How to model many-to-many relationships in plain C#?

Question

First of all I am making it clear that, I am not using any OR Mapper framework to solve this problem, coz I need to understand the model from the ground up.

OK. Let's see the table designs:

Course - table

------------------
 ID | CourseName
------------------
 1  |  C++
 2  |  Java
------------------

Teacher - table

-----------------------
 ID |   TeacherName
-----------------------
 1  |   Professor X
 2  |   Professor Y  
-----------------------

CourseTeacher - table

---------------------------------------------
 ID | CourseID | TeacherID | ClassTime
---------------------------------------------
 1  |   1      |     1     | Monday    10:55
 2  |   1      |     2     | Thursday  10:55
 3  |   2      |     1     | Tuesday   11:45
 4  |   2      |     2     | Wednesday 11:45
---------------------------------------------

How should I design my classes in C# and using List<T>?

N.B. Please note the ID in the CourseTeacher - table. I think the main challenge of the class design is to maintain unique IDs in CourseTeacher - table.

N.B. Please note that there is also a requirement to manipulate data from within a TransactionContext.

There can be another problem like is:

User - table

------------------
 ID | UserName
------------------
 1  |  a
 2  |  b
------------------

Role - table

-----------------------
 ID |   RoleName
-----------------------
 1  |   c
 2  |   d
-----------------------

UserRole - table

---------------------------------------------
 ID | UserID | RoleID | Remarks
---------------------------------------------
 1  |   1      |     1     | xy
 2  |   1      |     2     | yz
 3  |   2      |     1     | zx
 4  |   2      |     2     | xx
---------------------------------------------

Answer 1

I thinkt that you actually have a hidden third concept there: a ClassSession, or something like that. Each ClassSession would have a Teacher, a Course and a DateTime.

public class ClassSession
{
    public Teacher Teacher { get; set; }

    public Course Course { get; set; }

    public DateTime Time { get; set; }
}

Answer 2

In general, when you have a many-to-many relationship in your domain model, the access patterns in your code will either be from one side of the M2M or from the other...

For e.g. If you have a structure containing people, each of who has made a number of purchases at your retail outlets, you might as a rule want to access those transactions thrpugh a person, not through the product. in other words, you might more often want to know what products a specific person purchased, not what people purchased a specific product. So in this example you would simply have each person class contain a property which is a collection of products. There would be no need to access the person class through the Product class.

Another e.g. If you have a a structure containing people, each of whom subscribes to a number of magazines.

Now if you are the publisher, you might as a rule want to access those transactions through the magazine, not through the person. in other words, you would much more often want to know what people subscribe to a specific magazine, not what magazines a specific person subscribes to. So in this example you would put a Subscribers collection property in the Magazine class.

Answer 3

My sample:

interface ICourse
{
    string Name { get; set; }
}

interface ITeacher
{
    string Name { get; set; }
}

interface IClassSession
{
    ICourse Course { get; set; }
    ITeacher Teacher { get; set; }
    DateTime Scheduled { get; set; }
}

interface ITransactionalEntity
{
    int ID { get; set; }
}

//sample class implementation
class Teacher : ITeacher, ITransactionalEntity
{
    private int _ID;
    public int ID
    {
        get { return _ID; }
        set { _ID = value; }
    }
    //implement interfaces
    public void Save(SqlTransaction tran)
    {
        if (this.ID > 0)
            //insert
        else
            //update
    }
}

class ClassSession : IClassSession, ITransactionalEntity
{
    //implement interfaces
    public void Save(SqlTransaction tran)
    {
        this.Course.Save(tran);
        this.Teacher.Save(tran);
        if (this.ID > 0)
            //insert
        else
            //update
    }
}

...
List<IClassSession> list = new List<IClassSession>();
ClassSession cs = new ClassSession();
cs.Course = new Course(courseID);
cs.Teacher = new Teacher(teacherID);
classlist.Add(cs);
SqlTransaction tran;
...
foreach(IClassSession classsession in list)
    classsession.Save(tran);
...

and you can do whatever you want, Linq it, etc. Use your imagination. Not everything is written here, just as an idea.