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I just read an article that describes how HttpApplicationState has AcquireRead() / AcquireWrite() functions to manage concurrent access. It continues to explain, that in some conditions however we need to use an explict Lock() and Unlock() on the Application object to avoid a Race condition.

I am unable to understand why a race condition should exist for Application state if concurrent access is implicitly handled by the object.

Could someone please explain this to me ? Why would I ever need to use Application.Lock() and Application.Unlock() ? Thank You !

Preets
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2 Answers2

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The AcquireRead and AcquireWrite methods are in the internal HttpApplicationStateLock class, so you don't use them yourself. They synchronise access, but only for a single read or write. From your code you use the Lock and Unlock methods if you need to synchronise access.

You would typically need to synchonise the access if you are changing something that is not a single read or write, like adding two application items that rely on each other, or first checking if an item exist and then add it:

Application.Lock()
if (Application["info"] == null) {
   Application.Add("info", FetchInfoFromDatabase());
}
Application.Unlock();
Guffa
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  • It's confusing. What kind of thread safety Microsoft assures if this is the case? Should `ReaderWriterLock` not handle a situation when write operation (using `Add`, `Set`) is being performed, no other thread should be able to read (ensuring situation of invalid data being provided to the readers)? I may be wrong but I am not able to think of any such situation. Can you please provide an example too? – Learner Mar 26 '13 at 07:06
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    @CSharpLearner: In the example above there is a read and a write. Each is protected in itself, but two threads could do the read after each other before the other thread does the write, both would fetch the data and then try to add it. – Guffa Mar 26 '13 at 07:32
  • Thanks. I was thinking something different. In `ReaderWriterLock`, only one thread can write at a time, blocking other threads from reading or writing. Correct? So the parallel thread hitting the same code would wait until first thread finishes writing. When first thread finishes, the write operation on other thread would resume. Am I right? In this case, should explicit locking/unlocking matter a lot if `FetchInfoFromDatabase()` in your illustration is not heavy, time consuming operation? – Learner Mar 26 '13 at 07:48
  • If you could post your expert comment to http://stackoverflow.com/questions/15629478/is-httpapplicationstate-removeall-thread-safe, it would be great. – Learner Mar 26 '13 at 07:53
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HttpApplicationState - where the globally access variables those are visible to all the

users who are using the application. So in order to avoid the race condition while changing

the value of the variables. We need some precautionary, thats why we are using

Application.Lock() and after the job done releasing the same variable to others in the

queue using Application.Unlock()

Application.Lock()
Application("VisitorCount") = Convert.ToInt32(Application("VisitorCount")) + 1
Application.UnLock()
solairaja
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