FormTheMinimum.cs

using System.Linq;

class Kata
{
    public static long MinValue(int[] a)
    {
        var numbers = a
            .ToList()
            .Distinct()
            .OrderBy(n => n);
        return int.Parse(string.Join("", numbers));
    }
}

https://www.codewars.com/kata/form-the-minimum/train/csharp