!!! Design for user control
<%@ Control Language="vb" AutoEventWireup="false" CodeBehind="ucMembership.ascx.vb" Inherits="MembershipTest.ucMembership" %>

<%@ Register Assembly="ISNet.WebUI.WebEssentials" Namespace="ISNet.WebUI.WebEssentials" TagPrefix="ISWebEssentials" %>

<asp:Table ID="tblMembership" runat="server">
    <asp:TableRow>
        <asp:TableCell RowSpan="4">
            <ISWebEssentials:WebListBox ID="wlbAvailableItems" runat=server Width="45%" Height="100%" SelectedIndex="-1" SelectionMode="Multiple" DefaultStyleMode="Win7"
                EnableKeyboardSupport="true">
                <LayoutSettings ItemHeight="20" ScrollMode="ScrollBar" />
            </ISWebEssentials:WebListBox>
        </asp:TableCell>
        <asp:TableCell>
            <asp:Button ID="btnMoveRight" runat="server" Text =">" Width="30px" />
        </asp:TableCell>
        <asp:TableCell RowSpan="4">
            <ISWebEssentials:WebListBox ID="wlbSelectedItems" runat=server Width="45%" Height="100%" SelectedIndex="-1" SelectionMode="Multiple" DefaultStyleMode="Win7"
                EnableKeyboardSupport="true">
                <LayoutSettings ItemHeight="20" ScrollMode="ScrollBar" />
            </ISWebEssentials:WebListBox>
        </asp:TableCell>
    </asp:TableRow>
    <asp:TableRow>
        <asp:TableCell>
            <asp:Button ID="btnMoveAllRight" runat="server" Text =">>" Width="30px" />
        </asp:TableCell>
    </asp:TableRow>
    <asp:TableRow>
       <asp:TableCell>
            <asp:Button ID="btnMoveLeft" runat="server" Text ="<" Width="30px" />
        </asp:TableCell>
    </asp:TableRow>
    <asp:TableRow>
        <asp:TableCell>
            <asp:Button ID="btnMoveAllLeft" runat="server" Text ="<<" Width="30px" />
        </asp:TableCell>
    </asp:TableRow>
</asp:Table>

!!! Code behind for user control

Imports ISNet.WebUI.WebEssentials

Public Class ucMembership
    Inherits System.Web.UI.UserControl

    Public Property ControlHeight() As Double
        Get
            Return CType(ViewState("ControlHeight"), Double)
        End Get
        Set(ByVal value As Double)
            ViewState("ControlHeight") = value
        End Set
    End Property

    Public Property ControlWidth() As Double
        Get
            Return CType(ViewState("ControlWidth"), Double)
        End Get
        Set(ByVal value As Double)
            ViewState("ControlWidth") = value
        End Set
    End Property

    Public Property DefaultStyleMode() As WebListBoxDefaultStyleMode
        Get
            Return CType(ViewState("DefaultStyleMode"), WebListBoxDefaultStyleMode)
        End Get
        Set(value As WebListBoxDefaultStyleMode)
            ViewState("DefaultStyleMode") = value
        End Set
    End Property

    Private Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load

        If Not Page.IsPostBack Then
            If ControlHeight.ToString <> "" Then
                tblMembership.Height = New System.Web.UI.WebControls.Unit(ControlHeight, UnitType.Pixel)
                wlbAvailableItems.Height = New System.Web.UI.WebControls.Unit(ControlHeight, UnitType.Pixel)
                wlbSelectedItems.Height = New System.Web.UI.WebControls.Unit(ControlHeight, UnitType.Pixel)
            End If

            If ControlWidth.ToString <> "" Then
                tblMembership.Width = New System.Web.UI.WebControls.Unit(ControlWidth, UnitType.Pixel)
                wlbAvailableItems.Width = New System.Web.UI.WebControls.Unit(ControlWidth * 0.45, UnitType.Pixel)
                wlbSelectedItems.Width = New System.Web.UI.WebControls.Unit(ControlWidth * 0.45, UnitType.Pixel)
            End If

