ContentControl “bug” in WinRT

Background

I was recently on a WinRT project that required showing a hierarchy of assets that the user could browse through, ultimately leading to an “asset details” view where they can interact with the selected asset. One of the interactive features was to have a toggle button that would hide/show a details pane with the asset description. Throughout the asset hierarchy, the data templates would bind to Name and ImageUri properties on the assets to display them. When I got down to the asset details level, I needed to wrap the asset in a ViewModel to support the command that I needed to implement the description toggle.

Implementation

After messing around with the built in Transition API trying to get my description pane to animate in, I realized that I needed to expose a separate Asset property on my VM so my description pane could bind to it via a ContentControl with a ContentThemeTransition, and to do the toggling I would just null the object in the VM and let the Transition work its magic. I tested this without the ContentTemplate set on the ContentControl (just bound directly to the extra Asset property I added to my VM), and it worked as I expected…..the description pane was hidden, and when i clicked the toggle button the pane animated in and showed that it was bound to an Asset object.

Problem

The problem started when I added the DataTemplate that was bound to the ImageUri and Name of that Asset property. When I tested it, the description pane was instantly visible before I clicked the toggle button.

Solution

After a lot of breakpoints and head scratching, my theory was that, even though the Content property of the ContentControl was bound to this seperate Asset that I added to the VM, it still looked up the tree to find those properties when the object was null. Since the VM itself had those same properties, the DataTemplate was binding to that and displaying the pane when it shouldn’t have been. Sure enough, I made a separate little class to hold the Name and Description properties, and named them differently, and it worked fine. The surprise for me here was that the ContentControl was looking up the tree for property binding when it’s content was null, and it only did it when a DataTemplate was defined. I’m not sure if this is a bug, or by design, but it can cause a headache if you’re not expecting it.

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