interface IDBcontext
{
    List<Student> GetAllStudents();
    List<Klass> GetAllKlasses();

    Student FindStudentById(int studentID);
    void RemoveStudent(Student);
    void SaveStudent(Student);
    // Other methods if needed

    // The same for klass....
    Klass FindKlassById(int klassID);
}

class XmlDBContext : IDBContext
{
    XmlDBContext(string filename) {}
}

class Program
{
    public static void Main(string[] args)
    {
        IDBContext dbContext = new XmlDBContext("file.xml");

        // ...

        dbContext.SaveStudent(...);
    }
}