Validation xamarin forms

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Xamarin

Hello Goodnight,
I send this email since I need if you can post 2 queries that I need to do about xamarin to see if you can help me.

1: I am doing a validation in one of the texts that I need to do a validation I have the code that performs the validation but it is in c #.

The button does the validations in the methods and I have it like this

if (string.IsNullOrEmpty(this.RutTxt))
{
    await Application.Current.MainPage.DisplayAlert(
        "Error",
        "Ingrese el Rut",
        "Aceptar");
    return;
}
else if (this.RutTxt.Length != 7)
{
    await Application.Current.MainPage.DisplayAlert("Advertencia", "Faltan digitos, favor de ingresar el rut debe tener al menos 7 digitos.", "Aceptar");
    return;
}
else
{
    if (!this.RutTxt.ToCharArray().All(Char.IsDigit))
    {
        await Application.Current.MainPage.DisplayAlert("Advertencia", "El formato del rut es incorrecto, solo se aceptan numeros.", "Aceptar");
        return;
    }
}

I already have 3 different validations that validate the data of the rut that are only numbers but I need it to validate the check digit

this is the check digit button:

if (string.IsNullOrEmpty(this.DvTxt))
            {
                await Application.Current.MainPage.DisplayAlert(
                    "Error",
                    "Ingrese el Rut",
                    "Aceptar");
                return;
            }

that this is the code in C #

public static string Digito(int rut)
{
    int suma = 0;
    int multiplicador = 1;
    while (rut != 0)
    {
        multiplicador++;
        if (multiplicador == 8)
            multiplicador = 2;
        suma += (rut % 10) * multiplicador;
        rut = rut / 10;
    }
    suma = 11 - (suma % 11);
    if (suma == 11)
    {
        return "0";
    }
    else if (suma == 10)
    {
        return "K";
    }
    else
    {
        return suma.ToString();
    }
}

And the truth of the things that I do not know how to do the validation properly so that it really validates the information.

2: On the other hand I have 2 Piker and a texbox that I need to activate when I actually enter some information

For example

If I have in the Piker called user type and I have selected a Supplier, the supplier category texbox must be activated and if in the piker I have a worker selected, the other piker that is from the position must be activated.

I thank you in advance and I hope you have an excellent day.

What I have tried:

I have tried so far to try to call the class but it does not work for me and I really do not know how to solve it, if someone knows how to perform this validation I would appreciate it.