Datetime required (+18 years) - asp.net-mvc

Question

I have a little problem, here is my code:
public partial class Tourist
{
public Tourist()
{
Reserve = new HashSet<Reserve>();
}
public int touristID { get; set; }
[Required]
[StringLength(50)]
public string touristNAME { get; set; }
public DateTime touristBIRTHDAY { get; set; }
[Required]
[StringLength(50)]
public string touristEMAIL { get; set; }
public int touristPHONE { get; set; }
public virtual ICollection<Reserve> Reserve { get; set; }
}
}
How can I restrict touristBIRTHDAY to be +18 years old? I think that I have to use this function, but I don't know where to put it:
Note: this function it's an example.
DateTime bday = DateTime.Parse(dob_main.Text);
DateTime today = DateTime.Today;
int age = today.Year - bday.Year;
if(age < 18)
{
MessageBox.Show("Invalid Birth Day");
}
Thanks ;)
UPDATE:
I follow the solution of Berkay Yaylaci, but I'm getting a NullReferenceException. It's seems that my value parameter is default, and then my method is not posting, why? What is the solution to that?

Answer 1

You can write your own Validation. First, create a class.
I called MinAge.cs
public class MinAge : ValidationAttribute
{
private int _Limit;
public MinAge(int Limit) { // The constructor which we use in modal.
this._Limit = Limit;
}
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
DateTime bday = DateTime.Parse(value.ToString());
DateTime today = DateTime.Today;
int age = today.Year - bday.Year;
if (bday > today.AddYears(-age))
{
age--;
}
if (age < _Limit)
{
var result = new ValidationResult("Sorry you are not old enough");
return result;
}
return null;
}
}
SampleModal.cs
[MinAge(18)] // 18 is the parameter of constructor.
public DateTime UserBirthDate { get; set; }
IsValid runs after post and check the Limit. If age is not greater than the Limit (which we gave in modal!) than return ValidationResult

Answer 2

Implement IValidatableObject on your tourist class.
Put your logic in the Validate() method.
You are using MVC so there is no MessageBox.Show(). The MVC model binder will automatically call you validation routine.
Here's another SO question with the details How do I use IValidatableObject?
Also your age logic is wrong. it needs to be
DateTime now = DateTime.Today;
int age = now.Year - bday.Year;
if (now < bday.AddYears(age)) age--;