product.cs

using System;
using System.Linq;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var bestProduct = 0L;
for (var i = 0; i < 0x1 << 10; i++)
{
    
    var left = long.Parse("0" + string.Join("", Enumerable.Range(0, 10).Where(x => (i & (0x1 << x)) > 0).OrderByDescending(x => x)));
    var right = long.Parse("0" + string.Join("", Enumerable.Range(0, 10).Where(x => (i & (0x1 << x)) == 0).OrderByDescending(x => x)));

    if (left*right > bestProduct)
    {
        bestProduct = left*right;
    }
}
Console.WriteLine(bestProduct);

        }
    }
}