Validação de CPF/CNPJ

Antes.cs

private static bool IsValidCNPJ(string cnpj)
{
    var mul1 = new[] {5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2};
    var mul2 = new[] {6, 5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2};
    var cpnjWithoutCheckDigit = cnpj.Substring(0, 12);
    var sum = 0;
    for (var i = 0; i < 12; i++)
        sum += int.Parse(cpnjWithoutCheckDigit[i].ToString())*mul1[i];

    var rest = sum % 11;
    rest = rest < 2 ? 0 : 11 - rest;

    var digits = rest.ToString();
    cpnjWithoutCheckDigit += digits;
    sum = 0;
    for (var i = 0; i < 13; i++)
        sum += int.Parse(cpnjWithoutCheckDigit[i].ToString())*mul2[i];

    rest = sum%11;
    rest = rest < 2 ? 0 : 11 - rest;
    digits += rest.ToString();
    return cnpj.EndsWith(digits);
}

Depois.cs

public static class ValidadorDocumento
{
    static readonly int[] cpfMultipliers = new[] { 11, 10, 9, 8, 7, 6, 5, 4, 3, 2 };
    static readonly int[] cnpjMultipliers = new[] { 6, 5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2 };

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static string CleanString(this string cpf)
    {
        var sb = new StringBuilder();
        foreach (var item in cpf)
            if (char.IsDigit(item))
                sb.Append(item);
        return sb.ToString();
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool IsValidCPF(string cpf)
        => cpf.CleanString().IsValidModulo11(cpfMultipliers, 11);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool IsValidCNPJ(string cnpj)
        => cnpj.CleanString().IsValidModulo11(cnpjMultipliers, 14);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static bool IsValidModulo11(this string code, int[] multipliers, int length)
    {
        if (code.Length != length || VerficarSeTodosOsDigitosSaoIdenticos(code))
            return false;

        int multiplicar(char digit, int mult) => ObterDigito(digit) * mult;

        int sum = code.Zip(multipliers.Skip(1), multiplicar).Sum();
        var rest1 = sum % 11;
        rest1 = rest1 < 2 ? 0 : 11 - rest1;

        sum = code.Zip(multipliers, multiplicar).Sum();
        var rest2 = sum % 11;
        rest2 = rest2 < 2 ? 0 : 11 - rest2;

        return rest1 == ObterDigito(code[9]) && rest2 == ObterDigito(code[10]);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static int ObterDigito(char code) => code - '0';

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static bool VerficarSeTodosOsDigitosSaoIdenticos(string code)
    {
        var previous = -1;
        for (var i = 0; i < code.Length; i++)
        {
            var digito = code[i] - '0';
            if (previous == -1)
                previous = digito;
            else
            {
                if (previous != digito)
                    return false;
            }
        }
        return true;
    }
}