CalendarSerializer.SerializeToString is not thread safe

[IvaskevychYuriy]

Issue

Here's a small test:

[Test]
public void ConcurrentSerialization()
{
    var serializer = new CalendarSerializer();                      // (1)

    var results = Enumerable.Range(1, 100)
        .AsParallel()
        .Select(x =>
        {
            try
            {
                var cal = new Calendar();                                 // (2)
                cal.Events.Add(new CalendarEvent
                {
                    Start = new CalDateTime(DateTime.UtcNow),
                    End = new CalDateTime(DateTime.UtcNow.AddHours(1))
                });

                serializer.SerializeToString(cal);                       // (3)
                return true;
            }
            catch (Exception e)
            {
                return false;
            }
        })
        .ToList();

    Assert.IsEmpty(results.Where(x => !x));
}

Basically, having one serializer instance (line marked (1)) serialize (3) calendars (2) in different threads is not thread safe - exceptions are thrown all over the place (index out of range, null reference and others)
That's scenario A: (1) shared, (2) created every time

Other combinations are not thread safe too:
Scenario B: (1) shared, (2) also shared
Scenario C: (1) created every time, (2) shared

Workaround?

The only case it works is when new instances of serializer (1) and calendar (2) are created every time, so basically (1), (2) and (3) are all inside try block in above example.

Thoughts

I'm not sure if this can be addressed and fixed easily unfortunatelly. This happens as far as I can tell because serializer mutates a calendar, here's even an issue related to this

Anyway, I'll leave this here in case someone finds it helpful