// =================  Μέθοδος για εισαγωγη στοιχείων από το χρήστη ================
        public static void InputData(Database db)
        {
            try
            {
                string input;
                do
                {
                    Console.ForegroundColor = ConsoleColor.Yellow;
                    Console.WriteLine(new string('~', 50));
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.Write("  -- ");

                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("  If you want to enter ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.Write("student ");
                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("data, press ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine("1");
                    Console.WriteLine();

                    Console.Write("  -- ");
                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("  If you want to enter ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.Write("trainer ");
                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("data, press ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine("2");
                    Console.WriteLine();

                    Console.Write("  -- ");
                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("  If you want to enter ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.Write("course ");
                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("data, press ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine("3");
                    Console.WriteLine();

                    Console.Write("  -- ");
                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("  If you want to enter ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.Write("assignment ");
                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("data, press ");
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine("4");
                    Console.ForegroundColor = ConsoleColor.Yellow;
                    Console.WriteLine(new string('~', 50));
                    Console.ForegroundColor = ConsoleColor.White;
                    string choice = Console.ReadLine();
                    if (string.IsNullOrWhiteSpace(choice) || !(int.Parse(choice) > 0) || !(int.Parse(choice) < 5)) //ελεγχος αν δωσει λαθος αριθμο ή κενο χαρακτηρα ή τιποτα
                    {
                        Console.WriteLine("\nPlease give a number between 1-4 from the following menu\n");
                        input = "Y";
                        continue;
                    }


                    switch (choice)
                    {
                    case "1":
                        Student st1 = new Student();
                        st1.setPersonalInfo();
                        db.students.Add(st1);
                        break;

                    case "2":
                        Trainer tr1 = new Trainer();
                        tr1.setTrainerInfo();
                        db.trainers.Add(tr1);
                        break;

                    case "3":
                        Course c1 = new Course();
                        c1.setCourseInfo();
                        db.courses.Add(c1);
                        break;

                    case "4":
                        Assignment a1 = new Assignment();
                        a1.setAssignmentInfo();
                        db.assignments.Add(a1);
                        break;
                    }

                    input = continueGivingData(); //---- ASK USER TO CONTINUE GIVING DATA
                } while (input == "Y");           //end do while
            } //END OF TRY

            catch (Exception e)
            {
                Console.WriteLine(e);
            }
        }
        public Database()
        {
            Student s1  = new Student("Sotiris", "Papadopoulos", 1985, 2500);
            Student s2  = new Student("Maria", "Giannakopoulou", 1993, 2250);
            Student s3  = new Student("Giannis", "Golas", 1998, 2500);
            Student s4  = new Student("Kyriakh", "Nolemh", 1979, 1200);
            Student s5  = new Student("Giwrgos", "Ipatiou", 1995, 2500);
            Student s6  = new Student("Mixalis", "Kwnstantinou", 1988, 2500);
            Student s7  = new Student("Athanasios", "Liopidas", 1990, 2250);
            Student s8  = new Student("Panagiotis", "Sekinas", 1989, 2500);
            Student s9  = new Student("Hlias", "Liopis", 1982, 2500);
            Student s10 = new Student("Vasilikh", "Xouliara", 1991, 2500);

            students.Add(s1);
            students.Add(s2);
            students.Add(s3);
            students.Add(s4);
            students.Add(s5);
            students.Add(s6);
            students.Add(s7);
            students.Add(s8);
            students.Add(s9);
            students.Add(s10);

            Course c1 = new Course(title: "CB8", stream: "C#", type: "Part-time", startDate: new DateTime(2019, 2, 10), endDate: new DateTime(2019, 8, 1));
            Course c2 = new Course(title: "CB9", stream: "Java", type: "Part-time", startDate: new DateTime(2019, 4, 5), endDate: new DateTime(2019, 10, 1));
            Course c3 = new Course(title: "CB9", stream: "C#", type: "Full-time", startDate: new DateTime(2019, 4, 5), endDate: new DateTime(2019, 7, 11));
            Course c4 = new Course(title: "CB10", stream: "SQL", type: "Part-time", startDate: new DateTime(2019, 11, 07), endDate: new DateTime(2020, 1, 10));
            Course c5 = new Course(title: "CB10", stream: "Java", type: "Full-time", startDate: new DateTime(2019, 11, 5), endDate: new DateTime(2020, 6, 1));
            Course c6 = new Course(title: "CB11", stream: "HTML", type: "Full-time", startDate: new DateTime(2019, 12, 07), endDate: new DateTime(2020, 1, 10));

