working with lists in c# - without using database - many to many

Question

i already looked at How to model a Many to many-relationship in code?

and I don't quite understand the answer.

also looked at C# Mapping Many To Many but don't want to use entity framework/database

I have Students, and Subjects

public class Subject : Student
    {
        public string SubjectName { get; set; }
        public string StudentClassNumber { get; set; }
    }

    public class Student
    {
        public string StudentName { get; set; }
        public string isjoined { get; set; }


    }

then to instanciate

List<Subject> subject = new List<Subject>();

and to add values/properties

subject2.Add(new Subject2() { SubjectName = "Math", StudentName = "Ben", isjoined = "yes", StudentClassNumber ="2" });
            subject2.Add(new Subject2() { SubjectName = "Math", StudentName2 = "Andrew", isjoined = "yes", StudentClassNumber = "4" });
            subject2.Add(new Subject2() { SubjectName = "Science", StudentName2 = "Andrew", isjoined = "no", StudentClassNumber = "5" });
            subject2.Add(new Subject2() { SubjectName = "Science", StudentName2 = "Ben", isjoined = "no", StudentClassNumber = "3" });

and now my issue, The "Andrew" or "Ben" that are in "Math" are the same (students) that are in "Science" , this is not a new Andrew and Ben

Id like to have a WriteLine that writes: Math - StudentName - StudentClassNumber - Is joined, for both/all students

Ie Math : Ben 2 Yes Andrew 4 Yes

Science Ben 3 No Andrew 4 No

Here is an attempt using linq

class Students
    {
        public string StudentName { get; set; }

    }

    class Subject
    {
        public string Name { get; set; }
        public Students Occupier { get; set; }
    }

    class InnerJoin3
    {
        public static void DoJoin()
        {
            Students ben = new Students { StudentName = "Ben" };
            Students andrew = new Students { StudentName = "Andrew" };


            Subject math = new Subject { Name = "Math", Occupier = andrew };
            Subject science = new Subject { Name = "Science", Occupier = ben };

            // Create two lists.
            List<Students> people = new List<Students> { ben, andrew };
            List<Subject> subjects = new List<Subject> { math, science };

            // Create a collection of student-subject pairs. Each element in the collection
            // is an anonymous type containing both the student's name and their subject's name.
            var query = from student in people
                        join subject in subjects on student equals subject.Occupier
                        select new { OccupierName = student.StudentName, SubjectName = subject.Name };

            foreach (var ownerAndSubject in query)
            {
                Console.WriteLine($"\"{ownerAndSubject.SubjectName}\" is the subject of choice for {ownerAndSubject.OccupierName}");
            }
        }
    }

Answer 1

Well if it is like you said. You want the following result in your console Application:

Math: name, name
Science: name.. etc.

In your Main Class add the following:

static void Main(string[] args)
        {
            Students ben = new Students { StudentName = "Ben" };
            Students andrew = new Students { StudentName = "Andrew" };


            Subject mathAndrew = new Subject { Name = "Math", Students = andrew };
            Subject ScienceAndrew = new Subject { Name = "Science", Students = andrew };
            Subject scienceBen = new Subject { Name = "Science", Students = ben };

            List<Students> people = new List<Students> { ben, andrew };
            List<Subject> subjects = new List<Subject> { mathAndrew, ScienceAndrew, scienceBen };

            var query = from student in people
                        join subject in subjects on student equals subject.Students
                        select new SetQuery(student.StudentName, subject.Name);

            ScienceResult(query);
        }

And add this method:

note in order to create the ScienceResult Method we need a SetQuery class instance.

public static void ScienceResult(IEnumerable<SetQuery> query)
        {
            Console.Write("Science: ");
            foreach (var result in query)
            {
                if (result.Name == "Science")
                {
                    Console.Write(result.StudentName + " ");
                }
            }

            Console.WriteLine();

            Console.Write("Math: ");
            foreach (var result in query)
            {
                if (result.Name == "Math")
                {
                    Console.Write(result.StudentName + " ");
                }
            }

            Console.WriteLine();
        }

Set Query Class Instance with Dependency Injection

internal class SetQuery
    {
        public string StudentName;
        public string Name;

        public SetQuery(string studentName, string name)
        {
            this.StudentName = studentName;
            this.Name = name;
        }
    }

Result is:

Science: Ben Andrew
Math: Andrew

For best practice i would use a database to save your information and to GET your information from. Also the method ScienceResult is NOT DRY you would wanna change that. this is just for illustration purpose.

