed += new ProcessingCompletedHandler(LogFileStatus);
while (true)
{
try
{
var jobData = (JobData)fileMonitor.FileQueue.Dequeue();
if (jobData == null)
break;
_pool.WaitOne();
Application.Log(String.Format("{0}:{1}", DateTime.Now.ToString(CultureInfo.InvariantCulture), "Thread launched"));
Task.Factory.StartNew(() => processingManager.Process(jobData));
}
catch (Exception e)
{
Application.Log(String.Format("{0}:{1}", DateTime.Now.ToString(CultureInfo.InvariantCulture), e.Message));
}
}
}
What are are you suggestions on making the code multi-threaded while taking into consideration the possibility that two identical string paths may be added into the blocking queue? I have left the possibility that this might happen and in this case.. the file would be processed twice, the thing is that sometimes I get it twice, sometimes not, it is really awkward, if you have suggestions on this, please tell.
The null checking is for exiting the loop, I intentionally add a null from outside the threaded loop to determine it to stop.

For multi-threading this... I would probably add a "Completed" event to your FileProcessingManager and register for it. One argument of that event will be the "bool" return value you currently have. Then in that event handler, I would do the checking of the bool and re-queueing of the file. Note that you will have to keep a reference to the FileMonitorManager. So, I would have this ThreadProc method be in a class where you keep the FileMonitorManager and FileProcessingManager instances in a property.
To deduplicate, in ThreadProc, I would create a List outside of the while loop. Then inside the while loop, before you process a file, lock that list, check to see if the string is already in there, if not, add the string to the list and process the file, if it is, then skip processing.
Obviously, this is based on little information surrounding your method but my 2 cents anyway.
Rough code, from Notepad:
private static FileMonitorManager fileMon = null;
private static FileProcessingManager processingManager = new FileProcessingManager();
private static void ThreadProc(object param)
{
processingManager.RegisterProcessor(new ExcelFileProcessor());
processingManager.RegisterProcessor(new PdfFileProcessor());
processingManager.Completed += ProcessingCompletedHandler;
var procList = new List<string>();
while (true)
{
try
{
var path = (string)fileMon.FileQueue.Dequeue();
if (path == null)
break;
bool processThis = false;
lock(procList)
{
if(!procList.Contains(path))
{
processThis = true;
procList.Add(path);
}
}
if(processThis)
{
Thread t = new Thread (new ParameterizedThreadStart(processingManager.Process));
t.Start (path);
}
}
catch (System.Exception e)
{
Console.WriteLine(e.Message);
}
}
}
private static void ProcessingCompletedHandler(bool status, string path)
{
if (!status)
{
fileMon.FileQueue.Enqueue(path);
Console.WriteLine("\n\nError on file: " + path);
}
else
Console.WriteLine("\n\nSucces on file: " + path);
}

Serially process ConcurrentQueue and limit to one message processor. Correct pattern?

I'm building a multithreaded app in .net.
I have a thread that listens to a connection (abstract, serial, tcp...).
When it receives a new message, it adds it to via AddMessage. Which then call startSpool. startSpool checks to see if the spool is already running and if it is, returns, otherwise, starts it in a new thread. The reason for this is, the messages HAVE to be processed serially, FIFO.
So, my questions are...
Am I going about this the right way?
Are there better, faster, cheaper patterns out there?
My apologies if there is a typo in my code, I was having problems copying and pasting.
ConcurrentQueue<IMyMessage > messages = new ConcurrentQueue<IMyMessage>();
const int maxSpoolInstances = 1;
object lcurrentSpoolInstances;
int currentSpoolInstances = 0;
Thread spoolThread;
public void AddMessage(IMyMessage message)
{
this.messages.Add(message);
this.startSpool();
}
private void startSpool()
{
bool run = false;
lock (lcurrentSpoolInstances)
{
if (currentSpoolInstances <= maxSpoolInstances)
{
this.currentSpoolInstances++;
run = true;
}
else
{
return;
}
}
if (run)
{
this.spoolThread = new Thread(new ThreadStart(spool));
this.spoolThread.Start();
}
}
private void spool()
{
Message.ITimingMessage message;
while (this.messages.Count > 0)
{
// TODO: Is this below line necessary or does the TryDequeue cover this?
message = null;
this.messages.TryDequeue(out message);
if (message != null)
{
// My long running thing that does something with this message.
}
}
lock (lcurrentSpoolInstances)
{
this.currentSpoolInstances--;
}
}

