另一个“在使用ItemsSource之前，Items集合必须为空”

如何解决另一个“在使用ItemsSource之前，Items集合必须为空”

| 我对此xaml有一个控件：

        <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>

有了以下代码：

            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;
        }

而且我得到了众所周知的“使用ItemsSource之前，Items集合必须为空”的异常，但是我不明白为什么，xaml中没有添加的项！     

解决方法

        当然有：

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