using DevExpress.XtraEditors.Controls;

public Form1() {
    InitializeComponent();
    //...
    popupContainerEdit1.EditValue = new ArrayList();
}

private void popupContainerEdit1_QueryResultValue(object sender, QueryResultValueEventArgs e) {
    //Get the list box displayed in the editor's dropdown
    ListBox listBox = findChildListBox((sender as PopupContainerEdit).Properties.PopupControl);
    //Add selected items to the array specified by the editor's edit value
    ArrayList ar = e.Value as ArrayList;
    ar.Clear();
    ar.AddRange(listBox.SelectedItems);
}

private void popupContainerEdit1_QueryDisplayText(object sender, QueryDisplayTextEventArgs e) {
    //Return the string representing the selected item count
    int itemCount = (e.EditValue as ArrayList).Count;
    e.DisplayText = itemCount.ToString() + " item(s) selected";
}

private void buttonOk_Click(object sender, System.EventArgs e) {
    Control button = sender as Control;
    //Close the dropdown accepting the user's choice
    (button.Parent as PopupContainerControl).OwnerEdit.ClosePopup();
}

private void buttonCancel_Click(object sender, System.EventArgs e) {
    Control button = sender as Control;
    //Close the dropdown discarding the user's choice
    (button.Parent as PopupContainerControl).OwnerEdit.CancelPopup();
}

private void popupContainerEdit1_CloseUp(object sender, CloseUpEventArgs e) {
    //Restore selected items if the user's choice should be discarded
    if (! e.AcceptValue) {                
        PopupContainerEdit popupContainerEditor = sender as PopupContainerEdit;
        //Get the list box displayed in the dropdown of the popup container editor
        ListBox listBox = findChildListBox(popupContainerEditor.Properties.PopupControl);
        if (listBox != null) {
            //Clear existing selection
            listBox.ClearSelected();
            //Restore selection from the editor's edit value
            foreach(object o in popupContainerEditor.EditValue as ArrayList)
                listBox.SetSelected(listBox.Items.IndexOf(o), true);
        }
    }
}

//Locates and retrieves a child ListBox control contained within the specified control
private ListBox findChildListBox(Control parent) {
    foreach(Control control in parent.Controls)
        if (control is ListBox) return (ListBox)control;
    return null;
}

Imports DevExpress.XtraEditors.Controls

Public Sub New()
    MyBase.New()
    InitializeComponent()
    '...
    popupContainerEdit1.EditValue = New ArrayList
End Sub

Private Sub popupContainerEdit1_QueryResultValue(ByVal sender As Object, _
ByVal e As QueryResultValueEventArgs) _
Handles popupContainerEdit1.QueryResultValue
    'Get the list box displayed in the editor's dropdown
    Dim editor As PopupContainerEdit = CType(sender, PopupContainerEdit)
    Dim listBox As ListBox = findChildListBox(editor.Properties.PopupControl)
    'Add selected items to the array specified by the editor's edit value
    Dim ar As ArrayList = CType(e.Value, ArrayList)
    ar.Clear()
    ar.AddRange(ListBox.SelectedItems)
End Sub

Private Sub popupContainerEdit1_QueryDisplayText(ByVal sender As Object, _
ByVal e As QueryDisplayTextEventArgs) _
Handles popupContainerEdit1.QueryDisplayText
    'Return the string representing the selected item count
    Dim itemCount As Integer = CType(e.EditValue, ArrayList).Count
    e.DisplayText = itemCount.ToString() + " item(s) selected"
End Sub

Private Sub buttonOk_Click(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles buttonOk.Click
    Dim button As Control = CType(sender, Control)
    'Close the dropdown accepting the user's choice
    CType(button.Parent, PopupContainerControl).OwnerEdit.ClosePopup()
End Sub

Private Sub buttonCancel_Click(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles buttonCancel.Click
    Dim button As Control = CType(sender, Control)
    'Close the dropdown discarding the user's choice
    CType(button.Parent, PopupContainerControl).OwnerEdit.CancelPopup()
End Sub

Private Sub popupContainerEdit1_CloseUp(ByVal sender As Object, _
ByVal e As CloseUpEventArgs) Handles popupContainerEdit1.CloseUp
    'Restore selected items if the user's choice should be discarded
    If Not e.AcceptValue Then
        Dim popupContainerEditor As PopupContainerEdit = CType(sender, PopupContainerEdit)
        'Get the list box displayed in the dropdown of the popup container editor
        Dim listBox As ListBox = findChildListBox(popupContainerEditor.Properties.PopupControl)
        If Not listBox Is Nothing Then
            'Clear existing selection
            listBox.ClearSelected()
            'Restore selection from the editor's edit value
            For Each o As Object In CType(popupContainerEditor.EditValue, ArrayList)
                listBox.SetSelected(listBox.Items.IndexOf(o), True)
            Next
        End If
    End If
End Sub

'Locates and retrieves a child ListBox control contained within the specified control
Private Function findChildListBox(ByVal parent As Control) As ListBox
    For Each control As Control In parent.Controls
        If TypeOf control Is ListBox Then Return CType(control, ListBox)
    Next
    Return Nothing
End Function