Linq - Select 周围两个其他对象之间

Question

I've got a complexing puzzle for you.
I have have a situation where I have a collection of objects and I what to return all the objects between objects of a given type.
I also have a object that I know is inside that collection and I also know that it is inbetween the two objects.
So I want to return a set of surrounding objects (and self) that lay inbetween two objects of a given type.

Does this make sence?

Here is an example that I have so far:
There are two sets inside this collection.
This query returns the first set of objects, but I do not check if the known object is within this set and it will not continue searching the collection.

class Between { }
class Known{}

var bob = new Known();

var objs = new object[]
{
  "Outside",
  new Between(),
  "Inside",
  bob,
  "Inside",
  new Between(),
  "Outside",
  new Between(),
  "Inside",
  "Inside",
  new Between(),
  "Outside"
};
var objsBetween = objs
    .SkipWhile(x => x.GetType() != typeof(Between))
    .Skip(1)
    .TakeWhile(x => x.GetType() != typeof(Between)).ToList();
//...Result
/*
{
  "Inside",
  bob,
  "Inside"
}
*/

Do you understand what I am trying to do?
Is Linq the best way to do it or should I use a recersive function?

Thanks

Answer 1

You could get it to work with Aggregate() if you really want to use LINQ, but I'd probably just create an iterator method because I think the code is clearer:

IEnumerable<object> GetBetween(IEnumerable<object> items, Type betweenType)
{
    bool between = false;

    foreach (object item in items)
    {
        if (item.GetType() == betweenType)
            between = !between;
        else if (between)
            yield return item;
    }
}

Then you could do:

var objsBetween = GetBetween(objs, typeof(Between)).ToList();

Answer 2

I have come up with a solution, I stole the TakeUntil code from MoreLinq and created another recurrsive Extension method.
It also turnen out that for my requirements, checking for type is not enough, I needed a predicate:

static class Extensions
{
    public static IEnumerable<TSource> TakeUntil<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
    {
        if (source == null) throw new ArgumentNullException(nameof(source));
        if (predicate == null) throw new ArgumentNullException(nameof(predicate));

        return _(); IEnumerable<TSource> _()
        {
            for (int i=0;i< source.Count();i++)
            {
                yield return source.ElementAt(i);
                if (predicate(source.ElementAt(i), i))
                    yield break;
            }
        }
    }
public static List<List<T>> SetsBetween<T>(this List<T> collection, Func<T, bool> betweenPredicate, List<List<T>>  toReturn = null)
        {
            if (toReturn == null)
            {
                toReturn = new List<List<T>>();
            }
            var set = collection.SkipWhile(x => !betweenPredicate(x))
                .TakeUntil((x, i) => (i > 0) && betweenPredicate(x))
                .ToList();
            toReturn.Add(set.Skip(1).SkipLast(1).ToList());
            collection.RemoveAll(x => set.Contains(x));
            if (collection.Any(x => betweenPredicate(x)))
            {
                collection.SetsBetween(betweenPredicate, toReturn);
            }
            return toReturn;
        }

And then use it like:

var bob = new Person();

var objs = new List<object>()
{
"Outside",
new Between(),
"Inside",
bob,
"Inside",
new Between(),
"Outside",
new Between(),
"Not this",
"Not this either",
new Between(),
"Outside"
};

var sets = objs.SetsBetween(x => x.GetType() == typeof(Between));
var bobSet = sets.Where(x => x.Contains(bob)).SelectMany(x => x).ToList();

So if anyone wonders, the reason I am doing this is because I wan to create CSS selectors for each element in a Markdowwn document parsed by Markdig .
So eventually this code will allow me to get the nth-of-type of a element between two HTMLBlock s.