Thread Safe Async Locking

The lock(variable) method is not compatible with async await calls

lock(padlock)
{
    // This won't work
    await ComputeAsync(padlock);
}

So instead you need to use the SemaphoreSlim class with a value parameter of 1 to implement the same behaviour:

var semaphore = new SemaphoreSlim(1);
await DoWorkAsync(123);

...

async Task DoWorkAsync(int value)
{
    semaphore.WaitAsync();
    try
    {
        await ComputeAsync(value);
    }
    finally
    {
        semaphore.Release();
    }
}

The parameter value of 1 we used sets the count such that only one thread can access that code at a time.

Also call the release in a finally block to ensure the release code is called no matter if something happens, and thus avoid a potential deadlock.