Q:

LINQ - group specific types of classes

This question is similar to LINQ group one type of item but handled in a more generic way.
I have a List that has various derived classes. I may have something like this:
List<BaseClass> list = new List<BaseClass>() {
  new Class1(1),
  new Class2(1),
  new Class1(2),
  new Class3(1),
  new Class2(2),
  new Class4(1),
  new Class3(2)
};

I am trying to use LINQ to semi-sort the list so that the natural order is maintained EXCEPT for certain classes which have base.GroupThisType == true. All classes with GroupThisType should be grouped together at the place that the first class of the same type occurs. Here is what the output should be like:
List<BaseClass> list = new List<BaseClass>() {
  new Class1(1),
  new Class1(2),
  new Class2(1),
  new Class3(1),
  new Class3(2)
  new Class2(2),
  new Class4(1),
};

Edit:  Oops, forgot to say this result is assuming (Class1 and Class3).GroupThisType == true

A:

Like this:
list = list.Select((o, i) => new { Index = i * 10000, Value = o })
           .GroupBy(q => q.GetType())
           .SelectMany(g => {
               if (g.First().GroupThisType)
                   return g.Select((q, i) => 
                       new { Index = g.First().Index + i, Value = q.Value }
                   );
               else
                   return g;
           })
           .OrderBy(q => q.Index)
           .Select(q => q.Value)
           .ToList();

The i * 10000 allows up to 10,000 items from a group to be inserted between any two items.  
You can replace g.First().GroupThisType with typesToGroup.Contains(g.Key).

