It turned out that this is a limitation of the .NET framework and we reported it to MS (link). Currently you could workaround it as place the data template content in a UserControl and reference it in the window DataTemplate, i.e. instead of:

<Window.Resources>
    <DataTemplate x:Key=”foo”>
        <ig:XamDataTree SelectedDataItems=”{Binding SelectedFolders}” />
    </DataTemplate />
</Window.Resources>

use this:

<Window.Resources>
    <DataTemplate x:Key=”foo”>
        <local:TreeUserControl />
    </DataTemplate />
</Window.Resources>

Where the TreeUserControl just had something like:
<UserControl …>
    <ig:XamDataTree SelectedDataItems=”{Binding SelectedFolders}” />
</UserControl>

Thank you for helping us to improve our controls!

Best Regards,
Petia Koleva

Hi Michael,

SelectedDataItems is an object[] and it appears as though arrays are not allowed to be used inside DataTemplates.  I'm not sure why the platform has this limitation but I created a development issue (ID: 185101) so that we could address this.  Along with the development issue I also created a private support case for you.  The case number is CAS-146841-J7W5J4 and you can view it here: https://ko.infragistics.com/my-account/support-activity

You can view the status of the development issue connected to this case by going to the “My IG” tab on our website, hovering to the "My Support Activity" dropdown and selecting the "Development Issues" tab.