windows 8 MSDN Examples 中一個奇怪的問題

http://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.media.animation.repositionthemetransition.aspx

 1 <Button Content="Remove Rectangle" Click="RemoveButton_Click"/> 2  3 <ItemsControl Grid.Row="1" x:Name="rectangleItems"> 4     <ItemsControl.ItemContainerTransitions> 5         <TransitionCollection> 6              7             <!-- Without this, there would be no animation when items  8                  are removed. --> 9             <RepositionThemeTransition/>10         </TransitionCollection>11     </ItemsControl.ItemContainerTransitions>12     <ItemsControl.ItemsPanel>13         <ItemsPanelTemplate>14             <WrapGrid Height="400"/>15         </ItemsPanelTemplate>16     </ItemsControl.ItemsPanel>17     18     <!-- All these rectangles are just to demonstrate how the items19          in the grid re-flow into position when one of the child items20          are removed. -->21     <ItemsControl.Items>22         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>23         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>24         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>25         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>26         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>27         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>28         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>29         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>30         <Rectangle Fill="Red" Width="100" Height="100" Margin="10"/>31     </ItemsControl.Items>32 </ItemsControl>

 

1 private void RemoveButton_Click(object sender, RoutedEventArgs e)2 {3     if (rectangleItems.Items.Count > 0)4         rectangleItems.Items.RemoveAt(0);5 }

每點擊一次按鈕，將刪除一個紅色的方塊。但當方塊刪除的只剩一個時，再點擊按鈕刪除時會報錯“Value does not fall within the expected range.”
很奇怪的提示，明明還有一個，怎麼就刪不掉呢？難道rectangleItems.Items不可為空嗎？實驗一下，調用rectangleItems.Items.Clear()方法刪除rectangleItems.Items中的所有項，反應正常；又試了下，在XAML中<ItemsControl.Items>中的方塊直接都刪除，任然可以運行。

不知有沒有人也遇到了類似的問題，這到底是什麼原因呢？