另一個「Items集合必須在使用ItemsSource前空」

Question

我有此XAML控制：

 <Controls:TreeView x:Name="MachineGroupsTree" Style="{StaticResource MachineGroupStyle}" > 
      <i:Interaction.Triggers> 
       <i:EventTrigger EventName="SelectedItemChanged"> 
        <i:InvokeCommandAction Command="{Binding SetCurrentManagedObjectNodeCommand}" CommandParameter="{Binding SelectedItem, ElementName=MachineGroupsTree, Mode=OneWay}"/> 
       </i:EventTrigger> 
      </i:Interaction.Triggers> 
      <Controls:TreeViewItem> 
       <Controls:TreeViewItem.HeaderTemplate> 
        <DataTemplate> 
         <TextBlock Text="Loading..." Style="{StaticResource LoadingStyle}" /> 
        </DataTemplate> 
       </Controls:TreeViewItem.HeaderTemplate> 
      </Controls:TreeViewItem> 
     </Controls:TreeView> 

有了這個代碼隱藏：

  public static readonly DependencyProperty ItemsSourceProperty = 
DependencyProperty.Register("ItemsSource", typeof(IEnumerable), typeof( 
MachineGroupTreeViewControl), new PropertyMetadata(null, new PropertyChangedCallback( 
OnItemSourceChanged))); 


     public IEnumerable ItemsSource { 
      get { 
       return (IEnumerable)GetValue(ItemsSourceProperty); 
      } 
      set { 
       ClearValue(ItemsSourceProperty); 
       SetValue(ItemsSourceProperty, value); 
      } 
     } 

     static void OnItemSourceChanged(object sender, 
      DependencyPropertyChangedEventArgs args) { 
      // Get reference to self 
      MachineGroupTreeViewControl source = (MachineGroupTreeViewControl)sender; 

      // Add Handling Code  

      // --------------------------------------------- 
      // EXCEPTION HERE !!! 
      source.MachineGroupsTree.ItemsSource = (IEnumerable)args.NewValue; 
     } 

我也得到了著名的「項目集合在使用ItemsSource之前必須爲空「異常，但我不明白爲什麼，xaml中沒有添加項目！

Answer 1

肯定有：

<Controls:TreeViewItem> 
    <Controls:TreeViewItem.HeaderTemplate> 
     <DataTemplate> 
      <TextBlock Text="Loading..." Style="{StaticResource LoadingStyle}" /> 
     </DataTemplate> 
    </Controls:TreeViewItem.HeaderTemplate> 
</Controls:TreeViewItem>