public DataContext()
{
var fileName = ConfigurationManager.AppSettings[GROUP];
Groups = (List <Group>)XmlFileReader.Deserialize(fileName, typeof(List <Group>));
fileName = ConfigurationManager.AppSettings[STUDENT];
Students = (List <Student>)XmlFileReader.Deserialize(fileName, typeof(List <Student>));
fileName = ConfigurationManager.AppSettings[ABSENT];
Absents = (List <Absent>)XmlFileReader.Deserialize(fileName, typeof(List <Absent>));
fileName = ConfigurationManager.AppSettings[SCHEDULE];
Schedules = (List <Schedule>)XmlFileReader.Deserialize(fileName, typeof(List <Schedule>));
fileName = ConfigurationManager.AppSettings[LESSON];
Lessons = (List <Lesson>)XmlFileReader.Deserialize(fileName, typeof(List <Lesson>));
}