Efficient Linq Enumerable's 'Count() == 1' test

Question

Similar to this question but rephrased for Linq:

You can use Enumerable<T>.Any() to test if the enumerable contains data. But what's the efficient way to test if the enumerable contains a single value (i.e. Enumerable<T>.Count() == 1) or greater than a single value (i.e. Enumerable<T>.Count() > 1) without using an expensive count operation?

Answer 1

int constrainedCount = yourSequence.Take(2).Count();

// if constrainedCount == 0 then the sequence is empty
// if constrainedCount == 1 then the sequence contains a single element
// if constrainedCount == 2 then the sequence has more than one element

Answer 2

One way is to write a new extension method

public static bool IsSingle<T>(this IEnumerable<T> enumerable) {
  using (var enumerator = enumerable.GetEnumerator()) {
    if (!enumerator.MoveNext()) {
      return false;
    }
    return !enumerator.MoveNext();
  }
}

Answer 3

This code take's LukeH's excellent answer and wraps it up as an IEnumerable extension so that your code can deal in terms of None, One and Many rather than 0, 1 and 2.

public enum Multiplicity
{
    None,
    One,
    Many,
}

In a static class, e.g. EnumerableExtensions:

public static Multiplicity Multiplicity<TElement>(this IEnumerable<TElement> @this)
{
    switch (@this.Take(2).Count())
    {
        case 0: return General.Multiplicity.None;
        case 1: return General.Multiplicity.One;
        case 2: return General.Multiplicity.Many;
        default: throw new Exception("WTF‽");
    }
}

Answer 4

Another way:

bool containsMoreThanOneElement = yourSequence.Skip(1).Any();

Or for exactly 1 element:

bool containsOneElement = yourSequence.Any() && !yourSequence.Skip(1).Any();

Answer 5

Efficient Count() == n test:

public static bool CountIsEqualTo<T>(this IEnumerable<T> enumerable, int c) 
{
    using (var enumerator = enumerable.GetEnumerator()) 
    {
        for(var i = 0; i < c ; i++)
            if (!enumerator.MoveNext()) 
                return false;

        return !enumerator.MoveNext();
    }
}

Answer 6

With linq to objects, SingleOrDefault throws if there is more than one element, so you're probably best off if you roll your own.

EDIT: Now I've seen LukeH's answer, and I have to say I prefer it. Wish I'd thought of it myself!