ObservableCollection<HistoryEntry> Entries;
var serializer = new DataContractJsonSerializer(typeof(ObservableCollection<HistoryEntry>));
using (var stream = await ApplicationData.Current.LocalFolder.OpenStreamForWriteAsync(
                JsonFileName, CreationCollisionOption.ReplaceExisting)) {
    serializer.WriteObject(stream, Entries);
}
string _jsonContext;
var myStream = await ApplicationData.Current.LocalFolder.OpenStreamForReadAsync(JsonFileName);
using (var reader = new StreamReader(myStream)) {
    _jsonContent = await reader.ReadToEndAsync();
}
using (var ms = new MemoryStream(Encoding.Unicode.GetBytes(_jsonContent))) {
    Entries = serializer.ReadObject(ms) as ObservableCollection<HistoryEntry>;
}