public void AddSubjectToTeacher(string id, string nameSubject)
{
if (id == null)
{
throw new ArgumentNullException(nameof(id));
}
if (nameSubject == null)
{
throw new ArgumentNullException(nameof(nameSubject));
}
ITeacher t = GetTeacher(id);
ISubject s = GetSubject(nameSubject);
if (s != null)
{
s.AddTeacher(t);
t.AddSubject(s);
}
}