string localData = ObjectSerializer<myObject>.Serialize(_myObject);

            if (!string.IsNullOrEmpty(localData))
            {
                StorageFile localFile = await ApplicationData.Current.LocalFolder.CreateFileAsync("Data.xml", CreationCollisionOption.ReplaceExisting);
                await FileIO.WriteTextAsync(localFile, localData);
            }

public interface ILocalDataService {
  Task SaveToLocal(string localData, string filename)
  }