I have a situation where I might have multiple instances of a program running at once, and it's important that just one specific function not be executing in more than one of these instances at once.
Is this the proper way to use a mutex to prevent this from happening?
lock (this.GetType()) {
_log.Info("Doing Sync");
DoSync();
_log.Info("Sync Completed");
}
You said multiple instances of one application, so we're talking about two program.exe's running, right? The lock statement won't lock across multiple programs, just within the program. If you want a true Mutex, look at the System.Threading.Mutex object.
Here is a usage example:
bool createdNew;
using (Mutex mtx = new Mutex(false, "MyAwesomeMutex", out createdNew))
{
try
{
mtx.WaitOne();
MessageBox.Show("Click OK to release the mutex.");
}
finally
{
mtx.ReleaseMutex();
}
}
The createdNew variable will let you know whether or not it was created the first time. It only tells you if it has been created, though. If you want to acquire the lock, you need to call WaitOne and then call ReleaseMutex to release it. If you just want to see if you created a Mutex, just constructing it is fine.
TheSeeker is correct.
Jeff Richter's advice in Clr Via C# (p638-9) on locking is to create a private object specifically for the purpose of being locked.
private Object _lock = new Object();
// usage
lock( _lock )
{
// thread-safe code here..
}
This works because _lock cannot be locked by anything outside the current class.
EDIT: this is applicable to threads executing within a single process. #David Mohundro's answer is correct for inter-process locking.
Related
I have following code:
private static readonly ReaderWriterLockSlim Locker = new ReaderWriterLockSlim();
private void TryGetReadLock()
{
if (!Locker.IsWriteLockHeld && !Locker.IsReadLockHeld)
Locker.TryEnterReadLock(WaitTimeout);
}
Locker.TryEnterReadLock times out. If I stop the debugger before it times out, both Locker.IsWriteLockHeld and Locker.IsReadLockHeld are false.
Question: why would TryEnterReadLock wait in this particular case?
[EDIT]
Indeed, as indicated in the comments I have lock between checking your if condition and entering the if block. So, I have tried to remove the if and leave just Locker.TryEnterReadLock(WaitTimeout);, but I receive the following error:
Additional information: A read lock may not be acquired with the write
lock held in this mode.
Indeed some other thread has a write lock (Locker.IsWriteLockHeld = true).
What I want is obtain classic lock locking when one thread has the write lock, that is have the threads wait until one it finishes, but I cannot find a way to do it.
I'm thinking about Mutexes and I need some clarification about them. Should I try to open existing Mutex or create new one (of course both with the same name). Example of code:
First attempt - try open existing one:
private void AttemptOne()
{
Mutex myMutex;
if (!Mutex.TryOpenExisting("Mutex Name", out myMutex))
myMutex = new Mutex(false, "Mutex Name");
try
{
myMutex.WaitOne(3000);
// code
}
catch { }
finally
{
myMutex.ReleaseMutex();
// myMutex.Dispose(); - should I call that or it will be called automatically when returning from method?
}
}
Second attempt - create new Mutex:
private void AttemptTwo()
{
using (Mutex mutex = new Mutex(false, "Mutex Name"))
{
try
{
mutex.WaitOne(3000);
// code
}
catch { }
finally { myMutex.ReleaseMutex(); }
}
}
I have some questions, which bother me:
which attempt to use?
Let assume that I have a background process with created Mutex, and at the same time Main process tries to do some work, which should be blocked until backgorund process finishes. If background process created its own Mutex, should I try to open it?
Or create new Mutex in Main thread with the same name?
What is the difference between those attempts?
should I call Dispose() in first Attempt?
Or just assume that Mutex will be disposed when method returns? Will it be disposed then?
I assume that in second attempt Mutex will be Disposed (as its IDisposable) when using() ends. Am I right?
If you're planning to create the mutex if it doesn't exist anyway, then just go ahead and use the second method.
If you're going for a different behavior based on the existence of the mutex, then you should use TryOpenExisting to check if it exists.
As for your other questions:
Yes, you should call the Dispose or Close method on the mutex when you're done with it. In order to allow the operating system to destroy it once it's not in use.
Yes, using would call the Mutex object Dispose method.
I think you shouldn't use named mutex to synchronize threads in a single process. It's better to share a mutex (or semaphore) object between this threads (in a local variable or in a class field for example). So use named mutex to make process synchronization(for example running only one copy of the application).
About your questions. TryOpenExisting() represent a common pattern TryXxx and provide you opportunity to check the result of an operation without exceptions. Although there are some constructor overloads that have out createdNew boolean parameter you must declare another variable for this what is ugly. Your second variant means that you don't care about whether you have existing mutex or just create a new.
Calling Dispose() (or put the object into using) is always usefull practice. If you use named mutex like in your code all will be fine after disposing. But if you dispose a shared mutex variable (like I advice) in one thread all other threads which waits this mutex will fall with an exception. In this case you should dispose mutex only when you realy sure that you have no needs in it.
As you can see answers on your questions depends on the specific situation.
