Modulo11 para a validação de PIS

Antes.cs

public static string Mod11(string pis)
{
    var sum = 0;
    var fat = 2;
    for (var i = pis.Length-1; i >= 0; i--)
    {
        sum += Convert.ToInt32(pis[i].ToString()) * fat;
        fat++;
        if (fat > 9)
            fat = 2;
    }
    var mdl = sum % 11;
    if (mdl > 1)
        return (11 - mdl).ToString()[0].ToString();
    return "0";
}

Depois.cs

public static string Mod11(string pis)
{
    var fats = new[] { 2, 3, 4, 5, 6, 7, 8, 9, 2, 3, 4 };
    var mod = pis.Reverse().Zip(fats, (x, a) => (char.GetNumericValue(x) * a)).Sum() % 11;
    return mod > 1 ? (11 - mod).ToString().Substring(0, 1) : "0";
}