            If DefaultStyleMode.ToString <> "" Then
                wlbAvailableItems.DefaultStyleMode = DefaultStyleMode
                wlbSelectedItems.DefaultStyleMode = DefaultStyleMode
            End If
        End If

    End Sub

    Public Sub SetAvailableItems(ByVal ItemList As Collection)

        Dim x As WebListBoxItem
        Dim ListItem As ListItem

        wlbAvailableItems.Items.Clear()

        For Each ListItem In ItemList

            x = New WebListBoxItem
            x.Text = ListItem.Text
            x.Value = ListItem.Value

            wlbAvailableItems.Items.Add(x)

        Next

    End Sub

    Public Sub SetSelectedItems(ByVal ItemList As Collection)

        Dim x As WebListBoxItem
        Dim ListItem As ListItem

        wlbSelectedItems.Items.Clear()

        For Each ListItem In ItemList

            x = New WebListBoxItem
            x.Text = ListItem.Text
            x.Value = ListItem.Value

            wlbSelectedItems.Items.Add(x)

        Next

    End Sub

    Private Sub btnMoveRight_Click(sender As Object, e As EventArgs) Handles btnMoveRight.Click

        Dim ListItem As WebListBoxItem
        Dim SelectedItems As List(Of String)
        Dim SelectedValue As String
        Dim i As Integer
        Dim Moved As Boolean

        SelectedItems = wlbAvailableItems.GetValues()

        For Each SelectedValue In SelectedItems

            For Each ListItem In wlbAvailableItems.Items
                If ListItem.Value.ToLower = SelectedValue.ToLower Then
                    wlbSelectedItems.Items.Add(ListItem)
                    Exit For
                End If
            Next
        Next

        wlbAvailableItems.SetValues(New List(Of String))

        'try clearing all items
        wlbAvailableItems.Items.Clear()

        'try recreating client controls
        'wlbAvailableItems.RecreateChildControls()

        'try building a copy of the items and then adding them in after a clear
        'Dim AvailableListItems(wlbAvailableItems.Items.Count) As WebListBoxItem

        'wlbAvailableItems.Items.CopyTo(AvailableListItems, 0)

        'wlbAvailableItems.Items.Clear()

        'For i = LBound(AvailableListItems) To UBound(AvailableListItems) - 1
        '    Moved = False
        '    For Each SelectedValue In SelectedItems
        '        If AvailableListItems(i).Value.ToLower = SelectedValue.ToLower Then
        '            Moved = True
        '            Exit For
        '        End If
        '    Next
        '    If Not Moved Then
        '        wlbAvailableItems.Items.Add(AvailableListItems(i))
        '    End If
        'Next

    End Sub

End Class

!!! Design page that calls user control

        <div style="position: absolute; top:35px; left: 19px; height: 100px; width: 200px;">
            <uc1:ucMembership ID="ucMembership1" runat="server" ControlHeight="100" ControlWidth="400" DefaultStyleMode="OutlookBlue" />
        </div>  

!!! Code behind page for call to the user control

Public Class WebForm1
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        If Not Page.IsPostBack Then

                Dim x As New Collection
                Dim y As New ListItem
                Dim i As Integer

                For i = 1 To 10
                    y = New ListItem
                    y.Text = "test " + i.ToString
                    y.Value = i
                    x.Add(y)
                Next

                ucMembership1.SetAvailableItems(x)

                x = New Collection
                For i = 11 To 13
                    y = New ListItem
                    y.Text = "test " + i.ToString
                    y.Value = i
                    x.Add(y)
                Next

                ucMembership1.SetSelectedItems(x)

        End If

    End Sub

End Class

When I run the program I get items 1 through 10 in the Available Items list and items 11 to 13 in the Selected Items list. If I choose the first 3 items of the Available List and click the Move Right button (top one) the page crashes with the attached error. NOTE: There is commented code in the user control that tries to clear the items. You have to uncomment some of that to see the error.

Is there a way to remove items from a WebListBox without invoking a page crash?