Consider the following function that implements non-blocking access to only the one thread.
public bool TryCancelGroup()
{
if (Monitor.TryEnter(_locked))
{
if (_locked == false)
{
_locked = true;
try
{
// do something
}
catch (Exception ex)
{
_locked = false;
}
finally
{
Monitor.Exit(_locked);
}
}
return _locked;
}
else
{
return false;
}
}
And here is how _locked variable is defined.
bool _locked = false;
Now when program reaches Monitor.Exit(_locked); it throws an System.Threading.SynchronizationLockException saying that _locked variable was not synchronized before.
It all was working before when _locked variable was defined as object
object _locked = new object();
When I changed it to bool in order to use it as boolean flag I started getting this exception.
The reason why is that the Monitor methods all take a System.Object parameter. When you pass in a bool a box is required to convert to Object. The box operation produces a new System.Object value for each call. So the TryEnter and Exit methods see different objects and results in the exception.
When _locked was typed to Object there was no need for a box. Hence the TryEnter and Exit methods see the same object and can function correctly.
A few other comments about the code
TryEnter must be paired with Exit in all cases and for sanity sake the Exit call should be in a finally block. Otherwise you're inviting a deadlock scenario
The _locked variable is only set to false in the face of an exception. If execution does not produce an exception it will remain true and no thread will ever again enter the if block.
Setting the timeout on a monitor to 0 can help implement the behaviour you want. Use a globally declared object to lock against.
static object mylock = new object();
....
if (Monitor.TryEnter(mylock, 0))
{
try
{
// Do work
}
finally
{
Monitor.Exit(mylock);
}
}
To add to the success above - to be sure the lock is released - the TryEnter() and Exit() can be wrapped in a custom class as an extension to object taking a Delegate and Timeout as a parameter.
public static class MyMonitor
{
public static bool TryEnter(this object obj, Action action, int millisecondsTimeout)
{
if (Monitor.TryEnter(obj, millisecondsTimeout))
{
try
{
action();
}
finally
{
Monitor.Exit(obj);
}
return true;
}
else
{
return false;
}
}
}
And called like this waiting 1000 ms to obtain the lock or throw an error if timeout:
if (!_locked.TryEnter(() =>
{
//Exclusive access code placed here..
}, 1000)) {
throw new TimeoutException("Timeout waiting for exclusive access");
}
This way the forgetting the Monitor.Exit() is not an option.
Related
My challenge:
avoid race conditions in a method DoWorkWithId that's being triggered from the UI by multiple users for multiple ids, e.g. lock
re-entrance into DoWorkWithId permitted for different ids but not for the same id , e.g. using a ConcurrentDictionary where id is the key and value is an object
should be a non-blocking lock so threads skip critical section without waiting and letting others know that it's already running (at least was running when invoked a second time), e.g. Monitor.TryEnter or Interlocked.*
My attempt:
I guess 1. and 2. could be solved using a ConcurrentDictionary and lock
private static readonly ConcurrentDictionary<Guid, object> concurrentDictionaryMethodLock = new();
public string CallToDoWorkWithId(Guid id) // [Edited from DoWorkWithId]
{
concurrentDictionaryMethodLock.TryAdd(id, new object()); // atomic adding
lock (concurrentDictionaryMethodLock[id])
{
DoWorkWithId(id);
}
return "done";
}
Now, I don't want threads with the same id to wait. When the first thread is done, threads that waited would find out in the first line of DoWorkWithId that there's nothing to do as the object id refers to has been already modified.
I was hoping to tackle 3. with Monitor.TryEnter or Interlocked.*
3.1. MSDN says lock's basically
object __lockObj = x;
bool __lockWasTaken = false;
try
{
System.Threading.Monitor.Enter(__lockObj, ref __lockWasTaken);
// Your code...
}
finally
{
if (__lockWasTaken) System.Threading.Monitor.Exit(__lockObj);
}
3.2. MSDN example of Monitor.TryEnter
var lockObj = new Object();
bool lockTaken = false;
try
{
Monitor.TryEnter(lockObj, ref lockTaken);
if (lockTaken)
{
// The critical section.
