指定元素已经是另一个元素的逻辑子元素，当动态添加（绑定）自定义形状到面板列表框时

Question

This InvalidOperationException with message "Specified element is already the logical child of another element, disconnect it first" is strange cause it happens when adding some 8-9 shapes, but it is logged on some previous element fi shape7, when I scroll it down, or when elements are not so much when I scroll a few upside, after scrolling downwards. Interestingly that when I just create one main Grid/StackPanel instead ListBox and add Custom Control with wrapped panel with added 1 Shape, it doesnot happen. In every case one nested stackpanel should have 1 shapecontrol (shape). Here is Listbox xaml in MainWindow:

  <ListBox Height="500" ItemsSource="{Binding MyShapes}" Name = "shapeList" ScrollViewer.VerticalScrollBarVisibility="Visible"                   
                        <ListBox.ItemTemplate>
                            <DataTemplate>
                                <local:ShapeControl Height="auto" Width="auto" NestedShape="{Binding}"></local:ShapeControl>
                            </DataTemplate>
                        </ListBox.ItemTemplate>
  </ListBox>

Simple ShapeControl xaml:

<UserControl x:Class="ShapesProject.UserControls.Control"
    <StackPanel >
    </StackPanel>
</UserControl>

And more complex ShapeControl codebehind: I need to pass some shape (Path/GeometryGroup) with Dedendency Property, so without constructor parameter, adding it to Panel just in Loaded event, no other way around.

  public partial class ShapeControl : UserControl
  {
    private static int counter=0;
    public static readonly DependencyProperty NestedShapeProperty =
    DependencyProperty.Register(
        "NestedShape",
        typeof(Shape),
        typeof(ShapeControl));

    public Shape NestedShape
    {
        get { return (Shape)GetValue(NestedShapeProperty); }
        set { SetValue(NestedShapeProperty, value); }
    }

    public ShapeControl()
    {
        InitializeComponent();
            Loaded += (sender, args) =>
            {
                if (NestedShape.Parent != null) { Trace.WriteLine("Parent +" + ((StackPanel)NestedShape.Parent).Name + " " + NestedShape.Name); } //This produce parent in exception case
                ((StackPanel)this.Content).Name = "Stack" + counter++;
                ((StackPanel)this.Content).Children.Add(NestedShape); //HERE exception happes, indeed Children count changes from 0 to 1 always
                Trace.WriteLine("Parent1 +" + ((StackPanel)NestedShape.Parent).Name + " " + NestedShape.Name);
                // Here for unknown reason Names jumps Stack0,Stack2,Stack4... When customshapes 0,1,2,3,4
            };     
    }
}

I create and add the original shapes in main Panel, and simultaneously add in mainwindow to MyShapes ObservableCollection where it is binded to ItemsSource, and Item of it is binded to ShapesControl. I clone the shape with XamlReader.Parse(XamlWriter.Save(currentShape)) so it could not be the reason, and it is confirmed when passing to the same updated usercontrol, with parameter constructor dynamically, without xaml DP, that is simply added to Containg StackPanel but not to LB. When putting content of Loaded event in try-catch the StackTrace property produces such logs:

System.Windows.FrameworkElement.ChangeLogicalParent(DependencyObject newParent)
       in System.Windows.FrameworkElement.AddLogicalChild(Object child)
       in System.Windows.Controls.UIElementCollection.AddInternal(UIElement element)
       in System.Windows.Controls.UIElementCollection.Add(UIElement element)
       in ShapesHandler.UserControls.ShapeControl.<.ctor>b__0(Object sender, RoutedEventArgs args) in e:\ShapesProject\ShapesProject\UserControls\ShapeControl.xaml.cs:line 60

The new moment I have noticed after catching exception inside Loaded event (otherwise putting try block around Loaded did not catched nothing, so everything crashed) is that one, that just 2-3 last shapes stays in ListBox, of 8-9 added at the moment of exception. So the first ones disappear from inside of Shape Controls that comprises ListBoxItems, as just small ShapeControl void border is visible for item, instead of border that enveloped shape, and do that for last ones. After adding next several shapes, after exception, the exception repeats - just 2-3 last shapes stays in LB. Then I checked manually Trace.WriteLine-s and noticed, that at that case 10 shapes (wrapped in User Control) was added to ListBox, and Exception happened at Stack12-Shape6 (seven one), and shapes 8-10 was preserved, but previous have logged exceptions. So after adding 10 items to LB, the previous shape6 passed to Loaded event again. So I supposed it was due to size, but it happened in heigh dimension (400). But after I put Height 600/800, or deleted that tag, My UserControl in near left cell, slided down. Indeed for ListBox of 800 height I was able to add about 20 shapes without exception so it is more of size and whole layout. But my main Canvas in UC is slided down.

Answer 1

Indeed I do not know if Height of ListBox is the reason of exception, as I cannot find nothing similar in the internet. But why after some created items, some previous one or ones added to Loaded event, that is virtually create new item with the already added shape. With height of 800, the Exception indeed appears, but not at 10 addition, but at 15 addition in that case, and exception provoked by Shape9, but not Shape6 for example. But as 9 Shapes get disappeared in last case from UC-wrapper (transform in small border) I do undestand that this Binding: <local:ShapeControl Height="auto" Width="auto" NestedShape="{Binding}" does not work - or NestedShape (Binded to Item itself) get lost but ItemsSource is binded to MyShapes field in MainWindow.