For more info About DRY check the following: DRY (Don't Repeat Yourself) and if assignements

Answer 2

Not sure what you're looking for exactly, but best software practices be damned, here you go (assuming solution is called "stackoverflow61918396"):

Student.cs

using System.Collections.Generic;

namespace stackoverflow61918396
{
    class Student
    {
        public string Id { get; set; }

        public string Name { get; set; }

        public List<string> SubjectsEnrolledIn { get; set; }

        public Student()
        {
            SubjectsEnrolledIn = new List<string>();
        }
    }
}

Subject.cs

using System.Collections.Generic;

namespace stackoverflow61918396
{
    class Subject
    {
        public string Id { get; set; }

        public int ClassNumber { get; set; }

        public string Name { get; set; }

        public List<string> StudentsInSubject { get; set; }

        public Subject()
        {
            StudentsInSubject = new List<string>();
        }
    }
}

Manager.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace stackoverflow61918396
{
    class Manager
    {
        public static List<Student> Students { get; set; }

        public static List<Subject> Subjects { get; set; }

        public Manager()
        {
            Students = new List<Student>();
            Subjects = new List<Subject>();

            // Seed some students
            var ben = new Student() { Id = Guid.NewGuid().ToString(), Name = "Ben" };
            var andrew = new Student() { Id = Guid.NewGuid().ToString(), Name = "Andrew" };
            var amanda = new Student() { Id = Guid.NewGuid().ToString(), Name = "Amanda" };
            var greg = new Student() { Id = Guid.NewGuid().ToString(), Name = "Greg" };
            var peter = new Student() { Id = Guid.NewGuid().ToString(), Name = "Peter" };
            var jessica = new Student() { Id = Guid.NewGuid().ToString(), Name = "Jessica" };
            var james = new Student() { Id = Guid.NewGuid().ToString(), Name = "James" };
            var doug = new Student() { Id = Guid.NewGuid().ToString(), Name = "Doug" };
            var cory = new Student() { Id = Guid.NewGuid().ToString(), Name = "Cory" };
            var laura = new Student() { Id = Guid.NewGuid().ToString(), Name = "Laura" };
            Students.Add(ben);
            Students.Add(andrew);
            Students.Add(amanda);
            Students.Add(greg);
            Students.Add(peter);
            Students.Add(jessica);
            Students.Add(james);
            Students.Add(doug);
            Students.Add(cory);
            Students.Add(laura);

            // Seed a couple sections of math and science
            var math2 = new Subject() { Id = Guid.NewGuid().ToString(), Name = "Math", ClassNumber = 2 };
            var math4 = new Subject() { Id = Guid.NewGuid().ToString(), Name = "Math", ClassNumber = 4 };
            var science5 = new Subject() { Id = Guid.NewGuid().ToString(), Name = "Science", ClassNumber = 5 };
            var science3 = new Subject() { Id = Guid.NewGuid().ToString(), Name = "Science", ClassNumber = 3 };
            Subjects.Add(math2);
            Subjects.Add(math4);
            Subjects.Add(science5);
            Subjects.Add(science3);

