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
{
    IDBContext dbContext;
    IWindowOperations ops;


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

        // ...

        var student = dbContext.findStudentById(studentId);
        if (student == null)
        {
            ops.Error("Student doesn't exist");
        }
        else
        {
            ops.LoadStudentToEdit(student);
            dbContext.Save(student);
        }
    }
}