Problem trying to use TreeView Example Overrides from Examples

Hello. I'm trying to get a TreeListBox application running by following some of the example projects. Most of these projects use the classes here:

SampleBrowser\ProductSamples\GridsSamples\Common

such as:

namespace ActiproSoftware.ProductSamples.GridsSamples.Common {

public class DefaultTreeListBoxItemAdapter : TreeListBoxItemAdapter {

So, I figured I'd copy the file to my project and change the namespace to my project's default namespace (at least for now to get started). Here's what mine now looks like:

namespace ARTB_GUI_001 {

public class DefaultTreeListBoxItemAdapter : TreeListBoxItemAdapter {

 Back in my XAML, I have the namesapce:

xmlns:local="clr-namespace:ARTB_GUI_001"

 but when I try to do something like this:

<grids:TreeListBox.ItemAdapter>
<local:DefaultTreeListBoxItemAdapter TopLevelExpandabilityDefault="Never" />
</grids:TreeListBox.ItemAdapter>

 .. that second line gives me the error:

Error The name "DefaultTreeListBoxItemAdapter" does not exist in the namespace "clr-namespace:ARTB_GUI_001".

 The Intellisense seems to think there should be a <local:DefaultTreeListBoxItemAdapter .. but I can't seem to get rid of the error.

Any thoughts? Thanks in advance!