How to create Singleton with async method?

I want to have an si开发者_StackOverflow中文版ngleton, say Printer, with one method, say bool addToPrintQueue(Document document). addToPrintQueue should be async (should return immediately), return true if added to queue and false if document already in queue. How to write such method and what should I use for queue processing? (should it be separate thread?)

Have you consider doing Event-based Async pattern. Since each asynchronous event has corresponding completed event that is raised when the method finishes. This will tell if document is added to queue or it failed.

I would create Printer.addToPrintQueue event and let subscribers subscribe to it.

printer.addToPrintQueueCompleted += _addToPrinterQueueCompleted;
printer.addToPrintQueueAsync();


private void _addToPrinterQueueCompleted(object sender, addToPrintQueueCompletedEventArgs e)
{ /* ... */ }

Not really sure why you need a singleton here, an asynchronous thread with locking should do just fine here

 // create a document queue
    private static Queue<string> docQueue = new Queue<string>();

    // create async caller and result handler
    IAsyncResult docAdded = null;
    public delegate bool AsyncMethodCaller(string doc);

    // 
    public void Start()
    {
        // create a dummy doc
        string doc = "test";

        // start the async method
        AsyncMethodCaller runfirstCaller = new AsyncMethodCaller(DoWork);
        docAdded = runfirstCaller.BeginInvoke(doc,null,null);
        docAdded.AsyncWaitHandle.WaitOne();

        // get the result
        bool b = runfirstCaller.EndInvoke(docAdded);
    }


    // add the document to the queue
    bool DoWork(string doc)
    {

        lock (docQueue)
        {
            if (docQueue.Contains(doc))
            {
                return false;
            }
            else
            {
                docQueue.Enqueue(doc);
                return true;
            }
        }
    }
}