How to return selected item on listbox on the second form to the main form in visual basic? - vb.net

Question

So basically you have a main form that shows only a label (lblTotalPrice) and a button that when you click it, it opens a second form that contains a listbox with different prices packages (lstPackages). How do do you select an item from lstPackages that is on the second form and return it to lblTotalPrice, which is located on the main form?
I attempted this code using a function (thought it would be useful for calculations), but it seems like it didn't work:
On my second form, I populated the lstPackages with it's name using parallel arrays:
Dim strPackages As String = {"Package 1", "Package 2", "Package 3", "Package 4", "Package 5"}
Dim decPrice As String = {100D, 200D, 300D, 400D, 500D}
Private Sub SecondForm_Activated(sender As Object, e As EventArgs) Handles MyBase.Activate
Dim frmMain as New
Dim i As Integer
For i = 0 To strPackages.Length - 1
lstPackages.Items.Add(strPackages(i))
Next
End Sub
After I tried to select a value on lstPackages, but I just was not too sure if I was using the right code to select the item and return it to the main form:
Function CalcPackage()
Dim decPackages As Decimal = 0D
If lstPackages.SelectedIndex = -1 Then
MessageBox.Show("Select a package")
Else
For i = 0 To lstPackages.SelectedItem - 1
decPackages = lstPackages.SelectedItem(i)
Next i
End If
Return decPackages
End Function
When I have selected the item in the lstPackages, I tried to send the selected item back to the main form, but ran into a problem here:
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnClose.Click
Dim frmMainForm As New MainForm
Dim decTotal As Decimal
decTotal = CalcPackage()
'I was stuck in this part and wasn't sure how to return it to lblTotalPrice (and also close the second form too)
frmMainForm.lblTotalPrice.Text = decTotal.ToString("c")
frmMainForm.Show()
Me.Close()
End Sub
So yea sorry if my code is weird, but I'm hoping to get help on how to return the value of the item to lblTotalPrice on the main form, thanks guys

Answer 1

Part of your problem is that you are newing up an instance of the main form in the button exit click event handler of the second form, as shown in this code posted:
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnClose.Click
Dim frmMainForm As New MainForm
Dim decTotal As Decimal
decTotal = CalcPackage()
'I was stuck in this part and wasn't sure how to return it to lblTotalPrice (and also close the second form too)
frmMainForm.lblTotalPrice.Text = decTotal.ToString("c")
frmMainForm.Show()
Me.Close()
End Sub
Instead pass an instance of the main form to the second form via its constructor, like this:
Public Class SecondForm Inherits Form
Dim theMainForm As MainForm
Public Sub New(mainForm As MainForm)
theMainForm = mainForm
End Sub
End Class
Now you can reference the lblTotalPrice in the MainForm class via the theMainForm variable, like this:
theMainForm.lblTotalPrice.Text = lstPackages.SelectedItem
The theMainForm.lblTotalPrice.Text should only be updated in response to an item being selected from lstPackages not when the second form is closed; so handle the updating of the main form's label in the SelectedIndexChanged event of lstPackages, like this:
Protected Sub lstPackages_IndexChanged(sender As Object, e As EventArgs)
Dim decTotal As Decimal
decTotal = CalcPackage()
theMainForm.lblTotalPrice.Text = decTotal.ToString("c")
End Sub
All the exit button click event handler should do is to show the main form and close the second form, like this:
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnClose.Click
theMainForm.Show()
Me.Close()
End Sub

Answer 2

Just display SecondForm with ShowDialog(), then retrieve the value with the reference to it that you already have:
Public Class MainForm
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim sf As New SecondForm
If sf.ShowDialog = Windows.Forms.DialogResult.OK Then
lblTotalPrice.Text = sf.TotalPrice
End If
End Sub
End Class
Over in SecondForm:
Public Class SecondForm
Private _TotalPrice As Decimal
Public ReadOnly Property TotalPrice As Decimal
Get
Return _TotalPrice
End Get
End Property
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnClose.Click
If lstPackages.SelectedIndex = -1 Then
MessageBox.Show("Select a package")
Else
_TotalPrice = lstPackages.SelectedItem(0)
Me.DialogResult = Windows.Forms.DialogResult.OK
End If
End Sub
End Class