vb,net my collection property in my class is throwing an error - vb.net

Question

I am learning about collections, I have a Person class
Imports System
Imports System.Collections.Generic
Imports System.Text
Public Class Person
Public Sub New()
End Sub
Public Sub New(ByVal id As Integer, ByVal first_name As String, ByVal mid_name As String, ByVal last_name As String, ByVal age As Short, ByVal sex As Char)
Me.p_id = id
Me.first_name = first_name
Me.mid_name = mid_name
Me.last_name = last_name
Me.p_age = age
Me.p_sex = sex
End Sub
Private p_id As Integer = -1
Private first_name As String = String.Empty
Private mid_name As String = String.Empty
Private last_name As String = String.Empty
Private p_age As Short = 0
Private p_sex As Nullable(Of Char) = Nothing
Public Property ID() As Integer
Get
Return p_id
End Get
Set(ByVal value As Integer)
p_id = value
End Set
End Property
Public Property FirstName() As String
Get
Return first_name
End Get
Set(ByVal value As String)
first_name = value
End Set
End Property
Public Property MiddleName() As String
Get
Return mid_name
End Get
Set(ByVal value As String)
mid_name = value
End Set
End Property
Public Property LastName() As String
Get
Return last_name
End Get
Set(ByVal value As String)
last_name = value
End Set
End Property
Public Property Age() As Short
Get
Return p_age
End Get
Set(ByVal value As Short)
p_age = value
End Set
End Property
Public Property Sex() As Nullable(Of Char)
Get
Return p_sex
End Get
Set(ByVal value As Nullable(Of Char))
p_sex = value
End Set
End Property
End Class
and an Employee Class where I define a Person property
Imports System
Imports System.Collections.Generic
Imports System.Text
Public Class Employee
Public Sub New()
End Sub
Public Sub New(ByVal id As Integer, ByVal companyName As String, ByVal office As String, colPerson As Person)
'Me._employeeid = id
'Me._companyName = companyName
'Me._office = office
End Sub
Private _employeeid As Integer = -1
Private _companyName As String = String.Empty
Private _office As String = String.Empty
Public Property Empoyee_ID() As Integer
Get
Return _employeeid
End Get
Set(ByVal value As Integer)
_employeeid = value
End Set
End Property
Public Property CompanyName() As String
Get
Return _companyName
End Get
Set(ByVal value As String)
_companyName = value
End Set
End Property
Public Property Office() As String
Get
Return _office
End Get
Set(ByVal value As String)
_office = value
End Set
End Property
Property colPerson As List(Of Person)
End Class
How can i populate the persons class as well
Sub Main()
Dim pList As List(Of Person) = New List(Of Person)()
Dim thePerson As New List(Of Person) From
{
New Person With {.Age = 29, .FirstName = "John", .LastName = "Shields", .MiddleName = "", .Sex = "M", .ID = 1},
New Person With {.Age = 34, .FirstName = "Mary", .LastName = "Matthew", .MiddleName = "L", .Sex = "F", .ID = 2},
New Person With {.Age = 55, .FirstName = "Amber", .LastName = "Carl", .MiddleName = "P", .Sex = "M", .ID = 3},
New Person With {.Age = 12, .FirstName = "Kathy", .LastName = "Berry", .MiddleName = "O", .Sex = "F", .ID = 4}
}
'pList.Add(New Person(1, "John", "", "Shields", 29, "M"c))
'pList.Add(New Person(2, "Mary", "Matthew", "Jacobs", 35, "F"c))
'pList.Add(New Person(3, "Amber", "Carl", "Agar", 25, "M"c))
'pList.Add(New Person(4, "Kathy", "", "Berry", 21, "F"c))
'pList.Add(New Person(5, "Lena", "Ashco", "Bilton", 33, "F"c))
'pList.Add(New Person(6, "Susanne", "", "Buck", 45, "F"c))
'pList.Add(New Person(7, "Jim", "", "Brown", 38, "M"c))
'pList.Add(New Person(8, "Jane", "G", "Hooks", 32, "F"c))
'pList.Add(New Person(9, "Robert", "", "", 31, "M"c))
'pList.Add(New Person(10, "Cindy", "Preston", "Fox", 25, "F"c))
'pList.Add(New Person(11, "Gina", "", "Austin", 27, "F"c))
'pList.Add(New Person(12, "Joel", "David", "Benson", 33, "M"c))
'pList.Add(New Person(13, "George", "R", "Douglas", 55, "M"c))
'pList.Add(New Person(14, "Richard", "", "Banks", 22, "M"c))
'pList.Add(New Person(15, "Mary", "C", "Shaw", 39, "F"c))
'
'loop through the list
' PrintOnConsole(pList, "1. --- Looping through all items in the List<T> ---")
'
'Filtering List(T) using a single condition - (Age > 35)
