Question

without using GroupJoin:

var playersDictionary = players.ToDictionary(player => player.Id,
    element => new PlayerDto { Rounds = new List<RoundDto>() });

foreach (var round in rounds)
{
    PlayerDto playerDto;
    playersDictionary.TryGetValue(round.PlayerId, out playerDto);

    if (playerDto != null)
    {
        playerDto.Rounds.Add(new RoundDto { });
    }
}

var playerDtoItems = playersDictionary.Values;

using GroupJoin:

var playerDtoItems =
    from player in players
    join round in rounds
    on player.Id equals round.PlayerId
    into playerRounds
    select new PlayerDto { Rounds = playerRounds.Select(playerRound => new RoundDto {}) };

Which of these two pieces is more efficient?

Answer 1

Without testing, you really can’t know. As a rule of thumb, anything done in LINQ is slower than the same thing done manually; the extra iterators and delegate invocations do have their costs.

However, for most practical purposes I would consider them equal in performance. If the performance really makes a difference (i.e. you are in a very critical block, or you notice a slowdown), you will definitely want to measure.