string jsonFileName = "history.json";
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>;
}