Selecting according to a Min or Max on one value in LINQ

Question

This is equivalent to a correlated subquery, where you want the row which has the biggest or smallest X (and not the biggest or smallest X itself). I've only been able to get it through sorting and picking the first item.

public static TSource PickMin<TSource, TMin>(this IEnumerable<TSource> source, Func<TSource, TMin> selector) where TMin : IComparable<TMin>
{
    return source.OrderBy(selector).FirstOrDefault();
}

Which doesn't exactly sound like the most efficient approach. I think there's a combination of .Contains and .Min which could accomplish this? Or is this as good as LINQ gets?

Answer 1

You can use aggregation, which is O(n) (and which is afair used behind the scene, when you call Min() , Max() , etc.):

public static TSource PickMin<TSource, TMin>(this IEnumerable<TSource> source, Func<TSource, TMin> selector) where TMin : IComparable<TMin>
{
     var first = source.FirstOrDefault();
     return source.Aggregate(first, (min, current) => selector(current).CompareTo(selector(min)) < 0 ? current : min);
}

If you want it to throw exceptions on empty collections (instead of returning a default value), remove the first line.

Answer 2

There is no good way to do this directly in LINQ. What you could is to use MinBy() from MoreLINQ