            courses.Add(c1);
            courses.Add(c2);
            courses.Add(c3);
            courses.Add(c4);
            courses.Add(c5);
            courses.Add(c6);
            Assignment a1 = new Assignment("Eshop", "Create an e-shop", new DateTime(2019, 02, 20), 21, 95);
            Assignment a2 = new Assignment("Database", "Create a database", new DateTime(2019, 04, 05), 40, 81);
            Assignment a3 = new Assignment("Website", "Create a dynamic website", new DateTime(2019, 06, 10), 32, 92);
            Assignment a4 = new Assignment("School", "Create a school database", new DateTime(2020, 03, 12), 24, 90);
            Assignment a5 = new Assignment("Blog", "Create a blog", new DateTime(2020, 03, 25), 45, 100);
            Assignment a6 = new Assignment("Script", "Create a visual basic script", new DateTime(2020, 03, 25), 45, 100);
            Assignment a7 = new Assignment("Mobile", "Create a mobile app", new DateTime(2020, 03, 25), 45, 100);

            assignments.Add(a1);
            assignments.Add(a2);
            assignments.Add(a3);
            assignments.Add(a4);
            assignments.Add(a5);
            assignments.Add(a6);
            assignments.Add(a7);
            Trainer t1 = new Trainer("George", "Pikoulis", "Communications");
            Trainer t2 = new Trainer("Giannis", "Sotiriou", "Databases");
            Trainer t3 = new Trainer("Efthimios", "Dimitriou", "Unit Testing");
            Trainer t4 = new Trainer("Petros", "Afentras", "Mobile Architectures");
            Trainer t5 = new Trainer("Kyriakos", "Pampelas", "Security");
            Trainer t6 = new Trainer("Xaralampos", "Eleutheriou", "ECDL");

            trainers.Add(t1);
            trainers.Add(t2);
            trainers.Add(t3);
            trainers.Add(t4);
            trainers.Add(t5);
            trainers.Add(t6);
            List <Student> studentsC1 = new List <Student> {
                s1, s2, s5
            };                                                         //φτιαχνω λιστα με τους μαθητές στο course 1
            List <Student> studentsC2 = new List <Student> {
                s2, s4, s6
            };                                                         ////φτιαχνω λιστα με τους μαθητές στο course 2
            List <Student> studentsC3 = new List <Student> {
                s3, s7, s9
            };                                                           ////φτιαχνω λιστα με τους μαθητές στο course 3
            List <Student> studentsC4 = new List <Student> {
                s10, s8
            };                                                        ////φτιαχνω λιστα με τους μαθητές στο course 4
            List <Student> studentsC5 = new List <Student> {
                s1, s2, s4, s5, s7, s8
            };                                                                       ////φτιαχνω λιστα με τους μαθητές στο course 5
            List <Student> studentsC6 = new List <Student> {
                s1, s9, s6, s5
            };                                                           ////φτιαχνω λιστα με τους μαθητές στο course 6
            StudentsInCourse sc1 = new StudentsInCourse(studentsC1, c1); //φτιαχνω αντικειμενα που περιεχουν τους μαθητές στο course 1
            StudentsInCourse sc2 = new StudentsInCourse(studentsC2, c2); //φτιαχνω αντικειμενα που περιεχουν τους μαθητές στο course 2
            StudentsInCourse sc3 = new StudentsInCourse(studentsC3, c3); //φτιαχνω αντικειμενα που περιεχουν τους μαθητές στο course 3
            StudentsInCourse sc4 = new StudentsInCourse(studentsC4, c4); //φτιαχνω αντικειμενα που περιεχουν τους μαθητές στο course 4
            StudentsInCourse sc5 = new StudentsInCourse(studentsC5, c5); //φτιαχνω αντικειμενα που περιεχουν τους μαθητές στο course 5
            StudentsInCourse sc6 = new StudentsInCourse(studentsC6, c6); //φτιαχνω αντικειμενα που περιεχουν τους μαθητές στο course 6

            studInCourse.Add(sc1);                                       // αποθηκεύω σε λίστα τα απραπανω αντικειμενα
            studInCourse.Add(sc2);
            studInCourse.Add(sc3);
            studInCourse.Add(sc4);
            studInCourse.Add(sc5);
            studInCourse.Add(sc6);