This would be easier using BlockingCollection<T> instead of ConcurrentQueue<T>.
Something like this should work:
class MessageProcessor : IDisposable
{
BlockingCollection<IMyMessage> messages = new BlockingCollection<IMyMessage>();
public MessageProcessor()
{
// Move this to constructor to prevent race condition in existing code (you could start multiple threads...
Task.Factory.StartNew(this.spool, TaskCreationOptions.LongRunning);
}
public void AddMessage(IMyMessage message)
{
this.messages.Add(message);
}
private void Spool()
{
foreach(IMyMessage message in this.messages.GetConsumingEnumerable())
{
// long running thing that does something with this message.
}
}
public void FinishProcessing()
{
// This will tell the spooling you're done adding, so it shuts down
this.messages.CompleteAdding();
}
void IDisposable.Dispose()
{
this.FinishProcessing();
}
}
Edit: If you wanted to support multiple consumers, you could handle that via a separate constructor. I'd refactor this to:
public MessageProcessor(int numberOfConsumers = 1)
{
for (int i=0;i<numberOfConsumers;++i)
StartConsumer();
}
private void StartConsumer()
{
// Move this to constructor to prevent race condition in existing code (you could start multiple threads...
Task.Factory.StartNew(this.spool, TaskCreationOptions.LongRunning);
}
This would allow you to start any number of consumers. Note that this breaks the rule of having it be strictly FIFO - the processing will potentially process "numberOfConsumer" elements in blocks with this change.
Multiple producers are already supported. The above is thread safe, so any number of threads can call Add(message) in parallel, with no changes.

I think that Reed's answer is the best way to go, but for the sake of academics, here is an example using the concurrent queue -- you had some races in the code that you posted (depending upon how you handle incrementing currnetSpoolInstances)
The changes I made (below) were:
Switched to a Task instead of a Thread (uses thread pool instead of incurring the cost of creating a new thread)
added the code to increment/decrement your spool instance count
changed the "if currentSpoolInstances <= max ... to just < to avoid having one too many workers (probably just a typo)
changed the way that empty queues were handled to avoid a race: I think you had a race, where your while loop could have tested false, (you thread begins to exit), but at that moment, a new item is added (so your spool thread is exiting, but your spool count > 0, so your queue stalls).
private ConcurrentQueue<IMyMessage> messages = new ConcurrentQueue<IMyMessage>();
const int maxSpoolInstances = 1;
object lcurrentSpoolInstances = new object();
int currentSpoolInstances = 0;
public void AddMessage(IMyMessage message)
{
this.messages.Enqueue(message);
this.startSpool();
}
private void startSpool()
{
lock (lcurrentSpoolInstances)
{
if (currentSpoolInstances < maxSpoolInstances)
{
this.currentSpoolInstances++;
Task.Factory.StartNew(spool, TaskCreationOptions.LongRunning);
}
}
}
private void spool()
{
IMyMessage message;
while (true)
{
// you do not need to null message because it is an "out" parameter, had it been a "ref" parameter, you would want to null it.
if(this.messages.TryDequeue(out message))
{
// My long running thing that does something with this message.
}
else
{
lock (lcurrentSpoolInstances)
{
if (this.messages.IsEmpty)
{
this.currentSpoolInstances--;
return;
}
}
}
}
}

Check 'Pipelines pattern': http://msdn.microsoft.com/en-us/library/ff963548.aspx
Use BlockingCollection for the 'buffers'.
Each Processor (e.g. ReadStrings, CorrectCase, ..), should run in a Task.
HTH..

Infrequent hangs in a multi-threaded C# console application when using Console.Writeline() or Console.Write()

