A ListBox conundrum - Xtreme Visual Basic Talk

07-10-2012, 08:00 AM

KrisDwyer

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Join Date: Jun 2008

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A ListBox conundrum

My apologies guys, I'm trying to learn vb.net as I go (I was semi-literate in VB6) what with all its nuances and all.

Ok, here's what I want to do and where I've come into a bit of confusion about listboxes.

Ok, I have a set of players in an array called Player(x,y) - x being the players ID, y being a set attribute of the player - in this case element 1 being the last name.

I wish to set up a listbox on launch will populate any given number of Players, that will allow the user to pick a player using the listbox - by name, rather than by ID# - and have it show the selected players stats.

My issue is the population and retrieving.

Yes, I am aware of how to attach individual items to a listbox (listbox.items.add), but is there anyway to populate the listbox items with an additional value?

E.g Player(1,1) Might be ID#1 and the name might be Reynolds - is it possible for an item to be added as 'Reynolds' with a value of 1?

Thanks in advance?

07-10-2012, 08:15 AM

DrPunk

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You can put anything in a listbox. Not just a string.

So you can add any class you want to a listbox and the listbox will use the class's ToString method to populate the listbox with.

Code:

Public Class Player
    Public ID as Integer
    Public FirstName as String
    Public LastName as string

    Public Function FullName as String
        Return FirstName & " " & LastName
    End Sub

    Public Overrides Function ToString() As String
        Return FullName
    End Function

    Public Sub New(byval id as integer, byval firstName as string, byval lastName as string)
        me.id = id
        me.firstname = firstname
        me.lastname = lastname
    End Sub
End Class

Then, a form with a listbox showing how it's used...

Code:

Public Class Form1
    
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim p As Player

        p = New Player(1, "Bob", "Smith")
        ListBox1.Items.Add(p)

        p = New Player(2, "Ted", "Rogers")
        ListBox1.Items.Add(p)

        p = New Player(3, "John", "Manford")
        ListBox1.Items.Add(p)
    End Sub


    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        Dim p As Player = CType(ListBox1.SelectedItem, Player)
        MessageBox.Show(p.ID & ", " & p.FirstName & ", " & p.LastName)
    End Sub
End Class

The messagebox on selecting an item demonstrates that you can get to all the properties of the class that was added to the listbox.

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07-10-2012, 08:36 AM

KrisDwyer

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Join Date: Jun 2008

Posts: 20

Thanks so much for that, very insightful!

07-10-2012, 10:45 AM

KrisDwyer

Newcomer

Join Date: Jun 2008

Posts: 20

Object reference not set to an instance of an object

I keep getting that however when I'm trying to access the messagebox though. Albeit, I've used the code on a button, not a listbox - albeit on the same form.

07-10-2012, 04:31 PM

snarfblam

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Posts: 866

You should post the code that's throwing the error. I would imagine the null reference is within the expression that you pass to the message box.

Or better yet, set a breakpoint on the line that throws the error, and examine the variables to see which one is null.

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07-10-2012, 10:44 PM

KrisDwyer

Newcomer

Join Date: Jun 2008

Posts: 20

Quote:

Originally Posted by snarfblam

I've set up the listbox to be populated, and the PlayerList Class is in a public module.

Below is the code pulling up the error however when I'm clicking a button to retrieve the information from the listbox.

Dim p As PlayerList = CType(LBP.SelectedItem, PlayerList)
MessageBox.Show(p.ID & ", " & p.FirstName & ", " & p.LastName)

It would seem p is totally NULL, after setting the breakpoint, which is confusing.

07-10-2012, 10:47 PM

KrisDwyer

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Join Date: Jun 2008

Posts: 20

I've also traced this back to LBP.SelectedItem coming up as 'Nothing' - I don't know how this is possible as there is something selected within the listbox.

EDIT: I have discovered you have to CLICK the listbox to get the value for .selected item - is there a way where you can get the value if it is not 'clicked'.

Cheers.

Last edited by KrisDwyer; 07-10-2012 at 11:21 PM.

07-11-2012, 06:05 AM

Cerian Knight

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Location: Michigan

Posts: 3,739

Maybe try something like:

Code:

If LBP.SelectedItem Is Nothing Then
    If LBP.Items.Count > 0 Then
        LBP.SelectedIndex = LBP.TopIndex
    Else
        'Throw Error
    End If
End If

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