Using WCF service via async interface from worker thread, how do I ensure that events are sent from the client “in order?

The reason you are getting that kind of sequencing is because you are trying to use the same wait handle that the producer-consumer queue is using for a different purpose. That is going to cause all kinds of chaos. At some point things will go from bad to worse and the queue will get live-locked eventually.

You really should create a separate WaitHandle to wait for completion of the logging service. Or if the BeginLoggingEvents fits the standard pattern it will return a IAsyncResult that contains a WaitHandle that you can use instead of creating your own.

Have a sequence using a cheap monotonically increasing ID (e.g. With Interlocked.Increment()) so that no matter what order things happen at the client or server, you can regenerate the original ordering later on.

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