public static bool IsValidIranianNationalCode(string input)
        {
            input = input.PadLeft(10, '0');

            if (!Regex.IsMatch(input, @"^\d{10}$"))
                return false;

            var check = Convert.ToInt32(input.Substring(9, 1));
            var sum = Enumerable.Range(0, 9)
                .Select(x => Convert.ToInt32(input.Substring(x, 1)) * (10 - x))
                .Sum() % 11;

            return sum < 2 && check == sum || sum >= 2 && check + sum == 11;
        }

public static bool IsValidIranianNationalCode(string input)
{
    // input has 10 digits that all of them are not equal
    if (!System.Text.RegularExpressions.Regex.IsMatch(input, @"^(?!(\d)\1{9})\d{10}$"))
        return false;

    var check = Convert.ToInt32(input.Substring(9, 1));
    var sum = Enumerable.Range(0, 9)
        .Select(x => Convert.ToInt32(input.Substring(x, 1)) * (10 - x))
        .Sum() % 11;

    return sum < 2 && check == sum || sum >= 2 && check + sum == 11;
}