How to ask “Is there exactly one element satisfying condition” in LINQ?

Question

Quick question, what\'s the preferable way to programmaticaly ask \"Is there exactly one element in this sequence that satisfies X condition?\" using Linq?

i.e.

Answer 1

You could use this extension method, which I think should have been included in the standard extension methods for linq:

public static int CountUpTo(this IEnumerable sequence, int maxCount) {
    if (sequence == null) throw new ArgumentNullException("sequence");
    if (maxCount < 0) throw new ArgumentOutOfRangeException("maxCount");

    var count = 0;
    var enumerator = sequence.GetEnumerator();
    while (count < maxCount && enumerator.MoveNext())
        count += 1;
    return count;
}

Used like so:

return sequence.CountUpTo(2) == 1;