问ComboBox错误:集合被修改，枚举操作可能不会执行

EN

<StackPanel Orientation="Vertical">
    <ComboBox x:Name="selector">
        <ComboBoxItem IsEnabled="False" Content="---" />
        <ComboBoxItem FontStyle="Italic" Content="Add New" Selected="New_Selected" />
    </ComboBox>
</StackPanel>

public partial class MainWindow : Window
{
    List<string> files = new List<string>();

    public MainWindow()
    {
        InitializeComponent();
    }

    private void RepopulateResourceSelector()
    {
        // Remove all but the bottom 2 items
        while (selector.Items.Count > 2)
        {
            selector.Items.RemoveAt(0);
        }

        int index = 0;

        // Add all strings in the list to combo box
        foreach (var file in files)
        {
            selector.Items.Insert(index, file);
            index++;
        }
    }

    private void New_Selected(object sender, RoutedEventArgs e)
    {
        var dlg = new OpenFileDialog();
        dlg.Filter = "Image Files (.bmp, .jpg, .gif, .png, .tiff)|*.bmp;*.jpg;*.gif;*.png;*.tiff";

        if (dlg.ShowDialog(this) == true)
        {
            // Add selected file to the list
            string name = System.IO.Path.GetFileNameWithoutExtension(dlg.FileName);
            files.Add(name);

            RepopulateResourceSelector();
        }

        // Deselect `Add New` item
        selector.SelectedIndex = -1;
    }
}

System.InvalidOperationException occurred
    HResult=0x80131509
    Message=Collection was modified; enumeration operation may not execute.
Source=<Cannot evaluate the exception source>
StackTrace:
    at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
    at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
    at System.Collections.Generic.List`1.Enumerator.MoveNext()
    at System.Windows.Controls.Primitives.Selector.SelectionChanger.CreateDeltaSelectionChange(List`1 unselectedItems, List`1 selectedItems)
    at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
    at System.Windows.Controls.Primitives.Selector.SelectionChanger.SelectJustThisItem(ItemInfo info, Boolean assumeInItemsCollection)
    at System.Windows.Controls.ComboBox.NotifyComboBoxItemMouseUp(ComboBoxItem comboBoxItem)
    at System.Windows.Controls.ComboBoxItem.OnMouseLeftButtonUp(MouseButtonEventArgs e)
    at System.Windows.UIElement.OnMouseLeftButtonUpThunk(Object sender, MouseButtonEventArgs e)
    at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
    at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
    at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
    at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
    at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
    at System.Windows.UIElement.OnMouseUpThunk(Object sender, MouseButtonEventArgs e)
    at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
    at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
    at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
    at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
    at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
    at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
    at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
    at System.Windows.Input.InputManager.ProcessStagingArea()
    at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
    at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
    at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
    at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
    at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
    at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
    at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
    at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
    at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
    at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
    at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
    at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
    at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
    at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
    at System.Windows.Application.RunDispatcher(Object ignore)
    at System.Windows.Application.RunInternal(Window window)
    at System.Windows.Application.Run(Window window)
    at System.Windows.Application.Run()
    at WpfApp1.App.Main()