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?

score 68 · Accepted Answer · answered May 19 '11 at 13:48

68

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

answered May 19 '11 at 13:48

LukeH

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make sure you don't work with the yourSequence data itself because skipping or taking might lose/comsume the data in some cases. it is ok if you are only concerned about the count. – juFo Jan 07 '19 at 10:09

score 17 · Answer 2 · answered May 19 '11 at 13:50

17

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();
  }
}

answered May 19 '11 at 13:50

JaredPar

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@CodeArtist One measurement is worthless. You need to calculate an average and standard deviation. On my machine the difference is meaningless. – BartoszKP Mar 21 '17 at 10:11

Paul Ruane · Answer 3 · 2011-08-04T11:16:59.953

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‽");
    }
}

CodeNaked · Answer 4 · 2011-05-19T13:59:47.417

9

Another way:

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

Or for exactly 1 element:

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

edited May 19 '11 at 13:59

answered May 19 '11 at 13:53

CodeNaked

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This is what I've tended to use but LukeH's constrained count is pretty nifty. – Paul Ruane May 19 '11 at 13:54
@Paul - Yeah, it wraps up the `seq.Any()` and `seq.Skip(1).Any()` into one nice statement. – CodeNaked May 19 '11 at 13:56
You'll need to pair this up with something else to see if the count is 1. This covers the > 1 scenario, however. – Anthony Pegram May 19 '11 at 13:58

3dGrabber · Answer 5 · 2023-05-01T09:07:18.080

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();
    }
}

Edit, 11 years later:
Nowadays I'd probably write this as

public static bool CountIsEqualTo<T>(this IEnumerable<T> enumerable, int c) 
{
    return enumerable.Take(c + 1).Count() == c;
}

It's a generalized version of the accepted answer.

score 1 · Answer 6 · edited Oct 20 '12 at 09:50

1

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!

edited Oct 20 '12 at 09:50

Paul Fleming

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answered May 19 '11 at 13:50

phoog

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Single() throws - SingleOrDefault() will return the default value for the generic type (null for reference types). – David Neale May 19 '11 at 14:01
3

@David: Nope, `SingleOrDefault` will throw if there is more than one element. It only returns `default(T)` if there are no elements. – LukeH May 19 '11 at 14:04
3

@David, his comment about `SingleOrDefault` is true. The difference between `Single` and `SingleOrDefault` is not whether they throw, only when. – Anthony Pegram May 19 '11 at 14:04

score 0 · Answer 7 · answered Jan 07 '19 at 10:12

0

bool hasTwo = yourSequence.ElementAtOrDefault(1) != default(T);

...in case of class where values can be null this could maybe we useful.

answered Jan 07 '19 at 10:12

juFo

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

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