How do I check if IEnumerable has a single element?

var exactlyOne = sequence.Take(2).Count() == 1;

The Take extension method will not throw if there is less elements, it will simply return only those available.

more direct:

public static bool HasSingle<T>(this IEnumerable<T> sequence) {
    if (sequence is ICollection<T> list) return list.Count == 1; // simple case
    using(var iter = sequence.GetEnumerator()) {
        return iter.MoveNext() && !iter.MoveNext();
    }
}

Note, however, that you can only guarantee that you can read a sequence once, so in those cases: by the simple fact of checking that there is a single item, you can no longer get the item. So you might prefer something that gives you the value if there is one:

public static bool HasSingle<T>(this IEnumerable<T> sequence, out T value)
{
    if (sequence is IList<T> list)
    {
        if(list.Count == 1)
        {
            value = list[0];
            return true;
        }
    }
    else
    {
        using (var iter = sequence.GetEnumerator())
        {
            if (iter.MoveNext())
            {
                value = iter.Current;
                if (!iter.MoveNext()) return true;
            }
        }
    }

    value = default(T);
    return false;
}

To avoid the extra iteration in the other answers, you could implement your own extension:

public static bool HasExactlyOneElement<T>(this IEnumerable<T> source)
{
    using (var enumerator = source.GetEnumerator())
        return enumerator.MoveNext() && !enumerator.MoveNext();
}