Question - Strategy Design Pattern

public abstract class BaseUser
{
        protected List<Perm> permissions;

        public abstract void AddPerm(Perm perm);
}

public class NormalUser : BaseUser
{
        public override void AddPerm(Perm perm)
        {
                throw new InvalidOperationException("A normal user cant add permissions");
        }
}

public class SpecialUser : BaseUser
{
        public override void AddPerm(Perm perm)
        {
                if(permissions==null) permissions=new List<Perm>();

                this.permissions.Add(perm);
        }
}

class Container
{
        List<BaseUser> users;
}