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

Once I had the issue to determine if exactly one item is contained I decided to create following extension. In my optionion it fits good into the set of already existing extensions (e.g.: SingleOrDefualt, FirstOrDefault ...).

 public static class IEnumerableExtension
{
    public static TProject OneOrDefault(this IEnumerable sequence, Func projection)
    {
        var enumerator = sequence.GetEnumerator();
        bool firstMoveNext = enumerator.MoveNext();
        TInput current = enumerator.Current;
        if (firstMoveNext && !enumerator.MoveNext())
        {
            return projection(current);
        }
        else
        {
            return default;
        }
    }
}