How can I efficiently determine if an IEnumerable has more than one element?

Question

Given an initialised IEnumerable:

IEnumerable<T> enumerable; 

I would like to determine if it has more than one element. I think the most obvious way to do this is:

enumerable.Count() > 1 

However, I believe Count() enumerates the whole collection, which is unnecessary for this use case. For example, if the collection contains a very large amount of elements or provided its data from an external source, this could be quite wasteful in terms of performance.

How can I do this without enumerating any more than 2 elements?

Answer 1

You can test this in many ways by combining the extension methods in System.Linq... Two simple examples are below:

bool twoOrMore = enumerable.Skip(1).Any(); bool twoOrMoreOther = enumerable.Take(2).Count() == 2; 

I prefer the first one since a common way to check whether Count() >= 1 is with Any() and therefore I find it more readable.

Answer 2

For the fun of it, call Next() twice, then get another IEnumerable.

Or, write a small wrapper class for this specific goal: EnumerablePrefetcher : IEnumerable<T> to try and fetch the specified amount of items upon initialization.

Its IEnumerable<T> GetItems() method should use yield return in this fashion

foreach (T item in prefetchedItems) // array of T, prefetched and decided if IEnumerable has at least n elements {   yield return item; } foreach (T item in otherItems) // IEnumerable<T> {   yield return item; }