Storing & recovering 2-dimensional Array in a Class - arrays

Question

I just recently moved from VB6 to VB.NET and I'm recoding an old app. So I'm pretty unexperienced with .NET so far.
I have multiple (lets say 4 in this code example) twodimensional string arrays (or actually an array of arrays) which I want to store as a ComboBox items ie. one twodimensional array is one item.
Public Class MyItem
Private sName As String
Private sArr As Array()
Public Sub New(ByVal Name As String, ParamArray Arr As Array())
sName = Name
sArr = Arr
End Sub
Public Property Arr() As Array()
Get
Return sArr
End Get
Set(ByVal sValue As Array())
sArr = sValue
End Set
End Property
Public Overrides Function ToString() As String
Return sName
End Function
End Class
---
Dim sMyArray as Array()
For i As Integer = 0 to 3
sMyArray = Nothing ' resetting the array before refilling it
'
' No code here but filling sMyArray by reading a text file, each line
' number as dim 1 and splitted each line into dim 2 with ";" using Split(sRead, ";")
' so Debub.Print(sMyArray(0)(0)) prints the beginning of the first line until first ";" <- this works fine
'
' Then passing sMyArray to a ComboBox item
'
ComboBox.Items.Add(New MyItem("item" & i, sMyArray))
Next i
The problem is that when recovering the arrays from ComboCox items only the last ComboBox item has array data. So for example
Dim sMyNewArray As Array() = ComboBox.Items.Item(0).Arr
Debug.Print(sMyNewArray(0)(0))
throws an error while
Dim sMyNewArray As Array() = ComboBox.Items.Item(3).Arr
Debug.Print(UBound(sMyNewArray(UBound(sMyNewArray))))
does not and prints the last item's last row's ubound
Can anyone figure out what is it I'm missing or tell me a better way to do this? I'm pretty sure there is one..

Answer 1

I'm not 100% sure, but I think the problem is in this section:
Dim sMyArray as Array()
For i As Integer = 0 to 3
sMyArray = Nothing ' resetting the array before refilling it
Arrays are technically reference types, but like strings, there's some extra compiler magic to make them feel at times more like value types, and I have a sense in this case the actual sMyArray reference was used (perhaps because of a ParamArrays optimzation), such that setting it to Nothing broke things. The more idiomatic way to write this code for .Net it like this:
For i As Integer = 0 to 3
Dim sMyArray as Array()
.Net has a much more sophisticated garbage collector than was available for VB6. We don't often set variables to Nothing any more, but instead just re-assign them or let them fall out of scope. In fact, setting a variable to Nothing can in rare cases be actively harmful. Moreover, we want to see the Dim keyword inside the loop, so you're working with a different variable on each iteration, with the smallest possible scope.
While I'm here, in .Net we pretty much never use the base Array type. Instead of this:
Private sArr As Array()
You pretty much always do this:
Private arr As String()()
or this, for true two-dimensional (non-jagged) arrays:
Private arr As String(,)
or, best of all, this:
Private arr As New List(Of String())
Since VB.Net has more collection types than just array.
Also, I don't have the link handy, but Microsoft's coding guidelines now explicitly ask you not to use hungarian warts for variable and class names (so sArr can just be arr). This is a change from the VB6 era because of changes to the language where the type is more likely to be implicit with the variable and improvements to the tooling, where the prefixes usually no longer add much utility and have been shown to hurt readability.

Answer 2

Not really sure why you have a 2 dimensional array, but here is a small sample NOT using the Array type. It uses just plain strings and string arrays. Let me know if this helps. This splits a few strings, then reads out the results after populating.
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim sMyArray()() As String
Dim line1 As String = "a;b;c;d 1;2;3;4;5"
Dim line2 As String = "z;x;y;w 99;65;32;21;18"
sMyArray = ParseString(line1)
cboBox1.Items.Add(New MyItem("System0", sMyArray))
sMyArray = ParseString(line2)
cboBox1.Items.Add(New MyItem("System1", sMyArray))
For i As Integer = 0 To cboBox1.Items.Count - 1
For j As Integer = 0 To UBound(cboBox1.Items(i).arr)
For k As Integer = 0 To UBound(cboBox1.Items(i).arr(j))
Debug.Write(cboBox1.Items(i).arr(j)(k) & " ")
Next
Next
Debug.WriteLine("")
Next
End Sub
Private Function ParseString(s As String) As String()()
Dim rows As String() = s.Split(" ")
Dim matrix As String()() = New String(rows.Length - 1)() {}
For i As Integer = 0 To rows.Length - 1
matrix(i) = rows(i).Split(";")
Next
Return matrix
End Function
End Class
Public Class MyItem
Private sName As String
Private sArr As String()()
Public Sub New(ByVal Name As String, ByVal ParamArray Arr As String()())
sName = Name
sArr = Arr
End Sub
Public Property Arr() As String()()
Get
Return sArr
End Get
Set(ByVal sValue As String()())
sArr = sValue
End Set
End Property
Public Overrides Function ToString() As String
Return sName
End Function
End Class