Check if a date is valid

I think you can split the MaskedTextBox string by giving a split, using the '/' character, convert the numbers to integer, and then validate.

  

In MaskedTextBox , property TextMaskFormat must be IncludeLiterals which is the default setting.

Here's an example:

public static void Main()
{
    string data = "31/03/2018"; //Aqui você coloca: seuMaskedTextBox.Text;

    int maiorAnoPermitido = 2050;
    int menorAnoPermitido = 2000;

    string[] val = data.Split('/');

    int dia, mes, ano;

    if (int.TryParse(val[0] , out dia) && int.TryParse(val[1], out mes) && int.TryParse(val[2], out ano))
    {
                if (ano >= menorAnoPermitido && ano <= maiorAnoPermitido)
                {
                    if (mes >=1 && mes <=12)
                    {

                        int maxDia = (mes==2 ? ( ano % 4 ==0 ? 29 : 28  ) : mes <=7 ? (mes%2==0?30 : 31) : (mes%2==0?31:30));                       

                        if (dia >=1 && dia <=maxDia)
                        {
                            Console.WriteLine("Data Válida");
                        }
                        else
                        {
                            Console.WriteLine("Dia inválido");
                        }
                    }
                    else
                    {
                        Console.WriteLine("Mês inválido");
                    }
                }
                else
                {
                    Console.WriteLine("Ano inválido");
                }
    }
    else
    {
        Console.WriteLine("Data inválida");
    }

}

Now, with more time, I made the validation even of the days next to the month, besides defining variables for the greater and smaller year that the system must accept.

I put it in .NETFiddle: link

Although the object returned by Convert.ToDateTime is of type DateTime , it was not used directly from class DateTime ... I think it could be like this:

bool validaData;

try {
    Convert.ToDateTime(txtData.Text);
    validaData = true;
} catch (Exception exx) {
    validaData = false;
}

if(validaData){
    //Válida!
} else {
    //Não é válida!
}

You can get the text portions of the field and transform them into numbers:

string textoBruto = this.TextBoxN.Text;
int dia = Convert.ToInt32(textoBruto.Substring(0, 2), 10); // onde 10 significa base decimal
int mes = Convert.ToInt32(textoBruto.Substring(2, 2), 10);
int ano = Convert.ToInt32(textoBruto.Substring(4, 4), 10);

I suppose that the bars are not part of the text ... I do not remember how this component behaves, but if there are bars, just take with a Replace .

And if these conversions fail, it's because they were not valid integers anyway.

Now just do two simple checks: if the month is between 1 and 12, and if the days are between 1 and the maximum of the month.

if (mes < 1 || mes > 12) {
    throw new DataInvalidaMeuParsaException();
}

A hint for getting the biggest possible day of the month: you can create a hashmap, or cheat with an Array:

int[] maioresDias = new int[] {
    31,
    ano % 4 == 0 ? 29 : 28, // verificação de bissexto
    31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};

And now:

if (dia < 1 || dia > maioresDias[mes - 1]) {
    throw new ParaCeTaLokException();
}

Ex .: merely illustrative exceptions. .NET has some more appropriate exception types that already come with it, such as ArgumentException or ArgumentOutOfRangeException .