Sharing View Model instance in different collections - vb.net

Question

Are there best practices for creating and sharing a View Model instance between different collection objects? For example, here's a scenario of School and Class classes that both have collections of Student objects:
Public Class Student
Public Property FirstName As String
Public Property LastName As String
Public Property Id As Integer
End Class
Public Class [Class]
Public Property Id As Integer
Public Property Name As String
Public Property Students As New Dictionary(Of Integer, Student)
Public Event StudentAdded(sender As Object, e As StudentEventArgs)
Public Sub AddStudent(student As Student)
If Not Students.ContainsKey(student.Id) Then
Students.Add(student.Id, student)
RaiseEvent StudentAdded(Me, New StudentEventArgs(student))
End If
End Sub
End Class
Public Class School
Public Property Classes As New Dictionary(Of Integer, [Class])
Public Property Students As New Dictionary(Of Integer, Student)
Public Event ClassAdded(sender As Object, e As EventArgs)
Public Event StudentAdded(sender As Object, e As StudentEventArgs)
Public Sub AddClass([class] As [Class])
If Not Classes.ContainsKey([class].Id) Then
Classes.Add([class].Id, [class])
RaiseEvent ClassAdded(Me, EventArgs.Empty)
End If
End Sub
Public Sub AddStudent(student As Student)
If Not Students.ContainsKey(student.Id) Then
Students.Add(student.Id, student)
RaiseEvent StudentAdded(Me, New StudentEventArgs(student))
End If
End Sub
End Class
Public Class StudentEventArgs
Inherits EventArgs
Public Property Student As Student
Public Sub New(student As Student)
Me.Student = student
End Sub
End Class
When creating a View Model for School and Class, both could end up creating View Models for the same Student objects:
Public Class SchoolViewModel
Inherits BaseViewModel
Private WithEvents _school As School
Public Sub New(school As School)
_school = school
End Sub
Public Property Students As New ObservableCollection(Of StudentViewModel)
Public Property Classes As New ObservableCollection(Of ClassViewModel)
Private Sub _school_StudentAdded(sender As Object, e As StudentEventArgs) Handles _school.StudentAdded
' Create a new View Model for the Student
Students.Add(New StudentViewModel(e.Student))
End Sub
End Class
Public Class ClassViewModel
Inherits BaseViewModel
Private WithEvents _class As [Class]
Public Sub New([class] As [Class])
_class = [class]
End Sub
Public Property Students As New ObservableCollection(Of StudentViewModel)
Private Sub _class_StudentAdded(sender As Object, e As StudentEventArgs) Handles _class.StudentAdded
' Create a new View Model or try to get the one created by the SchoolViewModel?
Students.Add(New StudentViewModel(e.Student))
End Sub
End Class
Public Class StudentViewModel
Inherits BaseViewModel
Private _student As Student
Public Sub New(student As Student)
_student = student
With _student
Me.FirstName = .FirstName
Me.LastName = .LastName
Me.Id = .Id
End With
End Sub
Public Property FirstName As String
Public Property LastName As String
Public Property Id As Integer
End Class
My question is regarding the creation of StudentViewModel instances in both SchoolViewModel and ClassViewModel StudentAdded event handlers. Let's assume that the students are first added to the School and then assigned to their classes. The StudentViewModel is pretty simple and creating duplicates might not be a problem but it nonetheless seems wasteful when a View Model for the same student has already been created in the SchoolViewModel.
If I want to share the SchoolViewModel's StudentViewModel, what's a best practices approach for that?
Passing a reference to SchoolViewModel's Students ObservableCollection when creating a new ClassViewModel?
Creating some GetStudentViewModelDelegate function?
Creating a Shared collection of StudentViewModel?
I usually try to limit references between objects and creating new View Models would be a tradeoff for that in this example.

Answer 1

Although I can't help specifically in VB, but you may be best looking into GENERICS.
https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/data-types/generic-types
It is a way to create more of a top-level class where the internal operations represent a given "type". Ex: Student, School, Subject (instead of a Class type named [Class]. I just see ALL types of confusion downstream.
The article covers some basics, but I think is what you are looking for. You would want the context of things and functions generic too, so no matter what "type" you want, you can nest common functionality.
For example, for each of your student, class, school, you have an "AddStudent", "AddClass", "AddSchool", or similar. If you have a generic with an "Add" method, then it would have the "Add" place-holder / operations once. Then, if you have this ViewModel working as type "Student", and another of "School" that HAS an instance of ViewModel "Student", you could do something like
SchoolObject.StudentObject.Add()
vs
SchoolObject.Add()
The "Of" type can also be declared as a class type that qualifies based on an "Interface" or some parentClassType too. So, if you have a base-class that is common to all Student, Class (Subject), School, then you can generic apply to the INTERFACE. So, please forgive my basic capacities at VB.
https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/interfaces/walkthrough-creating-and-implementing-interfaces
Hopefully this may help you in utilizing a concept to better fit your need that you just were not familiar with.