}
else
{
// The lock was not acquired.
}
}
finally
{
// Ensure that the lock is released.
if (lockTaken)
{
Monitor.Exit(lockObj);
}
}
In order to correctly release the lock I thought I need to keep track who's acquired the lock, e.g.
private static readonly ConcurrentDictionary<Guid, bool> bools = new();.
So, I tried:
private static readonly ConcurrentDictionary<Guid, object> concurrentDictionaryMethodLock = new();
private static readonly ConcurrentDictionary<Guid, bool> bools = new();
public string CallToDoWorkWithId(Guid id) // [Edited from DoWorkWithId]
{
concurrentDictionaryMethodLock.TryAdd(id, new object());
bools.TryAdd(id, false);
try
{
Monitor.TryEnter(concurrentDictionaryMethodLock[id], ref bools[id]); // error
if (bools[id])
{
DoWorkWithId(id);
}
else
{
return "Process already running. Wait, refresh page and try again.";
}
}
finally
{
if (bools[id])
{
Monitor.Exit(concurrentDictionaryMethodLock[id]);
}
}
return "done";
}
This gives me CE CS0206: A property or indexer may not be passed as an out or ref parameter.
Same thing when using Interlocked.*. I need to keep track of the id based booleans, here called usingResource, MSDN example.
My question:
How can I use the verbose syntax of lock and store the bools used to identify who has the lock/ressource when using a lock on a ConcurrentDictionary?
PS:
I'm also interested in better suited solutions for what I'm trying to do. I was thinking of using a ConcurrentDictionary without a lock:
private static readonly ConcurrentDictionary<Guid, string> concurrentDictionaryMethodLock = new();
public string CallToDoWorkWithId(Guid id) // [Edited from DoWorkWithId]
{
var userName = GetUserName();
if (!concurrentDictionaryMethodLock.TryAdd(id, userName)) // false means unable to add, was already added, please skip and let UI know
{
if (concurrentDictionaryMethodLock.TryGetValue(id, out var value))
{
return $"Please wait. {value} started process already.";
}
else // in case id has been removed in the meantime, even TryGetOrAdd is not atomic (MSDN remarks about valueFactory)
{
return "Process was running and finished by now. Please refresh and try again.";
}
}
try
{
DoWorkWithId(id);
}
finally
{
concurrentDictionaryMethodLock.TryRemove(id, out _);
}
return "done";
}
Any pitfalls using this approach?
I've ran into this problem a few times on various projects, and I've wondered if there's a better solution than the one I normally end up using.
Say we have a series of methods that need to execute, and we want to know if something goes wrong within one of the methods and break out gracefully (potentially undo-ing any previous changes...), I typically do the following (pseudo C# because it's what I'm most familiar with):
private bool SomeMethod()
{
bool success = true;
string errorMessage = null;
success = TestPartA(ref errorMessage);
if (success)
{
success = TestPartB(ref errorMessage);
}
if (success)
{
success = TestPartC(ref errorMessage);
}
if (success)
{
success = TestPartD(ref errorMessage);
}
//... some further tests: display the error message somehow, then:
return success;
}
private bool TestPartA(ref string errorMessage)
{
// Do some testing...
if (somethingBadHappens)
{
errorMessage = "The error that happens";
return false;
}
return true;
}
I just wondered (and this is my question) if there's a better methodology for coping with this kind of thing. I seem to end up writing a lot of if statements for something that seems like it should be slicker.
I've been suggested having a loop over a set of delegate functions, but I'd be worried that would be over-engineering the solution, unless there's a clean way to do it.
I think you should probably be using exceptions. Note you should generally only be catching exceptions at the "top level" in your application.
private void TopLevelMethod()
{
try
{
SomeMethod();
}
catch (Exception ex)
{
// Log/report exception/display to user etc.
