score:15

Accepted answer 
int numOfEvents = eventsForAllMonths.SelectMany(m => m.AllDays)
                                    .Select(d => d.CalEvents.Count)
                                    .Sum();

score:0

I ForEach() for a living, so:

eventsForallMonths.ForEach(em=>em.ForEach(ed=>numOfEvents+=ed.CalEvents.Count));

score:0

How about transforming this in lambda?

List<Item> listItem = new List<Item>();
foreach (StackPanel sp in spPhotos.Children)
{
    foreach(Item item in sp.Children)
    {
        listItem.Add(item);
    }
    sp.Children.Clear();
}
spPhotos.Children.Clear();

score:2

Try this:

    private int getNextEventId()
    {
        int numOfEvents = eventsForAllMonths.SelectMany(eventMonth => eventMonth.AllDays).Aggregate(0, (current, eventDay) => current + eventDay.CalEvents.Count);

        return numOfEvents + 1;
    }

score:2

How about this?

return
    (from m in eventsForAllMonths
    from d in m.AllDays
    select d.CalEvents.Count).Sum() + 1;

score:3 

return (eventsForAllMonths.SelectMany(eventMonth => eventMonth.AllDays).Sum(eventDay => eventDay.CalEvents.Count) + 1);

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