Serialize object to json file

public async Task ObjectToFile(Object obj, string fileName, string path = null)
        {
            string json = await Task.Factory.StartNew(() => JsonConvert.SerializeObject(obj));

            StorageFolder sf = null;

            if (path != null)
            {
                sf = await ApplicationData.Current.LocalFolder.GetFolderAsync(path);
            }
            else
            {
                sf = ApplicationData.Current.LocalFolder;
            }

            StorageFile file = await sf.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

            using (StreamWriter sw = new StreamWriter(await file.OpenStreamForWriteAsync()))
            {
                await sw.WriteAsync(json);
            }
        }