using System;

using System.Diagnostics;

using System.Collections;

using System.Collections.Generic;

// see: https://stackoverflow.com/a/59784094/1016343

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Iterator example");

        // checking on runtime

        Console.WriteLine("Example 1");

        if (Iterator() is IEnumerable)

            foreach (var item in Iterator())

            {

                Console.WriteLine(item.ToString());

            }

        else

            Console.WriteLine("Cannot iterate!");

        // declarative way

        Console.WriteLine("Example 2");

        Iterate(Iterator());

        // this works too

        Console.WriteLine("Example 3a");

        var intArray = new int[] {1,2,3};

        Iterate(intArray);

        Console.WriteLine("Example 3b");

        Iterate2(intArray);

    }

    public static void Iterate<T>(T iterator)

        where T: System.Collections.IEnumerable

    {

        foreach (var item in iterator)

        {

            Console.WriteLine(item.ToString());

        }

    }

    public static void Iterate2<T>(T collection)

        where T: IEnumerable

    {

        var enumerator = collection.GetEnumerator();

        if (enumerator is IEnumerator)

            while (enumerator.MoveNext())

            {

                var item = enumerator.Current;

                Console.WriteLine(item.ToString());

            }

    }

    public static IEnumerable<int> Iterator()

    {

        for (int i = 0; i < 10; i++)

        {

            yield return i;

        };

    }

}