Declare and and define Property in VB

Question

I have two Class and I want to save my data into arrays form text box like this:
Students.Name(txtID.Text-1).MathMark = txtMark.Text
but I get error: Object reference not set to an instance of an object

my code is:

Dim StudentsNumber as Integer = txtstdnum.Text
Dim Students as New StudentsInf(StudentsNumber)

Students.Name(txtID.Text-1).MathMark = txtMark.Text


Public Class StudentsInf

    Private mName() As String

    Sub New(ByVal StudentNumbers As Integer)
        ReDim mName(StudentNumbers-1)
    End Sub

    Public Property Name(ByVal Index As Integer) As LessonsMark
        Get
            Return mName(Index)
        End Get
        Set(ByVal Value As LessonsMark)
            mName(Index) = Value
        End Set
    End Property

End Class

Public Class LessonsMark

    Private mMathMark() As Object

    Public Property MathMark() As Object
        Get
            Return mMathMark
        End Get
        Set(ByVal Value As Object)
            mMathMark = Value
        End Set
    End Property

End Class

rather than internal arrays, you could save each student to a `List(of Student)`; your code would be much easier to use and read — Ňɏssa Pøngjǣrdenlarp, Oct 15 '14 at 12:30
possible duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Ňɏssa Pøngjǣrdenlarp, Oct 15 '14 at 12:31
Exactly I write this code to know about my problem, What's your idea about a easier code ? — farzad89, Oct 16 '14 at 22:45
a student class would be a name and the mathmark; studentS would be a List)Of Student) to hold many student objects. — Ňɏssa Pøngjǣrdenlarp, Oct 16 '14 at 22:48
@Plutonix Yes,it's possible but I don't have only name in StudentInf Class, I need to write and read some another informations too, but I think about it. — farzad89, Oct 16 '14 at 23:20
it doesnt matter *what* is in student, it can have many many properties. your `StudentInf` is just a wrapper around an array pr set of arrays - it is inside out from how it is normally done today — Ňɏssa Pøngjǣrdenlarp, Oct 16 '14 at 23:24
[see this link](http://stackoverflow.com/a/23659023/1070452) for one, it mainly deal with seruialization but uses a `List(of T)` in the example — Ňɏssa Pøngjǣrdenlarp, Oct 17 '14 at 00:05
I got it, that's useful, I didn't use that never before. Can I define that as a public ? — farzad89, Oct 17 '14 at 00:26
yes, but the default is Friend which is usually enough unless you are using it from a Class Lib. the important part is keeping the data together for ONE student or animal or employee, then use a `List(Of T)` to hold a bunch of them — Ňɏssa Pøngjǣrdenlarp, Oct 17 '14 at 00:34

score 1 · Accepted Answer · answered Oct 15 '14 at 12:29

1

This:

Private mName() As String

needs to be:

Private mName() As LessonsMark

then you have to create the objects in your constructor, something like:

Sub New(ByVal StudentNumbers As Integer)
  ReDim mName(StudentNumbers - 1)
  For i As Integer = 0 To StudentNumbers - 1
    mName(i) = New LessonsMark()
  Next
End Sub

then it looks like your LessonsMark class is declaring an array of objects when it looks like it should be just a string property:

Public Class LessonsMark
  Private mMathMark As String

  Public Property MathMark As String
    Get
      Return mMathMark
    End Get
    Set(ByVal Value As String)
      mMathMark = Value
    End Set
  End Property
End Class

answered Oct 15 '14 at 12:29

LarsTech

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When I use 'Dim Students as New StudentsInf(StudentsNumber)' in another module to read my datas again ... I can't. I lose my datas. what can I do ? – farzad89 Oct 16 '14 at 22:33
@farzad89 useing *new* creates a new instance, so you would have to reference the existing variable. – LarsTech Oct 16 '14 at 22:39
Exactly I want to write and read my datas for each student as a array, and yes when I useing _new_ it creates a new instance. What can I do ? – farzad89 Oct 16 '14 at 22:50
@farzad89 You would have to pass the Students array to whatever class or function you are calling. – LarsTech Oct 16 '14 at 23:00
That's not possible for me to pass the students array to a Class in every module. I need **Students.Name(ID).MathMark** as a public data ? – farzad89 Oct 16 '14 at 23:17
@farzad89 Then make it public shared, but that's a code smell. Global variables lead to tough to debug errors and is widely considered a bad programming practice. – LarsTech Oct 16 '14 at 23:20

Declare and and define Property in VB

1 Answers1