Mailbox try

type MailboxMessage =
    | AddMessage of Message
    | GetMessage of AsyncReplyChannel<Message>

type Consumer() =    

    let queue = new ConcurrentQueue<Message>()
    let connectionHandler = ConnectionHandler()    

    let mb = MailboxProcessor.Start(fun inbox ->
        let mutable channel: AsyncReplyChannel<Message> = Unchecked.defaultof<AsyncReplyChannel<Message>>
        let rec loop ()=
            async {
                let! msg= inbox.Receive()
                match msg with
                | AddMessage x->
                    if isNull (box channel)
                    then queue.Enqueue(x)
                    else channel.Reply(x)
                | GetMessage ch->
                    match queue.TryDequeue() with
                    | true, msg ->
                        ch.Reply msg
                    | false, _ ->
                        channel <- ch
                return! loop ()             
            }
        loop ()
    )    

    do connectionHandler.MessageReceived.Add(fun msg -> 
        mb.Post(AddMessage msg)
    )

    member this.ReceiveAsync() =
        task {
            return! mb.PostAndAsyncReply(MailboxMessage.GetMessage)
        }