}
}
private void SomeMethod()
{
TestPartA();
TestPartB();
TestPartC();
TestPartD();
}
private void TestPartA()
{
// Do some testing...
try
{
if (somethingBadHappens)
{
throw new Exception("The error that happens");
}
}
catch (Exception)
{
// Cleanup here. If no cleanup is possible,
// do not catch the exception here, i.e.,
// try...catch would not be necessary in this method.
// Re-throw the original exception.
throw;
}
}
private void TestPartB()
{
// No need for try...catch because we can't do any cleanup for this method.
if (somethingBadHappens)
{
throw new Exception("The error that happens");
}
}
I have used the built-in System.Exception class in my example; you can create your own derived exception classes, or use the built-in ones derived from System.Exception.
You could perhaps try looking at the "Open/Closed" section of the SOLID Principle. In your example you could perhaps create an ITestRule interface which contains a method called CheckRule() that will updated your message and return a bool. You would then create an interface implementation for each rule you want to test, and add that class to a List<ITestRule> object. From the Redmondo example above, I would change to the following:
var discountRules =
new List<ITestRule>
{
new TestPartA(),
new TestPartB(),
new TestPartC(),
new TestPartD(),
};
You would then pass the new List<ITestRule> to an evaluator which will loop through each of the classes and runs the CheckRule() method.
I try to stick to a principle known as 'Fail Fast'; methods should fail when they are supposed to, and return immediately with details of the error. The calling method then responds appropriately (re-throw the exception to its caller, log the details, show an error if it's a UI-bound method, etc): -
http://en.wikipedia.org/wiki/Fail-fast
However, this does not mean using exceptions to control the flow of your application. Just raising an exception when you could deal with it is generally bad practice: -
http://msdn.microsoft.com/en-us/library/dd264997.aspx
In your case, I'd re-write your code as (for example): -
private bool SomeMethod()
{
bool success = false;
try
{
TestPartA();
TestPartB();
TestPartC();
TestPartD();
success = true;
}
catch (Exception ex)
{
LogError(ex.Message);
}
//... some further tests: display the error message somehow, then:
return success;
}
private void TestPartA()
{
// Do some testing...
if (somethingBadHappens)
{
throw new ApplicationException("The error that happens");
}
}
Say I have this function (assume I'm accessing Cache in a threadsafe way):
object GetCachedValue(string id)
{
if (!Cache.ContainsKey(id))
{
//long running operation to fetch the value for id
object value = GetTheValueForId(id);
Cache.Add(id, value);
}
return Cache[id];
}
I want to prevent two threads from running the "long running operation" at the same time for the same value. Obviously I can wrap the whole thing in a lock(), but then the whole function would block regardless of value and I want two threads to be able to perform the long running operation as long as they're looking for different id's.
Is there a built-in locking mechanism to lock based on a value so one thread can block while the other thread completes the long running operation so I don't need to do it twice (or N times)? Ideally as long as the long running operation is being performed in one thread, no other thread should be able to do it for the same id value.
I could roll my own by putting the id's in a HashSet and then removing them once the operation completes, but that seems like a hack.
I would use Lazy<T> here. Below code will lock the cache, put the Lazy into the cache and return immediately. Long-running-operation will be executed once in a thread safe manner.
new Thread(() => Console.WriteLine("1-" + GetCachedValue("1").Value)).Start();
new Thread(() => Console.WriteLine("2-" + GetCachedValue("1").Value)).Start();
Lazy<object> GetCachedValue(string id)
{
lock (Cache)
{
if (!Cache.ContainsKey(id))
{
Lazy<object> lazy = new Lazy<object>(() =>
{
Console.WriteLine("**Long Running Job**");
Thread.Sleep(3000);
return int.Parse(id);
},
true);
Cache.Add(id, lazy);
Console.WriteLine("added to cache");
}
return Cache[id];
}
}
In this case I would like to have interface like this
using (SyncDispatcher.Enter(id))
{
//any code here...
}
so I could execute any code and it would be thread safe if id is the same.
If I need to get value from Cache I get it straight forward, as just there is no concurrency calls.
