Question

I want to create an extension method like this

public static bool AllConsecutives(this IEnumerable<int> intValues )

This method should return true if all items in the list are consecutive (with no gaps)

some test cases

(new List<int>() {2, 3, 4, 5, 6}).AllConsecutives() == true
(new List<int>() {3, 7, 4, 5, 6}).AllConsecutives() == true //as it is not sensitive to list order
(new List<int>() {2, 3, 4, 7, 6}).AllConsecutives() == false //as the five is missing

Answer 1

int expected = intValues.Min();
foreach(int actual in intValues.OrderBy(x => x))
{
    if (actual != expected++)
       return false;
}

return true;

You can also verify, that collection has at least one item, before executing Min. Or you can sort items prior to taking min (in this case it will be first one, or will not be any, if collection is empty). Also in this case you will save one iteration for finding minimal value:

var sortedValues = intValues.OrderBy(x => x);
int expected = sortedValues.FirstOrDefault();

foreach (int actual in sortedValues)
{
    if (actual != expected++)
        return false;
}

return true;