Can I select multiple objects in a Linq query

Question

Can I return more than one item in a select? For instance I have a List of Fixtures (think football (or soccer for the yanks) fixtures). Each fixture contains a home and away t

Answer 1

The following will return an IEnumerable<Team>:

IEnumerable<Team> drew =
    from fixture in fixtures
    where fixture.Played && (fixture.HomeScore == fixture.AwayScore)
    from team in new[]{fixture.HomeTeam, fixture.AwayTeam}
    select team;

Or, with the fluent style of LINQ:

IEnumerable<Team> drew =
    fixtures
    .Where(fxtr => fxtr.Played && (fxtr.HomeScore == fxtr.AwayScore))
    .SelectMany(fixture => new[]{fixture.HomeTeam, fixture.AwayTeam});

Flattening and FlatMap

This requirement is often called 'flattening'. That is, taking a <Collection of <Collections of Things>> and converting it to a <Collection of Things>.

SelectMany both maps (a fixture to an Array of Teams) and flattens (a sequence of Team Arrays to a sequence of Teams). It is similar to the "flatMap" function in other languages such as Java and JavaScript.

It is possible to separate the Mapping and the Flattening:

IEnumerable<Team> drew =
    fixtures
    .Where(fxtr => fxtr.Played && (fxtr.HomeScore == fxtr.AwayScore))
    // map 
    .Select(fixture => new[]{fixture.HomeTeam, fixture.AwayTeam})
    // flatten
    .SelectMany(teams => teams);

Other Approaches

Iterator Block

The same can be achieved with an iterator block, but I suspect this is rarely the best approach:

IEnumerable<Team> Drew(IEnumerable<Fixture> fixtures){
    var draws = 
      fixtures
      .Where(fxtr => fxtr.Played && (fxtr.HomeScore == fxtr.AwayScore));

    foreach(var fixture in draws){
        yield return fixture.HomeTeam;
        yield return fixture.AwayTeam;
    }
}

Union

Union is also an option but has the potential to produce different results from the above:

1. The order of results will be different. All Home results are returned then all Away results.

2. Union enumerates fixtures twice, so, depending on how fixtures is implemented, there is the potential for fixtures to be updated between calls. E.g., if a new drawn fixture were added between calls then the Away team could be returned but not the Home team.

As Mike Powell describes:

IEnumerable<Team> drew =
    ( from fixture in fixtures
      where fixture.Played && (fixture.HomeScore == fixture.AwayScore)
      select fixture.HomeTeam
    ).Union(
      from fixture in fixtures
      where fixture.Played  && (fixture.HomeScore == fixture.AwayScore)
      select fixture.AwayTeam );

Depending on how fixtures is sourced/implemented it may be worth considering 'caching' the drawn fixtures to avoid having to enumerate fixtures twice.

var draws = 
    ( from fixture in fixtures
      where fixture.Played  && (fixture.HomeScore == fixture.AwayScore)
      select fixture
    ).ToList();

IEnumerable<Team> drew =
    (from draw in draws select draw.HomeTeam)
    .Union(from draw in draws select draw.AwayTeam);

Or using the fluent style:

var draws = 
    fixtures
    .Where(fxtr => fxtr.Played && (fxtr.HomeScore == fxtr.AwayScore))
    .ToList();

IEnumerable<Team> drew =
    draws.Select(fixture => fixture.HomeTeam)
    .Union(draws.Select(fixture => fixture.AwayTeam));

Modifying the Fixture class

One could consider adding "ParticipatingTeams" to the Fixture class to get:

IEnumerable<Team> drew =
    from fixture in fixtures
    where fixture.Played && (fixture.HomeScore == fixture.AwayScore)
    from team in fixture.ParticipatingTeams
    select team;

but as @MattDeKrey points out that requires a contract change.

Code Samples

Code samples are available on Repl.it