Help with NULL error, when trying to add from a form, a list, to another form and list.

Question

Public Sub AddPrintBook()
Select Case ShoppingCart.lstCart.Items.Add(lstPrintBooks.SelectedIndex) '"""""Getting a NULL error here. """""

        Case 0
            strSelectedBook = strYourWay
            decSubTotal += decYourWay
            decShippingTotal += decShippingTotal
        Case 1
            strSelectedBook = strHistoryScotland
            decSubTotal += decHistoryScotland
            decShippingTotal += decShippingTotal
        Case 2
            strSelectedBook += decCalculus
            decSubTotal += decHistoryScotland
            decShippingTotal += decShippingTotal
        Case 3
            strSelectedBook = strRelax
            decSubTotal += decRelax
            decShippingTotal += decShippingTotal

    End Select

End Sub

Answer 1

Hi @Hekzdaddy ,
Take a look at the following example:
I have listbox1 on form1 and listbox1 on form2.
Form1:

Public Class Form1  
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load  
        Dim frm2 As Form2 = New Form2(Me)  
        frm2.Show()  
    End Sub  
End Class  

Form2:

Public Class Form2  
    Private ReadOnly frm1 As Form1  
    Public Sub New(ByVal form1 As Form1)  
        frm1 = form1  
        InitializeComponent()  
    End Sub  
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click  
        ListBox1.Items.Add(frm1.ListBox1.SelectedIndex)  
    End Sub  
End Class  

Best Regards,
Xingyu Zhao
*
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Answer 2

The following code sample demonstrates passing information from one form to another form along with using assertion and being pro-active to ensure no runtime issues. I didn't match up your current code, wanted to give you a clear example that is easy to understand and allow anyone to try it out. Feel free to tweak as needed but the idea is to simply learn and adapt to your code.

Full project source code

1 is parent form, 2 is the child form where in form2 pressing a button passes the current month name to the list box in the calling form.

Main form

Public Class Form1  
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles ShowChildForm.Click  
        Dim childForm As New Form2  
  
        AddHandler Form2.ListBoxSelectionChanged, AddressOf AddMonth  
  
        childForm.ShowDialog()  
  
        RemoveHandler Form2.ListBoxSelectionChanged, AddressOf AddMonth  
    End Sub  
  
    Private Sub AddMonth(monthName As String)  
        If MonthsListBox.FindStringExact(monthName, 0) = -1 Then  
            MonthsListBox.Items.Add(monthName)  
            MonthsListBox.SelectedIndex = MonthsListBox.Items.Count - 1  
        End If  
    End Sub  
  
    Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles Me.Shown  
        MonthsListBox.Sorted = True  
    End Sub  
  
    Private Sub GetCurrentMonthButton_Click(sender As Object, e As EventArgs) Handles GetCurrentMonthButton.Click  
        If MonthsListBox.SelectedIndex > -1 Then  
            MessageBox.Show($"{MonthsListBox.SelectedIndex} -> {MonthsListBox.Text}")  
        End If  
    End Sub  
End Class  
  

Child form

Imports System.Globalization  
  
Public Class Form2  
  
    Public Delegate Sub OnListBoxSelectDelegate(month As String)  
    Public Shared Event ListBoxSelectionChanged As OnListBoxSelectDelegate  
  
    Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load  
  
        MonthsListBox.DisplayMember = "MonthName"  
        MonthsListBox.DataSource = Enumerable.Range(1, 12).  
            Select(Function(index)  
                       Return New With {Key .MonthName =  
                        DateTimeFormatInfo.CurrentInfo.GetMonthName(index)}  
                   End Function).ToList()  
  
    End Sub  
    Private Sub SendToParentFormButton_Click(sender As Object, e As EventArgs) _  
        Handles SendToParentFormButton.Click  
  
        If MonthsListBox.SelectedIndex > -1 Then  
            RaiseEvent ListBoxSelectionChanged(MonthsListBox.Text)  
        End If  
    End Sub  
End Class