ControlTag's _factory Dicttionary is not threadsafe

[jespersh]

In an attempt to scale out, I found that

RtfPipe/RtfPipe/Tokens/ControlTag.cs

Line 38 in 9851ab9

private static Dictionary<Type, Func<bool, ControlWord<bool>>> _factory = new Dictionary<Type, Func<bool, ControlWord<bool>>>();

is being accessed in a manner that isn't thread-safe inside of

RtfPipe/RtfPipe/Tokens/ControlTag.cs

Lines 40 to 51 in 9851ab9

	public static ControlWord<bool> Negate(ControlWord<bool> word)
	{
	var type = word.GetType();
	if (!_factory.TryGetValue(type, out var factory))
	{
	var ctor = type.GetConstructor(new[] { typeof(bool) });
	var boolParam = Expression.Parameter(typeof(bool), "value");
	factory = (Func<bool, ControlWord<bool>>)Expression.Lambda(Expression.Convert(Expression.New(ctor, boolParam), typeof(ControlWord<bool>)), boolParam).Compile();
	_factory[type] = factory;
	}
	return factory(!word.Value);
	}

It can show up as various exceptions, but in this case it's an System.IndexOutOfRangeException

   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at RtfPipe.ControlTag.Negate(ControlWord`1 word)
   at RtfPipe.Model.HtmlVisitor.GetNewStyles(IEnumerable`1 styles, HtmlTag tag)
   at RtfPipe.Model.HtmlVisitor.Visit(Element element)
   at RtfPipe.Model.HtmlVisitor.Visit(Element element)
   at RtfPipe.Model.HtmlVisitor.Visit(RtfHtml document)
   at RtfPipe.Rtf.ToHtml(RtfSource source, XmlWriter writer, RtfHtmlSettings settings)
   at RtfPipe.Rtf.ToHtml(RtfSource source, RtfHtmlSettings settings)