I have a .NET 4 WCF service that maintains a thread-safe, in-memory, dictionary cache of objects (SynchronizedObject). I want to provide safe, concurrent access to read and modify both the collection and the objects in the collection. Safely modifying the objects and the cache can be accomplished with reader-writer locks.
I am running into trouble providing read access to an object in the cache. My Read method returns a SynchronizedObject, but I do not know how to elegantly ensure no other threads are modifying the object while WCF is serializing the SynchronizedObject.
I have tried placing the Read return clause inside the read-lock and setting a breakpoint in a custom XmlObjectSerializer. When the XmlObjectSerializer::WriteObject(Stream,object) method is called, a read-lock is not held on the SynchronizedObject.
I am specifically concerned with the following scenario:
Thread A calls Read(int). Execution continues until just after the return statement. By this point, the finally has also been executed, and the read lock on the SynchronizedObject has been released. Thread A's execution is interrupted.
Thread B calls Modify(int) for the same id. The write lock is available and obtained. Sometime between obtaining the write lock and releasing it, Thread B is interrupted.
Thread A restarts and serialization continues. Thread B has a write-lock on the same SynchronizedObject, and is in the middle of some critical section, but Thread A is reading the state of the SynchronizedObject and thus returns a potentially invalid object to the caller of Read(int).
I see two options:
Maintain a custom XmlObjectSerializer that grabs the read-lock before calling the base.WriteObject(Stream, object) method, and releases it after. I do not like this option because sub-classing and overriding a framework serialization function to perform a certain action if a the object to be serialized matches a certain type smells to me.
Create a deep-copy of a SynchronizedObject in the Read method while the read-lock is held, release the lock, and return the deep copy. I do not like this option because there will be many sub-classes of SynchronizedObject that I would have to implement and maintain correct deep-copiers for and deep-copies could be expensive.
What other options do I have? How should I implement the thread-safe Read method?
I have provided a dummy Service below for more explicit references:
public class Service : IService
{
IDictionary<int, SynchronizedObject> collection = new Dictionary<int, SynchronizedObject>();
ReaderWriterLockSlim rwLock = new ReaderWriterLockSlim();
public SynchronizedObject Read(int id)
{
rwLock.EnterReadLock();
try
{
SynchronizedObject result = collection[id];
result.rwLock.EnterReadLock();
try
{
return result;
}
finally
{
result.rwLock.ExitReadLock();
}
}
finally
{
rwLock.ExitReadLock();
}
}
public void ModifyObject(int id)
{
rwLock.EnterReadLock();
try
{
SynchronizedObject obj = collection[id];
obj.rwLock.EnterWriteLock();
try
{
// modify obj
}
finally
{
obj.rwLock.ExitWriteLock();
}
}
finally
{
rwLock.ExitReadLock();
}
}
public void ModifyCollection(int id)
{
rwLock.EnterWriteLock();
try
{
// modify collection
}
finally
{
rwLock.ExitWriteLock();
}
}
}
public class SynchronizedObject
{
public ReaderWriterLockSlim rwLock { get; private set; }
public SynchronizedObject()
{
rwLock = new ReaderWriterLockSlim();
}
}
New answer
Based on your new information and clearer scenario, I believe you want to use something similar to functional programming's immutability feature. Instead of serializing the object that could be changed, make a copy that no other thread could possibly access, then serialize that.
Previous (not valuable) answer
From http://msdn.microsoft.com/en-us/library/system.threading.readerwriterlockslim.enterwritelock.aspx:
If other threads have entered the lock
in read mode, a thread that calls the
EnterWriteLock method blocks until
those threads have exited read mode.
When there are threads waiting to
enter write mode, additional threads
that try to enter read mode or
upgradeable mode block until all the
threads waiting to enter write mode
have either timed out or entered write
mode and then exited from it.
So, all you need to do is call EnterWriteLock and ExitWriteLock inside ModifyObject(). Your attempt to make sure you have both a read and a write lock is actually stopping the code from working.
I have two working threads.I have locked both with a same lock, but threadB is getting executed before threadA, so exception came.I locked both using the same lock object.Thread B is using delegate function.How can I solve the issue.
Detailed Information:
I have a class called StateSimulation.
Inside that there are two functions called
a) OnSimulationCollisionReset
b) OnSimulationProgressEvent
Implementation is like this:
private void OnSimulationCollisionReset()
{
Thread XmlReset = new Thread(XmlResetFn);
XmlReset.Start();
}
private void OnSimulationProgressEvent()
{
DataStoreSingleTon.Instance.IsResetCompleted = true;
Thread ThrdSimulnProgress = new Thread(SimulnProgress);
ThrdSimulnProgress.Start();
}
where SimulnProgress() and XmlResetFn() are as follows:
private void SimulnProgress()
{
//uses a delegate
UIControlHandler.Instance.ShowSimulationProgress();
}
private void XmlResetFn()
{
DataStoreSingleTon.Instance.GetFPBConfigurationInstance().ResetXmlAfterCollision();
}
In which OnSimulationProgressEvent() is using a delegate function.
Both showSimulationProgress and ResetXML...() uses a same resource FPBArrayList.