            // Seed some enrollments
            Enroll(ben.Id, math2.Id);
            Enroll(andrew.Id, math2.Id);
            Enroll(andrew.Id, science5.Id);
            Enroll(amanda.Id, math2.Id);
            Enroll(peter.Id, math2.Id);
            Enroll(jessica.Id, math4.Id);
            Enroll(james.Id, math2.Id);
            Enroll(doug.Id, math4.Id);
            Enroll(doug.Id, science5.Id);
        }

        public Student AddStudent(string name)
        {
            var student = new Student() { Id = Guid.NewGuid().ToString(), Name = name };
            Students.Add(student);

            return student;
        }

        public Subject AddSubject(string name, int classNumber)
        {
            var subject = new Subject() { Id = Guid.NewGuid().ToString(), Name = name, ClassNumber = classNumber };
            Subjects.Add(subject);

            return subject;
        }

        public bool DeleteStudent(string id)
        {
            try
            {
                var student = Students.Where(s => s.Id == id).FirstOrDefault();

                if (student == null) return false;

                Students.Remove(student);

                return true;
            }
            catch (Exception)
            {
                return false;
            }
        }

        public bool DeleteSubject(string id)
        {
            try
            {
                var subject = Subjects.Where(s => s.Id == id).FirstOrDefault();

                if (subject == null) return false;

                Subjects.Remove(subject);

                return true;
            }
            catch (Exception)
            {
                return false;
            }
        }

        public bool Disenroll(string studentId, string subjectId)
        {
            var student = Students.Where(s => s.Id == studentId).First();
            var subject = Subjects.Where(s => s.Id == subjectId).First();

            try
            {
                student.SubjectsEnrolledIn.Remove(subjectId);
                subject.StudentsInSubject.Remove(studentId);
            }
            catch (Exception)
            {
                return false;
            }

            return true;
        }

        public Student EditStudent(string id, string name)
        {
            var student = Students.Where(s => s.Id == id).FirstOrDefault();

            if (student == null) return null;

            student.Name = name;

            return student;
        }

        public Subject EditSubject(string id, string name, int classNumber)
        {
            var subject = Subjects.Where(s => s.Id == id).FirstOrDefault();

            if (subject == null) return null;

            subject.Name = name;
            subject.ClassNumber = classNumber;

            return subject;
        }

        public bool Enroll(string studentId, string subjectId)
        {
            var student = Students.Where(s => s.Id == studentId).First();
            var subject = Subjects.Where(s => s.Id == subjectId).First();

            try
            {
                student.SubjectsEnrolledIn.Add(subjectId);
                subject.StudentsInSubject.Add(studentId);
            }
            catch (Exception)
            {
                return false;
            }

            return true;
        }

        public Student GetStudent(string id)
        {
            return Students.Where(s => s.Id == id).FirstOrDefault();
        }

        public Subject GetSubject(string id)
        {
            return Subjects.Where(s => s.Id == id).FirstOrDefault();
        }

        public IEnumerable<IGrouping<string, Tuple<string, int, string>>> GetSubjectAllStudents(string id)
        {
            var tupleList = new List<Tuple<string, int, string>>();

            var subjectName = Subjects.Where(s => s.Id == id).FirstOrDefault()?.Name;
            var subjectList = Subjects.Where(s => s.Name == subjectName).ToList();

            if (subjectName == null) return null;

            foreach (Subject subject in subjectList)
            {
                foreach (Student student in Students)
                {
                    if (subject.StudentsInSubject.Contains(student.Id))
                    {
                        tupleList.Add(new Tuple<string, int, string>(student.Name, subject.ClassNumber, "yes"));
                    }
                    else
                    {
                        tupleList.Add(new Tuple<string, int, string>(student.Name, subject.ClassNumber, "no"));
                    }
                }
            }

            var sortedTupleList = tupleList.GroupBy(x => x.Item1);

            return sortedTupleList;
        }

        public List<Subject> GetSubjectAlternatives(string id)
        {
            var subjectName = Subjects.Where(s => s.Id == id).FirstOrDefault()?.Name;

            if (subjectName == null) return null;

            return Subjects.Where(s => s.Name == subjectName).ToList();
        }

        public List<Student> ListStudents()
        {
            return Students;
        }

        public List<Subject> ListSubjects()
        {
            return Subjects;
        }
    }
}

