Linq：确定序列中的项目数是否满足条件

Question

What I'm trying to do, exactly, is determine if there is only a single element in an IEnumerable and then act on that. I could do a .Count() and a comparison against the result, but that is extremely inefficient.

Is there a more efficient way of asking an IEnumerable if the count of items it contains meets some threshold without using .Count() ?

Answer 1

This would also take care of it:

if (seq.Skip(1).Any())
{
    // act on it
}

Answer 2

MyEnumerable.Take(2).Count() == 1 //to check if it has one element

Answer 3

ElementAtOrDefault(index) and comparing against null will work.

if (seq.ElementAtOrDefault(2) == null) 
{
    // act on it
}