Q: How do I import values to a ListView on a SubForm?

[joe_pool_is]

I am having a problem with ListViews. Here are the specifics of my problem:

Objective:

Pop up ShowLayers dialog box to allow input of new layers into ShowLayers.lsvLayers listbox. When the ShowLayers form is closed, the values in ShowLayers.lsvLayers save to Form1.lsvLayers (the main form).

Problem:

When I call the ShowLayers dialog box again from Form1, I don't know the correct way to get the Form1.lsvLayers values back into my ShowLayers.lsvLayers listbox.

Please help.

First Attempt:

   Private Sub frmShowLayers_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
       ' // Load any existing layers...
       Me.lsvLayers = Form1.lsvLayers
   End Sub

Problem with code above:

The imported layers do not show up on ShowLayers.lsvLayers listbox, and any other layers added to the listbox do not show up either. However, ShowLayers.lsvLayers.Count is correct!

 

Second Attempt:

   Private Sub frmShowLayers_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
       ' // Load any existing layers...
       Dim li As ListViewItem
       If Form1.lsvLayers.Items.Count > 0 Then
           For Each li In Form1.lsvLayers.Items
               lsvLayers.Items.Add(li)
           Next
       End If
   End Sub

Problem with above code:

Runtime gives me the Message:

An unhandled exception of type 'System.ArgumentException' occurred in system.windows.forms.dll

Additional information: Cannot add or insert the item 'Layer01' in more than one place. You must first remove it from its current location or clone it.

Calling the form is done via:

dim objShow as New frmShow

objShow.ShowDialog

How would I remove the item if the form has been created from New?

[Joe Mamma]

Not going to give you the answer here as you won't learn and learning is half the fun. ;)

 

 

but some questions to be asked are,

 

what are value types? what are reference types?

what type is Me.lsvLayers?

what methods does Me.lsvLayers have?

 

answer these questions and you will have your answer:)

[joe_pool_is]

I understand the difference between passing ByVal as opposed to ByRef. As to which Me.lsvLayers is, I would have to guess ByRef. How would I test this theory? (BTW: new to VB) I've spend a couple of days trying to figure this out on my own before replying again, but your answer has only left me more confused.

 

If one version of my program has ByRef variables and another has ByVal variables, what prevents assigning one's values to another? Why is this in place?