using static System.Console;

​

IEnumerable<int> iterate(int startingValue)

{

    if (startingValue is 2 or 5)

    {

        throw new ArgumentException("Starting from 2 or 5 is not allowed.");

    }

    while (true)

    {

        yield return startingValue++;

    }

}

​

try

{

    var fromOne = iterate(1);

    var fromTwo = iterate(2);

    var fromThree = iterate(3);

    WriteLine(

        string.Join(

            ',',

            fromThree.Take(4)));

    var fromFour = iterate(4);

    WriteLine(fromFour.Count());

}

catch {}