I’m currently working on a project that requires making several concurrent asynchronous calls to a web service.  Now, the service work fine synchrounously, but change the pattern slightly and I was getting the following error.

“There was an error during asynchronous processing. Unique state object is required for multiple asynchronous simultaneous operations to be outstanding.”

The solution, courtesy of a post on the MSDN forums was to pass a new GUID as a state object.

Problem solved.