VisualTreeHelper routines fail to enumerate contents of a ContentPresenter

[DHancock]

Describe the bug

I was trying to parse the contents of an Expander control but the expander drop down contents were not being returned. The Expander drop down is a ContentPresenter. The following code fails when the dependencey object is a ContentPresenter with VisualTreeHelper.GetChildrenCount() always returning zero.

    public void ParseVisualTree(DependencyObject reference)
    {
        // do work on reference here

        int count = VisualTreeHelper.GetChildrenCount(reference);

        for (int index = 0; index < count; index++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(reference, index);
            ParseVisualTree(child);
        }
    }

The following work around does enumerate the contents of an Expander:

    public void ParseVisualTree(DependencyObject reference)
    {
        // do work on reference here

        if ((reference is ContentPresenter cp) && (cp.Content is DependencyObject content))
        {
            ParseVisualTree(content);
        }
        else
        {
            int count = VisualTreeHelper.GetChildrenCount(reference);
    
            for (int index = 0; index < count; index++)
            {
                DependencyObject child = VisualTreeHelper.GetChild(reference, index);
                ParseVisualTree(child);
            }
        }
    }

The work around was derived from the following WindowsCommunityToolkit code:

https://github.com/CommunityToolkit/WindowsCommunityToolkit/blob/6bfe8a5d21feb8818a8048b9fc41a84a70af8b0f/Microsoft.Toolkit.Uwp.UI/Extensions/FrameworkElement/FrameworkElementExtensions.LogicalTree.cs#L182-L199C1

The live visual tree explorer in Visual Studio correctly shows the contents of the Expander drop down.

(Edit: improve work around)

Steps to reproduce the bug

Attempt to parse the visual tree to find the controls contained in an Expander's drop down.

Expected behavior

No response

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.4.3: 1.4.231115000

Windows version

Windows 11 (22H2): Build 22621

Additional context

No response

[DHancock]

Not a bug. My mistake, it's evident that the controls in the expander drop down are only added in to the visual tree when the expander is expanded for the first time, after that they are always there whether the expander is expanded or not.