ValidarCPF_CNPJ.cs

public bool IsValidCPF(string value)
{
    if (value == null)
        return false;

    value = value.NormalizeString();
    if (value.Length != 11)
        return false;

    var cpf = value.Select(c => c.ToInt()).ToList();
    var m1 = cpf.Take(9).Select((e, i) => e * (11 - (i + 1))).Sum() % 11;
    if (cpf[9] != ((m1 == 0 || m1 == 1) ? 0 : 11 - m1))
        return false;

    var m2 = cpf.Take(10).Select((e, i) => e * (11 - i)).Sum() % 11;
    if (cpf[10] != ((m2 == 0 || m2 == 1) ? 0 : 11 - m2))
        return false;

    return true;
}

public static bool IsValidCNPJ(string value)
{
	if (string.IsNullOrEmpty(value))
		return false;

	Func<int, int> func = somatorio => (somatorio % 11) < 2 ? 0 : 11 - (somatorio % 11);
	var multiplicador1 = new [] { 5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2 };
	var multiplicador2 = new [] { 6, 5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2 };

	value = value.Trim().Replace(".", "").Replace("-", "").Replace("/", "");
	if (value.Length != 14)
		return false;

	var cnpj = value.Trim().Replace(".", "").Replace("-", "").Replace("/", "");
	if (cnpj.Length != 14)
		return ValueResult.Failure();

	var cnpjBase = cnpj.Take(12).Select(c => int.Parse(c.ToString())).ToArray();
	var soma = multiplicador1.Select((x, index) => cnpjBase[index] * x).Sum();
	var resto = func(soma);
	var cnpjBaseComDigito = cnpjBase.Concat(new[] { resto }).ToArray();
	var soma2 = multiplicador2.Select((x, index) => cnpjBaseComDigito[index] * x).Sum();
	var resto2 = func(soma2);
	var digito = $"{resto}{resto2}";

	if (cnpj.EndsWith(digito.ToString()))
		return true;

	return false;
}