StorageItemQueryResult.ContentsChanged event not fired

I want to monitor folder for recursive changes. I use this code:

public async void StartMonitor(string folderPath)
{
    StorageFolder storageFolder = await StorageFolder.GetFolderFromPathAsync(folderPath);
    QueryOptions queryOptions = new QueryOptions();
    queryOptions.FolderDepth = FolderDepth.Deep;
    StorageItemQueryResult query = storageFolder.CreateItemQueryWithOptions(queryOptions);
    query.ContentsChanged += QueryContentsChanged;
    await query.GetItemsAsync();
}

private void QueryContentsChanged(IStorageQueryResultBase sender, object args)
{
    //to do
}

Event is fired once after StatMonitor executes. But then when I create file or folder in this directory, event is not fired. How can I get this event firing?

The query object was going out of scope. I made the query object a private class field and it works.