I have written a console application that makes use of console.write and console.writeline to provide some logging. The application is a server application that uses asynchronous beginacceptconnection() and beginread() ( Sockets ) for communication. Occasionally i get reports of it hanging and from the limited debug i can do i am able to see the problem being Console.Writeline() or Console.write().
Being multi-threaded I have been careful to have a lock around the logging class so only one thread can log a message at once.....when I've caught a hang all i get are threads blocking on the lock and VS reporting that the control has passed into Console.Write and it is waiting for it to come back....it never does.
A couple of days ago i got another report of a failure but this time during bootup....where no asynch connections have yet been kicked off( the main thread does spawn a thread to bootup though ) and I was sent a picture.....see below.( i added the begin and end critical section lines to prevent this and it did not )
// Logging Class
public class Logging
{
// Lock to make the logging class thread safe.
static readonly object _locker = new object();
public delegate void msgHandlerWriteLineDelegate(string msg, Color col);
public static event msgHandlerWriteLineDelegate themsgHandlerWriteLineDelegate;
public delegate void msgHandlerWriteDelegate(string msg, Color col);
public static event msgHandlerWriteDelegate themsgHandlerWriteDelegate;
public static void Write(string a, Color Col)
{
if (themsgHandlerWriteDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteDelegate(a, Col);
}
}
}
public static void Write(string a)
{
if (themsgHandlerWriteDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteDelegate(a, Color.Black);
}
}
}
public static void WriteLine(string a, Color Col)
{
if (themsgHandlerWriteLineDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteLineDelegate(a, Col);
}
}
}
public static void WriteLine(string a)
{
if (themsgHandlerWriteLineDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteLineDelegate(a, Color.Black);
}
}
}
// Console Methods That implement the delegates in my logging class.
public static void ConsoleWriteLine(string message, Color Col)
{
try
{
if (Col == Color.Black)
{
Console.ForegroundColor = ConsoleColor.Gray;
}
else
{
Console.ForegroundColor = (ConsoleColor)Enum.Parse(typeof(ConsoleColor), Col.Name);
}
Thread.BeginCriticalRegion();
Console.WriteLine(message);
Thread.EndCriticalRegion();
Console.ForegroundColor = ConsoleColor.Gray;
}
catch (ThreadAbortException ex)
{
Console.WriteLine("ThreadAbortException : " + ex.Message);
}
catch (Exception ex)
{
Console.WriteLine("Exception : " + ex.Message);
}
}
public static void ConsoleWrite(string message, Color Col)
{
try
{
if (Col == Color.Black)
{
Console.ForegroundColor = ConsoleColor.Gray;
}
else
{
Console.ForegroundColor = (ConsoleColor)Enum.Parse(typeof(ConsoleColor), Col.Name);
}
Thread.BeginCriticalRegion();
Console.Write(message);//**THIS IS WHERE IS HANGS...IT NEVER RETURNS **
Thread.EndCriticalRegion();
Console.ForegroundColor = ConsoleColor.Gray;
}
catch (ThreadAbortException ex)
{
Console.WriteLine("ThreadAbortException : " + ex.Message);
}
catch (Exception ex)
{
Console.WriteLine("Exception : " + ex.Message);
}
}
public static void ConsoleUpdate(string message)
{
try
{
Thread.BeginCriticalRegion();
Console.WriteLine(message);//**THIS IS WHERE IS HANGS...IT NEVER RETURNS **
Thread.EndCriticalRegion();
}
catch (ThreadAbortException ex)
{
Console.WriteLine("ThreadAbortException : " + ex.Message);
}
catch (Exception ex)
{
Console.WriteLine("Exception : " + ex.Message);
}
}
// The main method...subscribes to delegates and spawns a thread to boot HW..main thread then exits.
public static void Main()
{
Logging.themsgHandlerWriteDelegate += new Logging.msgHandlerWriteDelegate(ConsoleWrite);
Logging.themsgHandlerWriteLineDelegate += new Logging.msgHandlerWriteLineDelegate(ConsoleWriteLine);
Logging.themsgHandlerUpdateDelegate += new Logging.msgHandlerUpdateDelegate(ConsoleUpdate);
}
}
public class ClassOnOtherThread
{
// In a different class running on a different thread the following line occasionly invokes the error:
private void BootHw(string Resource, string Resource2)
{
Logging.Write("\t\t[");
}
}
My reading of the MSDN suggests Console.WriteLine and Console.Write are threadsafe and therefore i don't actually need a lock around it....i also can't believe it Microsoft's code is wrong(;-) and so I am guessing it is some interaction my code is doing which creates the error.
Now my question : Should i be doing anything to prevent Console.WriteLine and Console.Write being interrupted?...it is my guess that something it interrupting it...but i don't really know that!!
Any help would me very much appreciated.
Regards,
Gordon.

I had the same problem.
I was using console.readkey() in main thread to prevent closing application in debug mode.
After I replaced it with an infinite loop my problem was solved.

You should solve your problem by removing the locks around the logging. The logging is done via Console.WriteLine which is synchronized (and thread safe). You are possibly causing a deadlock through your own locking mechanism (though I can't verify without seeing the code).