My implementation for SyncDispatcher is this:
public class SyncDispatcher : IDisposable
{
private static object _lock = new object();
private static Dictionary<object, SyncDispatcher> _container = new Dictionary<object, SyncDispatcher>();
private AutoResetEvent _syncEvent = new AutoResetEvent(true);
private SyncDispatcher() { }
private void Lock()
{
_syncEvent.WaitOne();
}
public void Dispose()
{
_syncEvent.Set();
}
public static SyncDispatcher Enter(object obj)
{
var objDispatcher = GetSyncDispatcher(obj);
objDispatcher.Lock();
return objDispatcher;
}
private static SyncDispatcher GetSyncDispatcher(object obj)
{
lock (_lock)
{
if (!_container.ContainsKey(obj))
{
_container.Add(obj, new SyncDispatcher());
}
return _container[obj];
}
}
}
Simple test:
static void Main(string[] args)
{
new Thread(() => Execute("1", 1000, "Resource 1")).Start();
new Thread(() => Execute("2", 200, "Resource 2")).Start();
new Thread(() => Execute("1", 0, "Resource 1 again")).Start();
}
static void Execute(object id, int timeout, string message)
{
using (SyncDispatcher.Enter(id))
{
Thread.Sleep(timeout);
Console.WriteLine(message);
}
}
Move your locking down to where your comment is. I think you need to maintain a list of currently-executing long running operations, and lock accesses to that list, and only execute the GetValueForId if the id you're looking for isn't in that list. I'll try and whip something up.
private List<string> m_runningCacheIds = new List<string>();
object GetCachedValue(string id)
{
if (!Cache.ContainsKey(id))
{
lock (m_runningCacheIds) {
if (m_runningCacheIds.Contains(id)) {
// Do something to wait until the other Get is done....
}
else {
m_runningCacheIds.Add(id);
}
}
//long running operation to fetch the value for id
object value = GetTheValueForId(id);
Cache.Add(id, value);
lock (m_runningCacheIds)
m_runningCacheIds.Remove(id);
}
return Cache[id];
}
There's still the issue of what the thread is going to do while it waits on the other thread is getting the value.
I use in that cases the Mutex as:
object GetCachedValue(string Key)
{
// note here that I use the key as the name of the mutex
// also here you need to check that the key have no invalid charater
// to used as mutex name.
var mut = new Mutex(true, key);
try
{
// Wait until it is safe to enter.
mut.WaitOne();
// here you create your cache
if (!Cache.ContainsKey(Key))
{
//long running operation to fetch the value for id
object value = GetTheValueForId(Key);
Cache.Add(Key, value);
}
return Cache[Key];
}
finally
{
// Release the Mutex.
mut.ReleaseMutex();
}
}
Notes:
Some characters are not valid for the mutex name (like the slash)
If the Cache is different for every application (or web pool) that you use, and it is if we speak for the cache of asp.net, then the mutex is lock all threads and pools in the computer, in this case I also use a static random integer that I add it to the key, and not make the lock different for each key but also for each pool.
It's not the most elegant solution in the world, but I've gotten around this issue with a double check and a lock:
object GetCachedValue(string id)
{
if (!Cache.ContainsKey(id))
{
lock (_staticObj)
{
if (!Cache.ContainsKey(id))
{
//long running operation to fetch the value for id
object value = GetTheValueForId(id);
Cache.Add(id, value);
}
}
}
return Cache[id];
}
I recently had a interview question in a test that was similar to the below, I do not have very much experience of development using threads can someone please help advise me how to approach this question?:
public class StringQueue
{
private object _lockObject = new object();
private List<string> _items = new List<string>();
public bool IsEmpty()
{
lock (_lockObject)
return _items.Count == 0;
}
public void Enqueue(string item)
{
lock (_lockObject)
_items.Add(item);
}
public string Dequeue()
{
lock (_lockObject)
{
string result = _items[0];
_items.RemoveAt(0);
return result;
}
}
}
Is the following method thread safe with the above implementation and why?
public string DequeueOrNull()
{
if (IsEmpty())
return null;
return Dequeue();
}
It seems to me the answer is no.
