Primitive Ii

Posted : admin On 8/23/2021

In the last post I discussed an asynchronous version of the ManualResetEvent and as promised this time we will be looking at anasynchronous version of the AutoResetEvent. I’m using Stephen Toubs postas reference and we will be building a version that is functional in style that maps straight into asynchronous work flows without and conversionor adaptors.

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What is an AutoResetEvent?

An AutoResetEvent can be described as a turnstile mechanism, it lets a single waiting person through before re-latchingwaiting for the next signal. This is opposed to a ManualResetEvent which functions like an ordinary gate. Calling Set opensthe gate, allowing any number of threads that are waiting to be let through. Calling Reset closes the gate.



First of all here is the shape of the type that we will be building:

American Primitive Ii

Fairly simple: implied constructor, Set and WaitAsync members.

Implied Constructor

Thinking about this logically we may need the following items:

  • A queue mechanism to store asynchronous waiters - let mutable awaits = Queue<_>().
  • A way of knowing if a signal has been made in the absence of any waiters - let mutable signalled = false.
  • We can also declare a short-circuit asynchronous workflow for the situation that Set() is called before WaitAsync()

Primitive Vibes Iidx

  • let completed = async.Return true. This will save us constructing an AsyncResultCell<_> and going though therest of the asynchronous mechanism.

Also notice that an optional parameter called reusethread is defined, we use the ? prefix when defining it to make itoptional. We then make use of the defaultArg function to give it a default value of false if a one is not passed in. Thiswill be used in the Set operation to determine if the code will run on the same thread or a thread in the ThreadPool.


The first step is to use a locking construct to control access to the mutable queue awaits. Inside this lock wecheck to see if signalled is true and if so we reset it to false and return our pre-built completed asynchronous workflow. Ifsignalled is false then we create a new AsyncResultCell<_> and add it to the queue then return the AsyncResult to the caller.


We first declare a function called getWaiter(), we use this function to return an option typethat is either Some AsyncResultCell<bool> or None. We use the lock function to control access to the mutable queue lock awaits. Onceinside the lock we use pattern matching to capture awaits.Count and signalled:

Primitive id
  • The first pattern match (x,_) checks if there are any waiters (awaits.Count > 0) and then dequeues an AsyncResultCell<bool> from thequeue and returns it within an option type: Some < awaits.Dequeue().
  • The second pattern match (_,y) checks whether signalled is set to false before setting its value to true. This causes next WaitAsync()caller to get the short-circuited value completed. This means that an AsyncResultCell<bool> does not need to be created and go though thewhole async mechanism. We then return None as there is no waiter to be notified.
  • The final pattern match (_,_) is used when there are no waiting callers and signalled has already being set, there is simply nothing to do inthis situation so we return None.

We use the getWaiter() function via pattern match. If we have a result i.e. Some AsyncResultCell then we call RegisterResultpassing in AsyncOK(true) to indicate a completion. Notice that we also pass in the reuseThread boolean that was declared as part of theconstructor. If reuseThread is true then the notification to the waiter happens synchronously use this with care! Personally I would stickwith the default of false to ensure that the operation is completed via the thread pool, unless you have a performance critical reason and thewaiting code that executes is very fast.

The reason for using the getWaiter() function is to separate the locking function away from the notification, if RegisterResultwas called within the lock and reuseThreadGimp 64 bit windows 10willbrown. was true then the awaiting function would be called synchronously within the lock whichwould not be a very good situation to be in.

So there we have it, I could take this series further and convert the other primitives that Stephen Toub describes but there should beenough information in these two posts to set you on your way. If anyone would like me to complete the series then let me know. Imay well finish them off and post them on GitHub in the future, time permitting.

Thanks for tuning in, until next time…