c# - Can LINQ use binary search when the collection is ordered?

Question

Can I somehow "instruct" LINQ to use binary search when the collection that I'm trying to search is ordered. I'm using an ObservableCollection<T>, populated with ordered data, and I'm trying to use Enumerable.First(<Predicate>). In my predicate, I'm filtering by the value of the field my collection's sorted by.

Answer 1

As far as I know, it's not possible with the built-in methods. However it would be relatively easy to write an extension method that would allow you to write something like that :

var item = myCollection.BinarySearch(i => i.Id, 42);

(assuming, of course, that you collection implements IList ; there's no way to perform a binary search if you can't access the items randomly)

Here's a sample implementation :

public static T BinarySearch<T, TKey>(this IList<T> list, Func<T, TKey> keySelector, TKey key)
        where TKey : IComparable<TKey>
{
    if (list.Count == 0)
        throw new InvalidOperationException("Item not found");

    int min = 0;
    int max = list.Count;
    while (min < max)
    {
        int mid = min + ((max - min) / 2);
        T midItem = list[mid];
        TKey midKey = keySelector(midItem);
        int comp = midKey.CompareTo(key);
        if (comp < 0)
        {
            min = mid + 1;
        }
        else if (comp > 0)
        {
            max = mid - 1;
        }
        else
        {
            return midItem;
        }
    }
    if (min == max &&
        min < list.Count &&
        keySelector(list[min]).CompareTo(key) == 0)
    {
        return list[min];
    }
    throw new InvalidOperationException("Item not found");
}

(not tested... a few adjustments might be necessary) Now tested and fixed ;)

The fact that it throws an InvalidOperationException may seem strange, but that's what Enumerable.First does when there's no matching item.