While isEmpty() procedure locks the object, it is released as soon as the call is returned -
a different thread could potentially call DequeueOrNull() between the call to IsEmpty() and Dequeue() (at which time the object is unlocked), thus removing the only item that existed, making Dequeue() invalid at that time.
A plausible fix would be to put the lock over both statements in DequeueOrNull(), so no other thread could call DeQueue() after the check but before the DeQueue().
It is not threadsafe. At the marked line it is possible that the Dequeue method is called from another thread and thus, the consequent Dequeue return a wrong value:
public string DequeueOrNull()
{
if (IsEmpty())
return null;
/// << it is possible that the Dequeue is called from another thread here.
return Dequeue();
}
The thread safe code would be:
public string DequeueOrNull()
{
lock(_lockObject) {
if (IsEmpty())
return null;
return Dequeue();
}
}
No, because the state of _items could potentially change between the thread-safe IsEmpty() and the thread-safe Dequeue() calls.
Fix it with something like the following, which ensures that _items is locked during the whole operation:
public string DequeueOrNull()
{
lock (_lockObject)
{
if (IsEmpty())
return null;
return Dequeue();
}
}
Note: depending in the implementation of _lock, you may wish to avoid double-locking the resource by moving the guts of IsEmpty() and Dequeue into separate helper functions.
If I have an object that I would like to force to be accessed from within a lock, like so:
var obj = new MyObject();
lock (obj)
{
obj.Date = DateTime.Now;
obj.Name = "My Name";
}
Is it possible, from within the AddOne and RemoveOne functions to detect whether the current execution context is within a lock?
Something like:
Monitor.AreWeCurrentlyEnteredInto(this)
Edit: (for clarification of intent)
The intent here is to be able to reject any modification made outside of the lock, so that all changes to the object itself will be transactional and thread-safe. Locking on a mutex within the object itself does not ensure a transactional nature to the edits.
I know that it is possible to do this:
var obj = new MyObject();
obj.MonitorEnterThis();
try
{
obj.Date = DateTime.Now;
obj.Name = "My Name";
}
finally
{
obj.MonitorExitThis();
}
But this would allow any other thread to call the Add/Remove functions without first calling the Enter, thereby circumventing the protection.
Edit 2:
Here is what I'm currently doing:
var obj = new MyObject();
using (var mylock = obj.Lock())
{
obj.SetDate(DateTime.Now, mylock);
obj.SetName("New Name", mylock);
}
Which is simple enough, but it has two problems:
I'm implementing IDisposable on the
mylock object, which is a little bit
of an abuse of the IDisposable
interface.
I would like to change the SetDate and SetName functions to
Properties, for clarity.
I don't think that's possible without tracking the state yourself (e.g. by using some kind of semaphore). But even if it were, that'd be a gross violation of encapsulation. Your methods usually shouldn't care whether or not they're executing in a particular locking context.
There's no documented method of checking for this kind of condition at runtime, and if there were, I'd be suspicious of any code that used it, because any code that alters its behaviour based on the call stack would be very difficult to debug.
True ACID semantics are not trivial to implement, and I personally wouldn't try; that's what we have databases for, and you can use an in-memory database if you need the code to be fast/portable. If you just want forced-single-threaded semantics, that is a somewhat easier beast to tame, although as a disclaimer I should mention that in the long run you'd be better off simply providing atomic operations as opposed to trying to prevent multi-threaded access.
