A MessageEventHandler delegate object is used to receive asynchronously delivered messages. 
Each session must insure that it passes messages serially to the listener. This means that a listener assigned to one or more consumers of the same session can assume that the onMessage method is not called with the next message until the session has completed the last call. 
IMessageConsumer message sender
            MessageEventArgs is the message passed to the listener
            Namespace: WebLogic.Messaging
Assembly: WebLogic.Messaging (in WebLogic.Messaging.dll)