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megaadam wrote:

It might even be faster since AddRange() might do clever stuff under the hhod

foreach (var item in source)
{
  if (item is A a)
    list.Add(a);
  if (item is B b)
    DoThing(b);
}

foreach (var item in source)
{
  if (item is A a)
    list.Add(a);
  else if (item is B b)
    DoThing(b);
}

foreach (var item in source)
{
  if (item is B b)
    DoThing(b);
  else if (item is A a)
    list.Add(a);
}

static class Extensions
{
    public static IEnumerable<B> SplitList<T, A, B>(
        this IEnumerable<T> source, 
        IList<A> listOfA)
        where A : T
        where B : T
    {
        foreach (T item in source)
        {
            switch (item)
            {
                case A a:
                {
                    listOfA.Add(a);
                    break;
                }
                case B b:
                {
                    yield return b;
                    break;
                }
            }
        }
    }
}

List<A> list = new();
DoThings(source.SplitList<YourBaseType, A, B>(list));

static void DoThings(IEnumerable<B> items)
{
    foreach (var b in items)
    {
        // do something
    }
}

for(int i = 0; i < container.Count; ++i) { ...