WPF custom MultiSelector won't use ItemTemplateSelector (but will use ItemTemplate)

Question

I've searched for and found many questions similar mine but none of them seem to be quite my situation. Admittedly, mine is something of an edge-case. I'm hoping someone can spot what I'm missing here.

I've long been using a custom ItemsControl which derives from MultiSelector. I have a custom DataTemplate to draw the items in it. And they're drawn just fine if and only if I use the ItemTemplate property on the control.

But when I try instead to the ItemTemplateSelector property, my override of SelectTemplate is not being called. I've verified that it's creates and then set as the control's ItemTemplateSelector. But the breakpoint for its SelectTemplate override never gets hit.

The net effect is that the nice shapes that were previously being drawn beautifully by my one and only DataTemplate now just show up as string names.

The view I'm using is like this:

Custom DataTemplateSelector

public class ShapeTemplateSelector : DataTemplateSelector
{
    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        *** THIS NEVER EVEN GETS CALLED ***
        return item is PolyLineVm ? PolyLineTemplate : DefaultTemplate;
    }

    public DataTemplate PolyLineTemplate { get; set; }
    public DataTemplate DefaultTemplate { get; set; }
}

Custom MultiSelector ("ShapesControl")

using System.Collections.Specialized;
using System.Windows.Controls;

namespace MyApp.Shapes
{
    // Created By: 
    // Date: 2017-08-25

    using System.Linq;
    using System.Windows;
    using System.Windows.Controls.Primitives;
    using System.Windows.Input;


    /// 
    /// ShapesControl - our own version of a WPF MultiSelector.  Basically an
    /// ItemsControl that can select multiple items at once.  We need this to
    /// handle user manipulation of shapes on the ShapeCanvas and, potentially,
    /// elsewhere.
    /// 
    public class ShapesControl : MultiSelector
    {
        protected override bool IsItemItsOwnContainerOverride(object item)
        {
            return (item is ShapeItem);
        }

        protected override DependencyObject GetContainerForItemOverride()
        {
            // Each item we display is wrapped in our own container: ShapeItem
            // This override is how we enable that.

            return new ShapeItem();
        }

        // ...Other handlers are multi-selection overrides and removed for clarity
    }
}

Finally, the ShapeItemStyle I use to draw my custom ShapeItems

(Edit to add ShapeItem per request. Note that this includes selection code that interacts with the custom MultiSelector ("ShapesControl") above. I removed some of those functions from ShapesControl code for brevity as they're triggered by mouse-clicks and I couldn't see how they could possibly prevent a custom DataTemplateSelector from being invoked. But I've posted all of ShapeItem here)

namespace MyApp.Shapes
{
    [TemplatePart(Name="PART_Shape", Type=typeof(ContentPresenter))]
    public class ShapeItem : ContentControl
    {
        public ShapeVm ShapeVm => DataContext as ShapeVm;
        public ShapeType ShapeType => ShapeVm?.ShapeType ?? ShapeType.None;
        static ShapeItem()
        {
            DefaultStyleKeyProperty.OverrideMetadata
                (typeof(ShapeItem), 
                 new FrameworkPropertyMetadata(typeof(ShapeItem)));
        }
        private bool WasSelectedWhenManipulationStarted { get; set; }

        protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
        {
            base.OnMouseLeftButtonDown(e);
            ParentSelector?.NotifyItemClicked(this, true);
            e.Handled = true;

        }

        // The following ShapeItem manipulation handlers only handle the case of
        // moving the shape as a whole across the canvas.  These handlers do NOT
        // handle the case of resizing the shape.  Those handlers are on the
        // ShapeHandleThumb class.

        protected override void OnManipulationStarting(ManipulationStartingEventArgs e)
        {
            // The Canvas defines the coordinates for manipulation

            e.ManipulationContainer = this.FindVisualParent

Dipen Shah · Accepted Answer

The problem is with following code:

protected override DependencyObject GetContainerForItemOverride()
{
    // Each item we display is wrapped in our own container: ShapeItem
    // This override is how we enable that.

    return new ShapeItem();
}

When you override GetContainerForItemOverride method, it is your code's responsibility to use ItemTemplateSelector and attach it to ItemsControl.

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