'Dim filterOne As List(Of Person) = pList.FindAll(Function(p As Person) p.Age > 35)
'PrintOnConsole(filterOne, "2. --- Filtering List<T> on single condition (Age > 35) ---")
''
'' Filtering List(T) on multiple conditions (Age > 35 and Sex is Female)
'Dim filterMultiple As List(Of Person) = pList.FindAll(Function(p As Person) p.Age > 35 AndAlso p.Sex = "F"c)
'PrintOnConsole(filterMultiple, "3. --- Filtering List<T> on multiple conditions (Age > 35 and Sex is Female) ---")
''
''Sorting List(T) (Sort on FirstName)
'Dim sortFName As List(Of Person) = pList
'sortFName.Sort(Function(p1 As Person, p2 As Person) p1.FirstName.CompareTo(p2.FirstName))
'PrintOnConsole(sortFName, "4. --- Sort List<T> (Sort on FirstName) ---")
'
'Sorting List(T) descending (Sort on LastName descending)
'Dim sortLNameDesc As List(Of Person) = pList
'sortLNameDesc.Sort(Function(p1 As Person, p2 As Person) p2.LastName.CompareTo(p1.LastName))
'PrintOnConsole(sortLNameDesc, "5. --- Sort List<T> descending (Sort on LastName descending) ---")
''Add new List(T) to existing List(T)
'Dim newList As List(Of Person) = New List(Of Person)()
'newList.Add(New Person(16, "Geoff", "", "Fisher", 29, "M"c))
'newList.Add(New Person(17, "Samantha", "Carl", "Baxer", 32, "F"c))
'pList.AddRange(newList)
'PrintOnConsole(pList, "6. --- Add new List<T> to existing List<> ---")
''Remove multiple items from List(T) based on condition (remove male employees)
'Dim removeList As List(Of Person) = pList
'removeList.RemoveAll(Function(p As Person) p.Sex = "M"c)
'PrintOnConsole(removeList, "7. --- Remove multiple items from List<> based on condition ---")
'' Create Read Only List(T)
'Console.WriteLine("Create Read Only List<>")
'Dim personReadOnly As IList(Of Person) = pList
'Console.WriteLine("Before - Is List Read Only? True or False : " & personReadOnly.IsReadOnly)
'personReadOnly = pList.AsReadOnly()
'Console.WriteLine("After - Is List Read Only? True or False : " & personReadOnly.IsReadOnly & "</br>")
'
'Dim pList1 As New Person
Dim emp As New List(Of Employee)
Dim r As New Employee
r.CompanyName = "zac"
r.Office = "home"
r.Empoyee_ID = 1
'Dim pList1 = New Person(1, "John", "", "Shields", 29, "M"c)
r.colPerson.Add(New Person(1, "John", "", "Shields", 29, "M"c))---> Gives error
emp.Add(r)
'
Dim i As New Employee
i.CompanyName = "zac1"
i.Office = "home1"
i.Empoyee_ID = 2
i.colPerson.Add(New Person(3, "Amber", "Carl", "Agar", 25, "M"c))
pList.Add(New Person(4, "Kathy", "", "Berry", 21, "F"c))
emp.Add(i)
'
Dim t As New Employee
t.CompanyName = "zac2"
t.Office = "home2"
t.Empoyee_ID = 2
pList.Add(New Person(5, "Lena", "Ashco", "Bilton", 33, "F"c))
pList.Add(New Person(6, "Susanne", "", "Buck", 45, "F"c))
emp.Add(t)
For Each item In emp
'item.CompanyName = "zac"
'item.Office = "home"
'item.colperson.Where(Function(x) x.ID = 17)
'Console.WriteLine("employee with person collection: " & item.CompanyName & " " & item.Office & " " & item.colperson.Where(Function(x) x.ID = 17).ToString & "</br>")
Console.WriteLine("employee with person collection: " & item.CompanyName & " " & item.Office & "</br>")
Next
End Sub

Answer 1

r.colPerson.Add(New Person(1, "John", "", "Shields", 29, "M"c))---> Gives error
It gives an error because even though colPerson is declared to be capable of holing a list of people, it hasn't actually been set to be a list of people, so it's currently Nothing, and you can't call methods on something that is Nothing
Property colPerson As New List(Of Person)
^^^
Add a New directive to ensure it's declared and initialized to an instance of a List
Also, please:
Don't put "col" in names
Use a plural name for List(Of Thing) - this is a list of person so it should at least be called People, but also perhaps state what kind of people they are. For example if this Employee was recommended by a few people, call it RcommendedByPeople
Only use Collection in a name if you're writing a class that is a collection, such as Microsoft did when they wrote MatchCollection - a collection of regular expression Matches
Be definite about whether the property is public, private etc
i.e.
Public Property RcommendedByPeople As New List(Of Person)