Let's suppose that you have a very good reason for wanting to do this. Here is a proof-of-concept class you could use:
public interface ILock : IDisposable
{
}
public class ThreadGuard
{
private static readonly object SlotMarker = new Object();
[ThreadStatic]
private static Dictionary<Guid, object> locks;
private Guid lockID;
private object sync = new Object();
public void BeginGuardedOperation()
{
lock (sync)
{
if (lockID == Guid.Empty)
throw new InvalidOperationException("Guarded operation " +
"was blocked because no lock has been obtained.");
object currentLock;
Locks.TryGetValue(lockID, out currentLock);
if (currentLock != SlotMarker)
{
throw new InvalidOperationException("Guarded operation " +
"was blocked because the lock was obtained on a " +
"different thread from the calling thread.");
}
}
}
public ILock GetLock()
{
lock (sync)
{
if (lockID != Guid.Empty)
throw new InvalidOperationException("This instance is " +
"already locked.");
lockID = Guid.NewGuid();
Locks.Add(lockID, SlotMarker);
return new ThreadGuardLock(this);
}
}
private void ReleaseLock()
{
lock (sync)
{
if (lockID == Guid.Empty)
throw new InvalidOperationException("This instance cannot " +
"be unlocked because no lock currently exists.");
object currentLock;
Locks.TryGetValue(lockID, out currentLock);
if (currentLock == SlotMarker)
{
Locks.Remove(lockID);
lockID = Guid.Empty;
}
else
throw new InvalidOperationException("Unlock must be invoked " +
"from same thread that invoked Lock.");
}
}
public bool IsLocked
{
get
{
lock (sync)
{
return (lockID != Guid.Empty);
}
}
}
protected static Dictionary<Guid, object> Locks
{
get
{
if (locks == null)
locks = new Dictionary<Guid, object>();
return locks;
}
}
#region Lock Implementation
class ThreadGuardLock : ILock
{
private ThreadGuard guard;
public ThreadGuardLock(ThreadGuard guard)
{
this.guard = guard;
}
public void Dispose()
{
guard.ReleaseLock();
}
}
#endregion
}
There's a lot going on here but I'll break it down for you:
Current locks (per thread) are held in a [ThreadStatic] field which provides type-safe, thread-local storage. The field is shared across instances of the ThreadGuard, but each instance uses its own key (Guid).
The two main operations are GetLock, which verifies that no lock has already been taken and then adds its own lock, and ReleaseLock, which verifies that the lock exists for the current thread (because remember, locks is ThreadStatic) and removes it if that condition is met, otherwise throws an exception.
The last operation, BeginGuardedOperation, is intended to be used by classes that own ThreadGuard instances. It's basically an assertion of sorts, it verifies that the currently-executed thread owns whichever lock is assigned to this ThreadGuard, and throws if the condition isn't met.
There's also an ILock interface (which doesn't do anything except derive from IDisposable), and a disposable inner ThreadGuardLock to implement it, which holds a reference to the ThreadGuard that created it and calls its ReleaseLock method when disposed. Note that ReleaseLock is private, so the ThreadGuardLock.Dispose is the only public access to the release function, which is good - we only want a single point of entry for acquisition and release.
To use the ThreadGuard, you would include it in another class:
public class MyGuardedClass
{
private int id;
private string name;
private ThreadGuard guard = new ThreadGuard();
public MyGuardedClass()
{
}
public ILock Lock()
{
return guard.GetLock();
}
public override string ToString()
{
return string.Format("[ID: {0}, Name: {1}]", id, name);
}
public int ID
{
get { return id; }
set
{
guard.BeginGuardedOperation();
id = value;
}
}
public string Name
{
get { return name; }
set
{
guard.BeginGuardedOperation();
name = value;
}
}
}
All this does is use the BeginGuardedOperation method as an assertion, as described earlier. Note that I'm not attempting to protect read-write conflicts, only multiple-write conflicts. If you want reader-writer synchronization then you'd need to either require the same lock for reading (probably not so good), use an additional lock in MyGuardedClass (the most straightforward solution) or alter the ThreadGuard to expose and acquire a true "lock" using the Monitor class (be careful).
