我的treeView的可扩展加号总是存在的，即使我的节点没有任何子节点

我在这篇非常好的文章的帮助下构建了一个treeView WPF MVVM，然后我为一些节点创建了一个contextMenu，允许我从选定的父节点添加子节点。

问题是:当我创建一个新节点时，它带有可扩展的加号，或者我的新节点没有任何子节点!

我该如何纠正这个问题。

public class TreeViewItemViewModel : INotifyPropertyChanged
{ 
#region Data
    static readonly TreeViewItemViewModel DummyChild = new TreeViewItemViewModel();
readonly ObservableCollection<TreeViewItemViewModel> _children;
readonly TreeViewItemViewModel _parent;
bool _isExpanded;
bool _isSelected;
#endregion // Data
#region Constructors
protected TreeViewItemViewModel(TreeViewItemViewModel parent, bool lazyLoadChildren)
{
    _parent = parent;
    _children = new ObservableCollection<TreeViewItemViewModel>();
    if (lazyLoadChildren)
        _children.Add(DummyChild);
}
// This is used to create the DummyChild instance.
private TreeViewItemViewModel()
{
}
#endregion // Constructors
#region Presentation Members
#region Children
/// <summary>
/// Returns the logical child items of this object.
/// </summary>
public ObservableCollection<TreeViewItemViewModel> Children
{
    get { return _children; }
}
#endregion // Children
#region HasLoadedChildren
/// <summary>
/// Returns true if this object's Children have not yet been populated.
/// </summary>
public bool HasDummyChild
{
    get { return this.Children.Count == 1 && this.Children[0] == DummyChild; }
}
#endregion // HasLoadedChildren
#region IsExpanded
/// <summary>
/// Gets/sets whether the TreeViewItem 
/// associated with this object is expanded.
/// </summary>
public bool IsExpanded
{
    get { return _isExpanded; }
    set
    {
        if (value != _isExpanded)
        {
            _isExpanded = value;
            this.OnPropertyChanged("IsExpanded");
        }
        // Expand all the way up to the root.
        if (_isExpanded && _parent != null)
            _parent.IsExpanded = true;
        // Lazy load the child items, if necessary.
        if (this.HasDummyChild)
        {
            this.Children.Remove(DummyChild);
            this.LoadChildren();
        }
    }
}
#endregion // IsExpanded
#region IsSelected
/// <summary>
/// Gets/sets whether the TreeViewItem 
/// associated with this object is selected.
/// </summary>
public bool IsSelected
{
    get { return _isSelected; }
    set
    {
        if (value != _isSelected)
        {
            _isSelected = value;
            this.OnPropertyChanged("IsSelected");
        }
    }
}
#endregion // IsSelected
#region LoadChildren
/// <summary>
/// Invoked when the child items need to be loaded on demand.
/// Subclasses can override this to populate the Children collection.
/// </summary>
protected virtual void LoadChildren()
{
}
#endregion // LoadChildren
#region Parent
public TreeViewItemViewModel Parent
{
    get { return _parent; }
}
#endregion // Parent
#endregion // Presentation Members
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
    if (this.PropertyChanged != null)
        this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
#endregion // INotifyPropertyChanged Members

MyXml:

`<TreeView ItemsSource="{Binding Regions}" IsEnabled="{Binding EnableTree}" >
  <TreeView.ItemContainerStyle>
    <!-- 
    This Style binds a TreeViewItem to a TreeViewItemViewModel. 
    -->
    <Style TargetType="{x:Type TreeViewItem}">
      <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
      <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
       <Setter Property="FontWeight" Value="Normal" />
      <Style.Triggers>
        <Trigger Property="IsSelected" Value="True">
          <Setter Property="FontWeight" Value="Bold" />
        </Trigger>
      </Style.Triggers>
    </Style>
  </TreeView.ItemContainerStyle>
  <TreeView.Resources>
    <ContextMenu x:Key="AddCity"  ItemsSource="{Binding AddCityItems}"/>
    <HierarchicalDataTemplate 
      DataType="{x:Type local:StateViewModel}" 
      ItemsSource="{Binding Children}"
      >
      <StackPanel Orientation="Horizontal" ContextMenu="{StaticResource AddCity}">
        <Image Width="16" Height="16" Margin="3,0" Source="Images'Region.png" />
        <TextBlock Text="{Binding RegionName}" />
      </StackPanel>
    </HierarchicalDataTemplate>
  </TreeView.Resources>`

MyRegionViewModel(下面我添加了我的代码来创建用于添加子菜单的contextMenu和上面的所有代码，您可以在文章中找到它)

public class StateViewModel : TreeViewItemViewModel {
readonly State _state;
   public ICommand AddCityCommand { get; private set; }
   public List<MenuItem> AddCityItems { get; set; }
public StateViewModel(State state, RegionViewModel parentRegion)
    : base(parentRegion, true)
{
    _state = state;
    AddCityItems = new List<MenuItem>();
    AddCityCommand = new DelegateCommand<CancelEventArgs>(OnAddCityCommandExecute, OnAddCityCommandCanExecute);
    AddCityItems.Add(new MenuItem() { Header = "Add City", Command = AddCityCommand });
}
public string StateName
{
    get { return _state.StateName; }
}
protected override void LoadChildren()
{
    foreach (City city in Database.GetCities(_state))
        base.Children.Add(new CityViewModel(city, this));
}

bool OnAddCityCommandCanExecute(CancelEventArgs parameter)
{
    return true;
}
public void OnAddCityCommandExecute(CancelEventArgs parameter)
{
    var myNewCity = new city();
    Children.Add(new CityViewModel(myNewCity, this));
}
}