Multiple instances of a canvas user control throw an exception in Silverlight

Question

I have a user control that has a grid (one that you get automatically when you create a user control) and a canvas inside that.

<Grid x:Name="LayoutRoot" Background="White">
    <Canvas x:Name="SurfaceCanvas">
    </Canvas>
</Grid>

In the CS file, I defined the "Items" collection.

public ObservableCollection<TestItem> Items {
    get { return (ObservableCollection<TestItem>)GetValue(ItemsProperty); }
    set { SetValue(ItemsProperty, value); }
}

public static readonly DependencyProperty ItemsProperty =
    DependencyProperty.Register("Items", typeof(ObservableCollection<TestItem>),
    typeof(TestControl),
    new PropertyMetadata(new ObservableCollection<TestItem>()));

TestItem class declaration:

public class TestItem : ContentControl { ... }

Elements are added to it through XAML.

<my:TestControl x:Name="ControlOne" Height="100" Width="100">
    <my:TestControl.Items>
        <my:TestItem x:Name="ItemOne">One</my:TestItem>
    </my:TestControl.Items>
</my:TestControl>

<my:TestControl x:Name="ControlTwo" Height="100" Width="100">
    <my:TestControl.Items>
        <my:TestItem x:Name="ItemTwo">Two</my:TestItem>
    </my:TestControl.Items>
</my:TestControl>

When items are added to the collection, I add them to the canvas.

void Items_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e) {
    switch(e.Action) {
        case NotifyCollectionChangedAction.Add:
            foreach(Control item in e.NewItems) {
                SurfaceCanvas.Children.Add(item);
            }
        break;
    }
}

Now the problem.

If there is one instance of this control, all is well. But when I define the second instance, I get InvalidOperationException: "The element is already a descendant of another element" in the element added to ControlTwo.

, , , ItemOne ControlOne, ItemTwo ControlOne, ControlTwo. , , .

, - , , , "SurfaceCanvas", . , .

?

Answer 1

, :

DependencyProperty. DependencyProperty TestControl. , - TestControl.Items, ObservbleCollection, Events CollectionChanged, Canvas.

ItemsProperty. ObservbleCollection , xaml TestControl.

: DependencyProperty , .