And here's a test program to play with:
class Program
{
static void Main(string[] args)
{
MyGuardedClass c = new MyGuardedClass();
RunTest(c, TestNoLock);
RunTest(c, TestWithLock);
RunTest(c, TestWithDisposedLock);
RunTest(c, TestWithCrossThreading);
Console.ReadLine();
}
static void RunTest(MyGuardedClass c, Action<MyGuardedClass> testAction)
{
try
{
testAction(c);
Console.WriteLine("SUCCESS: Result = {0}", c);
}
catch (Exception ex)
{
Console.WriteLine("FAIL: {0}", ex.Message);
}
}
static void TestNoLock(MyGuardedClass c)
{
c.ID = 1;
c.Name = "Test1";
}
static void TestWithLock(MyGuardedClass c)
{
using (c.Lock())
{
c.ID = 2;
c.Name = "Test2";
}
}
static void TestWithDisposedLock(MyGuardedClass c)
{
using (c.Lock())
{
c.ID = 3;
}
c.Name = "Test3";
}
static void TestWithCrossThreading(MyGuardedClass c)
{
using (c.Lock())
{
c.ID = 4;
c.Name = "Test4";
ThreadPool.QueueUserWorkItem(s => RunTest(c, cc => cc.ID = 5));
Thread.Sleep(2000);
}
}
}
As the code (hopefully) implies, only the TestWithLock method completely succeeds. The TestWithCrossThreading method partially succeeds - the worker thread fails, but the main thread has no trouble (which, again, is the desired behaviour here).
This isn't intended to be production-ready code, but it should give you the basic idea of what has to be done in order to both (a) prevent cross-thread calls and (b) allow any thread to take ownership of the object as long as nothing else is using it.
Lets redisgn your class to make it actually work like transaction.
using (var transaction = account.BeginTransaction())
{
transaction.Name = "blah";
transaction.Date = DateTime.Now;
transaction.Comit();
}
Changes will not be propagated until commit is called.
In commit you can take a lock and set the properties on the target object.
You can override AddOne and RemoveOne to take a boolean flag that is set to true if it's being called from a lock. I don't see any other way.
You can also play with the ExecutionContext class if you want to know something about the current execution context. You can get the current context by calling ExecutionContext.Capture().
using thread local storage you can store the entering and exiting of a lock.
If your requirement is that the lock must be acquired for the duration of either method AddOne() or RemoveOne(), then why not simply acquire the lock inside each method? It shouldn't be a problem if the caller has already acquired the lock for you.
However, if your requirement is that the lock must be acquired before calling AddOne() and RemoveOne() together (because other concurrent operations performed on the instance are potentially unsafe), then maybe you should consider changing the public interface so that locking can be handled internally without concerning client code with the details.
One possible way to accomplish the later would be to provide methods for Begin- and End-Changes that have to be called before and after AddOne and RemoveOne. An exception should be raised if AddOne or RemoveOne is called outside of the Begin-End scope.
I ran into this same problem and created a helper class that looks like this:
public class BusyLock : IDisposable
{
private readonly Object _lockObject = new Object();
private int _lockCount;
public bool IsBusy
{
get { return _lockCount > 0; }
}
public IDisposable Enter()
{
if (!Monitor.TryEnter(_lockObject, TimeSpan.FromSeconds(1.0)))
throw new InvalidOperationException("Cannot begin operation as system is already busy");
Interlocked.Increment(ref _lockCount);
return this;
}
public bool TryEnter(out IDisposable busyLock)
{
if (Monitor.TryEnter(_lockObject))
{
busyLock = this;
Interlocked.Increment(ref _lockCount);
return true;
}
busyLock = null;
return false;
}
#region IDisposable Members
public void Dispose()
{
if (_lockCount > 0)
{
Monitor.Exit(_lockObject);
Interlocked.Decrement(ref _lockCount);
}
}
#endregion
}
You can then create an instance wrapped like this:
public sealed class AutomationManager
{
private readonly BusyLock _automationLock = new BusyLock();
public IDisposable AutomationLock
{
get { return _automationLock.Enter(); }
}
public bool IsBusy
{
get { return _automationLock.IsBusy; }
}
}
And use it like this:
public void DoSomething()
{
using (AutomationLock)
{
//Do important busy stuff here
}
}
For my particular case, I only wanted an enforcing lock (two threads shouldn't ever try to acquire the lock at the same time if they're well-behaved), so I throw an exception. You can easily modify it to perform more typical locking and still take advantage of the IsBusy.