My requirement is SimulationProgressEvent() should work only after Reset..(). In resetXML..() I clear the FPBList.
In SimulationProgress() I access FPBList[i] where i:0--->size;
I have locked both functions using a same lock object.I expected, reset() will complete first. But after entering to reset, before complete reset, showProgress() started and exception occured..
How to solve my issue?
This is how I locked the functions
public System.Object lockThis = new System.Object();
private void SimulnProgress()
{
lock (lockThis)
{
UIControlHandler.Instance.ShowSimulationProgress();
}
}
private void XmlResetFn()
{
lock (lockThis)
{
DataStoreSingleTon.Instance.GetFPBConfigurationInstance().ResetXmlAfterCollision();
}
}
Please give a solution.
Regards
Nidhin KR
It's not a good idea to write multithreaded code that assumes or requires that execution on different threads occurs in a particular order. The whole point of multithreading is to allow things to be executed independently of each other. Independently means no particular order is expressed or implied. CPU time might not be distributed evenly between the two threads, for example, particularly is one thread is waiting for an external signaling event and the other thread is in a compute loop.
For your particular code, it seems very odd that IsResetCompleted = true; is set in the OnSimulationProgressEvent handler. The completion state of the Reset activity should be set by the Reset activity, not by some other event executing in another thread assuming "If we're here, the work in the other thread must be finished."
You should review your design and identify your assumptions and dependencies between threads. If thread B must not proceed until after thread A has completed something, you should first reexamine why you're putting this work in different threads, and then perhaps use a synchronization object (such as an AutoResetEvent) to coordinate between the threads.
The key point here is if you take a sequential task and split it into multiple threads, but the threads use locks or synch objects to serialize their execution, then there is no benefit to using multiple threads. The operation is still sequential.
Locks are intended to prevent several threads from entering a given section of code simultaneously. They are not intended to synchronize the threads in any other way, like, making them execute code in some specific order.
To enforce the execution order you need to implement some signalling between your threads.
Have a look at Synchronization Primitives, specifically, Auto/ManualResetEvent is probably what you want.
I am not sure if I understand the question entirely, but if your requirement is simply that you want to prevent the body of SimulnProgress from executing before XmlResetfn has executed at least once, you can do:
public readonly object lockThis = new object();
private readonly ManualResetEvent resetHandle = new ManualResetEvent(false);
private void SimulnProgress()
{
resetHandle.WaitOne();
lock (lockThis)
{
UIControlHandler.Instance.ShowSimulationProgress();
}
}
private void XmlResetFn()
{
lock (lockThis)
{
DataStoreSingleTon.Instance.GetFPBConfigurationInstance().ResetXmlAfterCollision();
}
resetHandle.Set();
}
In my application I have a form that starts synchronization process and for number of reasons I want to allow only one synchronization to run at a time. So I've added a static bool field to my form indicating whether sync is in progress and added a lock to set this field to true if it wasn't already set so that first thread could start synchronization but when it's running every other thread that will try to start it will terminate.
My code is something like this:
internal partial class SynchronizationForm : Form
{
private static volatile bool workInProgress;
private void SynchronizationForm_Shown(object sender, EventArgs e)
{
lock (typeof(SynchronizationForm))
{
if (!workInProgress)
{
workInProgress = true;
}
else
{
this.Close();
}
}
}
}
This is working well but when I run Code Analysis on my project I'm getting the following warning message:
CA2002 : Microsoft.Reliability : 'SynchronizationForm.SynchronizationForm_Shown(object, EventArgs)' locks on a reference of type 'Type'. Replace this with a lock against an object with strong-identity.
Can anyone explain to me what's wrong with my code and how can I improve it to make the warning gone. What does it mean that object has a strong-identity?
What is wrong is that you are locking on something public (typeof(SynchronizationForm)) which is accessible everywhere from your code and if some other thread locks on this same thing you get a deadlock. In general it is a good idea to lock only on private static objects:
private static object _syncRoot = new object();
...
lock (_syncRoot)
{
}
This guarantees you that it's only SynchronizationForm that could possess the lock.
From the MSDN explanation of the rule
An object is said to have a weak identity when it can be directly accessed across application domain boundaries. A thread that tries to acquire a lock on an object that has a weak identity can be blocked by a second thread in a different application domain that has a lock on the same object.
Since you can't necessarily predict what locks another AppDomain might take, and since such locks might need to be marshalled and would then be expensive, this rule makes sense to me.
The problem is that typeof(SynchronizationForm) is not a private lock object, which means that any other piece of code could use it to lock on, which could result in deadlock. For example if some other code did this:
var form = new SynchronizationForm();
lock(typeof(SynchronizationForm))
{
form.SomeMethodThatCausesSynchronizationForm_ShownToBeCalled();
}
Then deadlock will occur. Instead you should delcare a private lock object in the SynchronizationForm class and lock on that instead.
The System.Type object of a class can conveniently be used as the mutual-exclusion lock for static methods of the class.
Source: http://msdn.microsoft.com/en-us/library/aa664735(VS.71).aspx
To add to Doug's answer, what you have here is a locking mechanism which should only be used in static methods, being used in an instance method.