Answer Submitted

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web;

namespace ExamenVoorbeeld.Models
{
    public class VluchtIDVerification : ValidationAttribute
    {
        public override bool IsValid(object value)
        {
            if (!(value is String))
                return false;
            String val = value.ToString();

            if (string.IsNullOrWhiteSpace(val))
                return false;

            String[] dat = val.Split('/');
            if (dat.Length != 3)
                return false;
            int errorCode = -1;
            if (!Int32.TryParse(dat[2],out errorCode))
                return false;
            int valNumberBack = Int32.Parse(dat[2]);
            if (!Int32.TryParse(dat[0], out errorCode))
                return false;
            int valNumberFront = Int32.Parse(dat[0]);

            if (999 - valNumberFront == valNumberBack)
                return true;

            return false;
        }
    }
}