I guess your application is started by another process which does redirect stderr and stdout. If your "watcher" process uses ReadToEnd() for both streams on the same thread you can deadlock.
Another option to deadlock is to send the child process input via stdin which in turn starts another process which has a console which waits for input indefinitely. This happend once to me with wmic.exe which does block when stdin is redirected.
If you have played with your log class I suspect that you change the underlying Stream of Console.Out with your own one. Please post at least the callstack where your application is hanging so we have something to analyze. There are many ways to shoot yourself in the foot if you replace the console stream with your own one.
Yours,
Alois Kraus

This is a bit of a long shot, but I wonder if you are calling Console.WriteLine with objects whose ToString() method takes a lock. If so, you can get yourself in a deadlock situation with respect to the lock taken internally by Console.WriteLine.
I once posted this bug report to Microsoft Connect, though sadly they declined to fix it.

Static Logger in separate thread?

I've made my Logger, that logs a string, a static class with a static
so I can call it from my entire project without having to make an instance of it.
quite nice, but I want to make it run in a separate thread, since accessing the file costs time
is that possible somehow and what's the best way to do it?
Its a bit of a short description, but I hope the idea is clear. if not, please let me know.
Thanks in advance!
By the way any other improvements on my code are welcome as well, I have the feeling not everything is as efficient as it can be:
internal static class MainLogger
{
internal static void LogStringToFile(string logText)
{
DateTime timestamp = DateTime.Now;
string str = timestamp.ToString("dd-MM-yy HH:mm:ss ", CultureInfo.InvariantCulture) + "\t" + logText + "\n";
const string filename = Constants.LOG_FILENAME;
FileInfo fileInfo = new FileInfo(filename);
if (fileInfo.Exists)
{
if (fileInfo.Length > Constants.LOG_FILESIZE)
{
File.Create(filename).Dispose();
}
}
else
{
File.Create(filename).Dispose();
}
int i = 0;
while(true)
{
try
{
using (StreamWriter writer = File.AppendText(filename))
{
writer.WriteLine(str);
}
break;
}
catch (IOException)
{
Thread.Sleep(10);
i++;
if (i >= 8)
{
throw new IOException("Log file \"" + Constants.LOG_FILENAME + "\" not accessible after 5 tries");
}
}
}
}
}
enter code here

If you're doing this as an exercise (just using a ready made logger isn't an option) you could try a producer / consumer system.
Either make an Init function for your logger, or use the static constructor - inside it, launch a new System.Threading.Thread, which just runs through a while(true) loop.
Create a new Queue<string> and have your logging function enqueue onto it.
Your while(true) loop looks for items on the queue, dequeues them, and logs them.
Make sure you lock your queue before doing anything with it on either thread.

sry, but you may not reinvent the wheel:
choose log4net (or any other (enterprise) logging-engine) as your logger!

Ok, simply put you need to create a ThreadSafe static class. Below are some code snippets, a delegate that you call from any thread, this points to the correct thread, which then invokes the WriteToFile function.
When you start the application that you want to log against, pass it the following, where LogFile is the filename and path of your log file.
Log.OnNewLogEntry += Log.WriteToFile (LogFile, Program.AppName);
Then you want to put this inside your static Logging class. The wizard bit is the ThreadSafeAddEntry function, this will make sure you are in the correct Thread for writing the line of code away.
public delegate void AddEntryDelegate(string entry, bool error);
public static Form mainwin;
public static event AddEntryDelegate OnNewLogEntry;
public static void AddEntry(string entry) {
ThreadSafeAddEntry( entry, false );
}
private static void ThreadSafeAddEntry (string entry, bool error)
{
try
{
if (mainwin != null && mainwin.InvokeRequired) // we are in a different thread to the main window
mainwin.Invoke (new AddEntryDelegate (ThreadSafeAddEntry), new object [] { entry, error }); // call self from main thread
else
OnNewLogEntry (entry, error);
}
catch { }
}
public static AddEntryDelegate WriteToFile(string filename, string appName) {
//Do your WriteToFile work here
}
}
And finally to write a line...
Log.AddEntry ("Hello World!");

What you have in this case is a typical producer consumer scenario - many threads produce log entries and one thread writes them out to a file. The MSDN has an article with sample code for this scenario.

For starters, your logging mechanism should generally avoid throwing exceptions. Frequently logging mechanisms are where errors get written to, so things get ugly when they also start erroring.
I would look into the BackgroundWorker class, as it allows you to fork off threads that can do the logging for you. That way your app isn't slowed down, and any exceptions raised are simply ignored.