Error when removing selected items from a listbox

Question

Scenario

I have 2 listboxes 1 for an order and another for the total cost. To make it user to use i used the following code to link up the selected items.

Dim Selecting As Boolean = False
Private Sub lstOrders_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lstOrders.SelectedIndexChanged
    If Not Selecting Then
        Selecting = True
        lstTotalsEachOrder.SetSelected(lstOrders.SelectedIndex, True)
        Selecting = False
    End If
End Sub
Private Sub lstTotalsEachOrder_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lstTotalsEachOrder.SelectedIndexChanged
    If Not Selecting Then
        Selecting = True
        lstOrders.SetSelected(lstTotalsEachOrder.SelectedIndex, True)
        Selecting = False
    End If
End Sub

The problem comes in when i attempt to remove an item use the following code.

        lstOrders.Items.RemoveAt(lstOrders.Items.Count - 1)
        lstTotalsEachOrder.Items.RemoveAt(lstTotalsEachOrder.Items.Count - 1)

The Problem

It works fine removing items, but when it reaches a selected item it throws the error: InvalidArgument=Value of '-1' is not valid for 'index'.

I attempted to fix this using the following code

lstOrders.ClearSelected()
lstTotalsEachOrder.ClearSelected()

But i still get the error.

Any help would be appreciated.

Answer 1

Are you sure that the problem is with the selected items? Seems to me that you are getting this exception after you try to remove items from an empty ListBox .

Use a simple condition to make sure the ListBox actually have items inside:

If lstOrders.Items.Count > 0 Then 
    lstOrders.Items.RemoveAt(lstOrders.Items.Count - 1)
End If
If lstTotalsEachOrder.Items.Count > 0 Then 
    lstTotalsEachOrder.Items.RemoveAt(lstTotalsEachOrder.Items.Count - 1)
End If

Of course, you can wrap it all in a sub like this:

Sub RemoveLastItem(byVal lst as ListBox)
    If lst.Items.Count > 0 Then 
        lst.Items.RemoveAt(lst.Items.Count - 1)
    End if
End Sub

and then simply call this sub from your code:

RemoveLastItem(lstOrders)
RemoveLastItem(lstTotalsEachOrder)

Answer 2

As your Error message says:

InvalidArgument=Value of '-1' is not valid for 'index'.

Which indicates that you are trying to remove the ListItem @ -1th index. which is not present in the collection. I hope that this will happens only when you are trying to delete some items when the list box is empty(ie., lstOrders.Items.Count=0 )

hence lstOrders.Items.RemoveAt(lstOrders.Items.Count - 1) it result in removing the item @ -1th index.

And also you have to notice that your code will always delete the last item from the Listbox.

My suggestion: You can avoid this by using the above suggestion, or in simple you can use [SelectedIndex][1] property of a ListBox to find and deleted the selected value.

Answer 3

I found that the problem occured because there was sum sort of mismatch in the code. The error was thrown in the following code

 Dim Selecting As Boolean = False
Private Sub lstOrders_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lstOrders.SelectedIndexChanged
    If Not Selecting Then
        Selecting = True
        lstTotalsEachOrder.SetSelected(lstOrders.SelectedIndex, True)
        Selecting = False
    End If
End Sub
Private Sub lstTotalsEachOrder_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lstTotalsEachOrder.SelectedIndexChanged
    If Not Selecting Then
        Selecting = True
        lstOrders.SetSelected(lstTotalsEachOrder.SelectedIndex, True)
        Selecting = False
    End If
End Sub

So i though if i attempted to deselect the times then remove the item it would be fine so i wrote this before removing the items

lstOrders.ClearSelected()
lstTotalsEachOrder.ClearSelected()

But still the error occured, so i attached the following code to a button and this was also triggering the error. So i fixed this by doing the following,

    Selecting = True

    lstOrders.ClearSelected()
    lstTotalsEachOrder.ClearSelected()

    Selecting = False

This allowed me to deselect the items and remove them from the listbox's.