using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var array = new int[] { 1, 2, 3, 4, 5, 3, 8, 2, 3, 6, 8, 8, 4, 3, 2, 8, 1, 8 };

​

        var maxNumber = array.Max();

        var indices =

            array.Select((number, index) => number == maxNumber ? index : -1)

                .Where(index => index != -1)

                    .ToArray();

        foreach (var index in indices)

        {

            Console.WriteLine(index);

        }

    }

}