Question

Consider the following class:

public class Score
{
    private static readonly Guid _relationId = Guid.NewGuid();

    public Guid Id { get; set; }
    public Guid RelationId { get { return _relationId; } }

    public Score()
    {
        Id = Guid.NewGuid();
    }
}

The following function:

public IEnumerable<Score> GetScores()
{
    yield return new Score();
    yield return new Score();
    yield return new Score();
}

Why is it that the following function returns 1:

public int Execute()
{
    var scores = GetScores();
    IList<Score> externalScores = new List<Score>();

    var filteredScores = scores.Where(score => externalScores.All(x => x.RelationId != score.RelationId));

    foreach (var score in filteredScores)
    {
        externalScores.Add(score);
    }

    return externalScores.Count();
}

But when I add .ToList() to filteredScores (either in the foreach loop or after the linq operation) the result of the function is 3.

I suspect it has something to do with Closing over the loop variable, but I can’t seem to figure this out.

Answer 1

With ToList, all items in scores are checked if they match the predicate, which returns always true because externalScores is empty.

Without ToList, each item in scores is checked interleaved with the foreach loop (see: Deferred Execution) which modifies externalScores, causing the predicate to return true for the first item and false for the other two items.