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NLog version: 4.6.3

Platform: .Net 4.7.2/ .NET Core 2

Current NLog config (xml or C#, if relevant)
It happen when set <default-wrapper xsi:type="AsyncWrapper" overflowAction="Block" forceLockingQueue="false" />(or netcore2.0 without forceLockingQueue). And can be reproducted with the below code.

            var queue = new NLog.Targets.Wrappers.ConcurrentRequestQueue(10, NLog.Targets.Wrappers.AsyncTargetWrapperOverflowAction.Block);

            for (var j = 0; j < 3; ++j)
            {
                int cur = j;
                _ = Task.Run(() =>
                {
                    for (int i = 0; i < 20; ++i)
                    {
                        queue.Enqueue(default(NLog.Common.AsyncLogEventInfo));
                        Console.WriteLine($"Thread {cur}: PostEnqueue {i + 1}");
                    }
                });
            }
            Task.Run(() =>
            {
                Thread.Sleep(1000);
                int left = 20 * 3;
                while (left > 0)
                {
                    var tmp = queue.DequeueBatch(3);
                    left -= tmp.Length;
                    if (tmp.Length != 0)
                        Console.WriteLine($"DequeueBatch left={left}");
                }
                Console.WriteLine("Finished");
            }).Wait();

You can see some threads are block on line133(Monitor.Enter(_logEventInfoQueue);), but the queue is empty at that time. Seems Monitor.Wait() should not change the value of lockTaken at line141.

• What is the current result?
  Still block even if the queue is empty at that time.
• What is the expected result?
  No block after the queue be empty.
• Did you checked the Internal log?
• Please post full exception details (message, stacktrace, inner exceptions)
• Are there any workarounds? yes/no
• Is there a version in which it did work?
• Can you help us by writing an unit test?