Why Can't I Use the 'Await' Operator Within the Body of a Lock Statement

Why can't I use the 'await' operator within the body of a lock statement?

I assume this is either difficult or impossible for the compiler team to implement for some reason.

No, it is not at all difficult or impossible to implement -- the fact that you implemented it yourself is a testament to that fact. Rather, it is an incredibly bad idea and so we don't allow it, so as to protect you from making this mistake.

call to Monitor.Exit within ExitDisposable.Dispose seems to block indefinitely (most of the time) causing deadlocks as other threads attempt to acquire the lock. I suspect the unreliability of my work around and the reason await statements are not allowed in lock statement are somehow related.

Correct, you have discovered why we made it illegal. Awaiting inside a lock is a recipe for producing deadlocks.

I'm sure you can see why: arbitrary code runs between the time the await returns control to the caller and the method resumes. That arbitrary code could be taking out locks that produce lock ordering inversions, and therefore deadlocks.

Worse, the code could resume on another thread (in advanced scenarios; normally you pick up again on the thread that did the await, but not necessarily) in which case the unlock would be unlocking a lock on a different thread than the thread that took out the lock. Is that a good idea? No.

I note that it is also a "worst practice" to do a yield return inside a lock, for the same reason. It is legal to do so, but I wish we had made it illegal. We're not going to make the same mistake for "await".

Unable to await in a lock, how do I ensure async variable and method are not accessed from multiple threads?

lock is a helper API around Monitor, which is a thread-bound synchronization primitive, which means it isn't suitable for use with await, because there is no guarantee what thread you'll be on when you come back from an incomplete asynchronous operation.

Ultimately, you need an async-aware synchronization primitive; the most readily available would be SemaphoreSlim, which has the WaitAsync() API that you would use to acquire the lock, with a try/finally that calls Release().

In the code in the question, depending on the code branch you either acquire (only) the semaphore, or release the semaphore; that is almost certainly wrong. Correct usage would be something more like:

await totalThreadLimiter.WaitAsync();
try
{
    // some code with "await" here
}
finally
{
    totalThreadLimiter.Release();
}

Await operator can only be used within an Async method

You can only use await in an async method, and Main cannot be async.

You'll have to use your own async-compatible context, call Wait on the returned Task in the Main method, or just ignore the returned Task and just block on the call to Read. Note that Wait will wrap any exceptions in an AggregateException.

If you want a good intro, see my async/await intro post.

C# Lock and Async Method

No it won't.

lock is syntactic sugar for Monitor.Enter and Monitor.Exit. lock will keep execution in the method until the lock is released. It does not function like await in any way, shape or form.

The 'await' operator can only be used within an async lambda expression error

The problem is in the lambda expression you pass to the Select. If you want to make use of the await inside the type create, RegionBreif, you have to pass an async lambda expression as below:

Select(async r => new RegionBreif
{
    IsServiceable = await RegionBreif.checkIfServicable(Context, r.SiteId, r.Id)
})

Why cant I use await?

You need to mark the method that's using await as being async:

async Task<UserAccount> DoStuff()
{
 var DatabaseContext = new ApplicationDbContext();
 var thisUserAccount = await DatabaseContext.Users.FirstAsync(u => u.Id == Id);

 return thisUserAccount;
}

That's what the error message is telling you.

await operator is being skipped

If it skips over your code like that, chances are it threw an error and went straight into the catch.

If you want to always execute this block

await this.reloadGridsOnAction(contractItemSubSystemIDsList, responce, true);

you might want to have a finally block after the catch.

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