A set of extension and helper methods to complement the ones from System.Linq.Enumerable.

Some of these methods are just shortcuts for common Linq operations (e.g. IsNullOrEmpty), or improvements to existing Linq methods (e.g. specify default value for FirstOrDefault, specify comparer for Max). Others do more complex things that have no equivalent in standard Linq (RankBy, DistinctUntilChanged).

Here are some methods of interest:

Same as Distinct, Intersect, Union, Except, SequenceEqual, but allow you to specify a key for equality comparison.

var result = items.DistinctBy(i => i.Name);

Returns a sequence with distinct contiguous items (i.e. removes contiguous duplicates).

var input = new[] { 1, 1, 1, 2, 3, 3, 1, 3, 2, 2, 1 };
var result = input.DistinctUntilChanged(); // 1, 2, 3, 1, 3, 2, 1

This is the enumerable equivalent of the Observable.DistinctUntilChanged method from Rx.

Return the item of a sequence that has the min or max value for the specified key.

var winner = players.MaxBy(p => p.Score);
Console.WriteLine("The winner is {0} with {1} points!", winner.Name, winner.Score);

Unlike the well known approach of sorting the list and taking the first item, this method doesn't need sorting and operates in O(n).

These methods associate a rank with each item of a collection, based on the specified key. The difference between the two is the same as between the RANK and DENSE_RANK functions in SQL: RankBy leaves "holes" in the ranks if some items are equal, while DenseRankBy does not.

This code:

var ranking = players.RankByDescending(player => player.Score, (player, rank) => string.Format("{0}. {1} ({2})", rank, player.Name, player.Score));

Produces the following results

1. Joe (42)
2. Liz (23)
2. Ben (23)
4. Ann (16)
5. Bob (15)

As the names imply.

The first two are for those who always forget how to do an outer join with Linq ;).

FullOuterJoin fills a gap, though, since there is no built-in way to do it with Linq.

var result = left.OuterJoin(right, x => x.Id, y => y.Id, (id, x, y) => new { x, y });

Transforms a flat sequence of items into a hierarchy. Each node is of type INode<T> and exposes its children and parent.

var roots = items.ToHierarchy(i => i.Id, i => i.ParentId);

Transforms a hierarchy of objects to a flat sequence.

var flat = roots.Flatten(node => node.Children, TreeTraversalMode.DepthFirst);