C# limit options of enum in a derived class

Question

So I'm trying to limit the options ,of an enum I declared in the base class, I can select in the derived class.

namespace FLYNET.Personeel
{
    public enum Graad
    {
        Captain,
        SeniorFlightOfficer,
        SecondOfficer,
        JuniorFlightOfficer,
        Steward,
        Purser
    };

public abstract class VliegendPersoneelslid : Personeelslid
{
    public VliegendPersoneelslid()
    {
    }

    public override decimal BerekenTotaleKostprijsPerDag()
    {
        return BasisKostprijsPerDag;
    }

    public List<Certificaat> Certificaten { get; set; }
}

The above is the enum I'm trying to use from the base class Staff. In the derived class Cabincrew I want to limit the options that can be selected in the constructor. If the wrong option is selected, it needs to throw a specified exception, so a bit like this :

namespace FLYNET.Personeel
{
    public class CockpitPersoneel : VliegendPersoneelslid
    {
        public int VliegUren { get; set; }

        public Graad Graad { get; set; }


            public CockpitPersoneel()
            {
                if (Grade = Graad.Steward || Grade = Graad.Purser)
                {
                    throw new Exception 
                }
            }

            public override decimal BerekenTotaleKostprijsPerDag()
            {
                decimal kostprijsPersoneel = 0m;
                decimal percentage;

                return kostprijsPersoneel;
            }
        }
    }

I'm aware this is probably a beginners question (and it is :p) but please bear with me.

Answer 1

I suggest using extension methods in order to hide options

public enum Grade {
   None, // <- zero option is often a good idea to include 
   Captain,
   SeniorFlightOfficer,
   SecondOfficer,
   JuniorFlightOfficer,
   Steward,
   Purser };

public static class GradeExtensions { 
  public static bool IsCabinCrue(this Grade grade) {
    return grade == Grade.Steward || grade == Grade.Purser;
  }

  public static bool IsCockpitPersonnel(this Grade grade) {
    return grade == Grade.Captain || 
           grade == Grade.SeniorFlightOfficer ||
           grade == Grade.SecondOfficer ||
           grade == Grade.JuniorFlightOfficer;
  } 
}  

Then you can use extension methods as if their are methods of the enum when validating Grade values provided:

public class CabinCrue {
  ...
  public CabinCrue(Grade grade) {
    // Validation: Cabin Crue grade only
    // not Exception but ArgumentException: it's argument grade that's wrong
    if (!grade.IsCabinCrue()) 
      throw new ArgumentException("Cabin crue only", "grade");

    Grade = grade;

    ...
  } 

  public Grade Grade {
    get;
    private set;
  }
  ...
}