Program.cs

using System;
using System.Collections.Generic;

namespace stackoverflow61918396
{
    class Program
    {
        static bool Again { get; set; }
        static Manager Manager { get; set; }

        static void Main()
        {            
            Again = true;
            Manager = new Manager();

            do
            {
                Console.WriteLine("Enter a command (type 'help' for options):");
                var selection = Console.ReadLine();
                Console.WriteLine("\n");

                CommandHandler(selection);
            } while (Again);
        }

        static void CommandHandler(string command)
        {
            switch (command)
            {
                case var s when s.Trim().ToLower() == "help":
                    Console.WriteLine("Commands:");
                    Console.WriteLine("'quit' - exits the program");
                    Console.WriteLine("'students' - lists all students");
                    Console.WriteLine("'subjects' - lists all subjects");                    
                    Console.WriteLine("'student [id]' - shows this student's record");
                    Console.WriteLine("'subject [id]' - shows this subject's record");
                    Console.WriteLine("'all students by subject [id]' - lists all students in this subject or not");
                    Console.WriteLine("'alternatives for subject [id]' - lists all class numbers of a subject");
                    Console.WriteLine("'new student [name]' - adds a new student");
                    Console.WriteLine("'new subject [name] [class number]' - adds a new subject");
                    Console.WriteLine("'edit student [id] [name]' - edits an existing student");
                    Console.WriteLine("'edit subject [id] [name] [class number]' - edits an existing subject");
                    Console.WriteLine("'delete student [id]' - deletes an existing student");
                    Console.WriteLine("'delete subject [id]' - deletes an existing subject");
                    Console.WriteLine("'enroll [student id] [subject id]' - enrolls a student into a subject");
                    Console.WriteLine("'disenroll [student id] [subject id]' - disenrolls a student from a subject");
                    Console.WriteLine("\n");
                    break;
                case var s when s.Trim().ToLower() == "quit":
                    Again = false;
                    break;
                case var s when s.Trim().ToLower() == "students":
                    try
                    {
                        List<Student> studentList = Manager.ListStudents();

                        if (studentList == null) throw new Exception();

                        Console.WriteLine("Students:");
                        foreach (Student student in studentList)
                        {
                            Console.WriteLine("Id: " + student.Id + ", Name: " + student.Name);
                        }
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower() == "subjects":
                    try
                    {
                        List<Subject> subjectList = Manager.ListSubjects();

                        if (subjectList == null) throw new Exception();

                        Console.WriteLine("Subjects:");
                        foreach (Subject subject in subjectList)
                        {
                            Console.WriteLine(
                                "Id: " + subject.Id +
                                ", Class Number: " + subject.ClassNumber +
                                ", Name: " + subject.Name
                            );
                        }
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("student"):
                    try
                    {
                        var studentId = s.Split(' ')[1];
                        var student = Manager.GetStudent(studentId);

                        if (student == null) throw new Exception();

                        Console.WriteLine("Id: " + student.Id + ", Name: " + student.Name);

                        Console.WriteLine("Subjects Enrolled In:");
                        foreach (string subjectId in student.SubjectsEnrolledIn)
                        {
                            var subject = Manager.GetSubject(subjectId);
                            Console.WriteLine(
                                "Id: " + subject.Id + 
                                ", Class Number: " + subject.ClassNumber + 
                                ", Name: " + subject.Name
                            );
                        }
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("subject"):
                    try
                    {
                        var subjectId = s.Split(' ')[1];
                        var subject = Manager.GetSubject(subjectId);

                        if (subject == null) throw new Exception();

                        Console.WriteLine("Id: " + subject.Id + ", Name: " + subject.Name);

                        Console.WriteLine("Students In Subject:");
                        foreach (string studentId in subject.StudentsInSubject)
                        {
                            var student = Manager.GetStudent(studentId);
                            Console.WriteLine(
                                "Id: " + student.Id +
                                ", Name: " + student.Name
                            );
                        }
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("all"):
                    try
                    {
                        var subjectId = s.Split(' ')[4];
                        var subjectName = Manager.GetSubject(subjectId)?.Name;
                        var subjectWithAll = Manager.GetSubjectAllStudents(subjectId);

