Question

I have an object that downloads a file from a server, saves it into Isolated Storage asynchronously and provides a GetData method to retrieve the data. Would I use a

IsolatedStorageFile storageObj; //initialized in the constructor

lock(storageObj)
{
   //save code
}

In the response and

lock(storageObj)
{
   //load code
}

In the GetData method?

Edit: I’ll give some context here.
The app (for Windows Phone) needs to download and cache multiple files from a server, so I’ve created a type that takes 2 strings (a uri and a filename), sends out for data from the given uri, and saves it. The same object also has the get data method. Here’s the code (simplified a bit)

public class ServerData: INotifyPropertyChanged
{
    public readonly string ServerUri;
    public readonly string Filename;
    IsolatedStorageFile appStorage;

    DownloadState _downloadStatus = DownloadState.NotStarted;
    public DownloadState DownloadStatus
    {
        protected set
        {
            if (_downloadStatus == value) return;
            _downloadStatus = value;
            OnPropertyChanged(new PropertyChangedEventArgs("DownloadStatus"));
        }
        get { return _downloadStatus; }
    }

    public ServerData(string serverUri, string filename)
    {
        ServerUri = serverUri;
        Filename = filename;
        appStorage = IsolatedStorageFile.GetUserStoreForApplication();
    }
    protected virtual void OnPropertyChanged(PropertyChangedEventArgs args)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, args);
    }

    public void RequestDataFromServer()
    {
        DownloadStatus = DownloadState.Downloading;
   //this first bit adds a random unused query to the Uri, 
   //so Silverlight won't cache the request
        Random rand = new Random();
        StringBuilder uriText = new StringBuilder(ServerUri);
        uriText.AppendFormat("?YouHaveGotToBeKiddingMeHack={0}", 
                             rand.Next().ToString());
        Uri uri = new Uri(uriText.ToString(), UriKind.Absolute);
        HttpWebRequest serverRequest = (HttpWebRequest)WebRequest.Create(uri);
        ServerRequestUpdateState serverState = new ServerRequestUpdateState();
        serverState.AsyncRequest = serverRequest;
        serverRequest.BeginGetResponse(new AsyncCallback(RequestResponse),
            serverState);
    }

    void RequestResponse(IAsyncResult asyncResult)
    {
        var serverState = (ServerRequestUpdateState)asyncResult.AsyncState;
        var serverRequest = (HttpWebRequest)serverState.AsyncRequest;
        Stream serverStream;
        try
        {
            // end the async request
            serverState.AsyncResponse = 
                (HttpWebResponse)serverRequest.EndGetResponse(asyncResult);
            serverStream = serverState.AsyncResponse.GetResponseStream();
            Save(serverStream);
            serverStream.Dispose();
        }
        catch (WebException)
        {
            DownloadStatus = DownloadState.Error;
        }

        Deployment.Current.Dispatcher.BeginInvoke(() =>
        {
            DownloadStatus = DownloadState.FileReady;
        });
    }

    void Save(Stream streamToSave)
    {
        StreamReader reader = null;
        IsolatedStorageFileStream file;
        StreamWriter writer = null;
        reader = new StreamReader(streamToSave);

        lock (appStorage)
        {
            file = appStorage.OpenFile(Filename, FileMode.Create);
            writer = new StreamWriter(file);
            writer.Write(reader.ReadToEnd());
            reader.Dispose();
            writer.Dispose();
        }
    }

    public XDocument GetData()
    {
        XDocument xml = null;
        lock(appStorage)
        {
            if (appStorage.FileExists(Filename))
            {
                var file = appStorage.OpenFile(Filename, FileMode.Open);
                xml = XDocument.Load(file);
                file.Dispose();
            }
        }
        if (xml != null)
            return xml;
        else return new XDocument();
    }
}

Answer 1

Your question doesn’t provide an awful lot of context, and with the amount of information given people could be inclined to simply tell you yes, maybe with small, but pertinent additions.

Practice generally sees locking occur on an instance of a dedicated object, being sure to stay away from locking on this since you lock the whole instance of the current object down, which is scarcely, if ever the intent – but, in your case, we don’t rightly know to the fullest extent, however, I hardly think locking your storage instance is the way to go.

Also, since you mention client and server interaction, it isn’t as straight forward.

Depending on the load and many other factors, you might want to provide many reads of the file from the server yet only a single write at any one time on the client that is downloading; for this purpose I would recommend using the ReaderWriterLockSlim class, which exposes TryEnterReadLock, TryEnterWriteLock and corresponding release methods.

For more detailed information on this class see this MSDN link.

Also, remember to use try, catch and finally when coding within the scope of a lock, always releasing the lock in the finally block.