LINQ equivalent of F# List.partition

Question

Using LINQ, how do you partition an IEnumerable in a way equivalent to F#'s List.parition function? The result should be the source sequence partitioned into two sequences based the predicate, with the predicate evaluated once per source item.

Answer 1

Enumerable.ToLookup comes close.

var l = new int[]{1,2,3,4,5};

var split = l.ToLookup( i => i > 2);

var a = split[true]; // 3,4,5
var b = split[false]; // 1,2

---

It's not too hard to wrap this in a function.

public static (IEnumerable<TSource> Yeses, IEnumerable<TSource> Nos) Partition<TSource>(IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
    //Null handling?
    var s = source.ToLookup(i => predicate(i));
    return (s[true], s[false]);
}

public static void Main(string[] args)
{
    var l = new int[] { 1,2,3,4,5 };

    var s = Partition(l, i => { Console.WriteLine($"Called: {i}"); return i > 2;});

    Console.WriteLine(string.Join(",", s.Yeses));
    Console.WriteLine(string.Join(",", s.Nos));
}

Called: 1
Called: 2
Called: 3
Called: 4
Called: 5
3,4,5
1,2