using System;

using System.Collections.Generic;

​

namespace testread

{

    internal static class Program

    {

        static void Main(string[] args)

        {

            var input1 = new[] { 1, 2 };

            Foo(input1);

​

            var input2 = new List<int> { 3, 4, 5 };

            Foo(input2);

        }

​

        private static void Foo(IReadOnlyCollection<int> bar)

        {

            Console.WriteLine(bar.Count);

        }

    }

}