            List <Trainer> trainersC1 = new List <Trainer> {
                t1, t6
            };                                                    // φτιαχνω λιστα με τους trainers σε κάθε course
            List <Trainer> trainersC2 = new List <Trainer> {
                t2, t3
            };
            List <Trainer> trainersC3 = new List <Trainer> {
                t1, t2
            };
            List <Trainer> trainersC4 = new List <Trainer> {
                t2, t3, t1
            };
            List <Trainer> trainersC5 = new List <Trainer> {
                t3, t5, t4
            };
            List <Trainer> trainersC6 = new List <Trainer> {
                t6, t4
            };

            TrainersInCourse trCour1 = new TrainersInCourse(c1, trainersC1);// φτιαχνω αντικείμενα με τους trainers του συγκεκριμένου course
            TrainersInCourse trCour2 = new TrainersInCourse(c2, trainersC2);
            TrainersInCourse trCour3 = new TrainersInCourse(c3, trainersC3);
            TrainersInCourse trCour4 = new TrainersInCourse(c4, trainersC4);
            TrainersInCourse trCour5 = new TrainersInCourse(c5, trainersC5);
            TrainersInCourse trCour6 = new TrainersInCourse(c6, trainersC6);

            trainInCourse.Add(trCour1);//αποθηκεύω σε λίστα τα απραπανω αντικειμενα
            trainInCourse.Add(trCour2);
            trainInCourse.Add(trCour3);
            trainInCourse.Add(trCour4);
            trainInCourse.Add(trCour5);
            trainInCourse.Add(trCour6);

            List <Assignment> assignmentsC1 = new List <Assignment> {
                a1, a2, a3
            };                                                                     //list containing assignments for course 1
            List <Assignment> assignmentsC2 = new List <Assignment> {
                a2,
            };                                                             //list containing assignments for course 2
            List <Assignment> assignmentsC3 = new List <Assignment> {
                a2, a3
            };                                                                //list containing assignments for course 3
            List <Assignment> assignmentsC4 = new List <Assignment> {
                a4
            };                                                            //list containing assignments for course 4
            List <Assignment> assignmentsC5 = new List <Assignment> {
                a5, a6,
            };                                                                 //list containing assignments for course 5
            List <Assignment> assignmentsC6 = new List <Assignment> {
                a6, a7
            };                                                                              //list containing assignments for course 6
            AssignmentsInCourse assignCourse1 = new AssignmentsInCourse(c1, assignmentsC1); // create object with the list of assignments for course 1
            AssignmentsInCourse assignCourse2 = new AssignmentsInCourse(c2, assignmentsC2); // create object with the list of assignments for course 2
            AssignmentsInCourse assignCourse3 = new AssignmentsInCourse(c3, assignmentsC3); // create object with the list of assignments for course 3
            AssignmentsInCourse assignCourse4 = new AssignmentsInCourse(c4, assignmentsC4); // create object with the list of assignments for course 4
            AssignmentsInCourse assignCourse5 = new AssignmentsInCourse(c5, assignmentsC5); // create object with the list of assignments for course 5
            AssignmentsInCourse assignCourse6 = new AssignmentsInCourse(c6, assignmentsC6); // create object with the list of assignments for course 6

            assignmentsInCourse.Add(assignCourse1);
            assignmentsInCourse.Add(assignCourse2);
            assignmentsInCourse.Add(assignCourse3);
            assignmentsInCourse.Add(assignCourse4);
            assignmentsInCourse.Add(assignCourse5);
            assignmentsInCourse.Add(assignCourse6);
        }