                        if (subjectWithAll == null) throw new Exception();

                        Console.WriteLine("All students with respect to " + (subjectName ?? subjectId) + ":");
                        foreach (var group in subjectWithAll)
                        {
                            foreach (var item in group)
                            {
                                Console.WriteLine(
                                    "\tName: " + item.Item1 +
                                    ", \t\tClass Number: " + item.Item2 +
                                    ", \t\tEnrolled: " + item.Item3
                                );
                            }
                        }
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("alternatives"):
                    try
                    {
                        var subjectId = s.Split(' ')[3];
                        List<Subject> subjectAlternatives = Manager.GetSubjectAlternatives(subjectId);

                        if (subjectAlternatives == null) throw new Exception();

                        Console.WriteLine("Alternatives for " + subjectAlternatives[0].Name + ":");
                        foreach (Subject subject in subjectAlternatives)
                        {
                            Console.WriteLine("Id: " + subject.Id + ", Class Number: " + subject.ClassNumber);
                        }
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("new student"):
                    try
                    {
                        var name = s.Split(' ')[2];
                        var student = Manager.AddStudent(name);

                        if (student == null) throw new Exception();

                        Console.WriteLine("Student " + student.Id + " created");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("new subject"):
                    try
                    {
                        var name = s.Split(' ')[2];
                        var classNumber = s.Split(' ')[3];
                        var subject = Manager.AddSubject(name, int.Parse(classNumber));

                        if (subject == null) throw new Exception();

                        Console.WriteLine("Subject " + subject.Id + " created");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("edit student"):
                    try
                    {
                        var studentId = s.Split(' ')[2];
                        var studentName = s.Split(' ')[3];
                        var student = Manager.EditStudent(studentId, studentName);

                        if (student == null) throw new Exception();

                        Console.WriteLine("Student " + student.Id + " was edited");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("edit subject"):
                    try
                    {
                        var subjectId = s.Split(' ')[2];
                        var subjectName = s.Split(' ')[3];
                        var subjectClassNumber = s.Split(' ')[4];
                        var subject = Manager.EditSubject(subjectId, subjectName, int.Parse(subjectClassNumber));

                        if (subject == null) throw new Exception();

                        Console.WriteLine("Subect " + subject.Id + " was edited");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("delete student"):
                    try
                    {
                        var studentId = s.Split(' ')[2];
                        var deleted = Manager.DeleteStudent(studentId);

                        Console.WriteLine("Student was " + (deleted ? "" : "not") + " deleted");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("delete subject"):
                    try
                    {
                        var subjectId = s.Split(' ')[2];
                        var deleted = Manager.DeleteSubject(subjectId);

                        Console.WriteLine("Subject was " + (deleted ? "" : "not") + " deleted");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("enroll"):
                    try
                    {
                        var studentId = s.Split(' ')[1];
                        var subjectId = s.Split(' ')[2];
                        var enrolled = Manager.Enroll(studentId, subjectId);

                        Console.WriteLine("Student was " + (enrolled ? "" : "not") + " enrolled");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                case var s when s.Trim().ToLower().StartsWith("disenroll"):
                    try
                    {
                        var studentId = s.Split(' ')[1];
                        var subjectId = s.Split(' ')[2];
                        var disenrolled = Manager.Disenroll(studentId, subjectId);

                        Console.WriteLine("Student was " + (disenrolled ? "" : "not") + " disenrolled");
                        Console.WriteLine("\n");
                    }
                    catch (Exception)
                    {
                        Console.WriteLine("Unrecognized command. Please try again.");
                        Console.WriteLine("\n");
                    }
                    break;
                default:
                    Console.WriteLine("Unrecognized command. Please try again.");
                    Console.WriteLine("\n");
                    break;
            }
        }
    }
}