Capture Screenshot include the Menu of WPF application

Hello all,

I am trying to capture the screenshot  in Basic theme of Win 7 rather than Aero themes. However, I see the problem that all popup menus inside the WPF application are not captured in the screen shot even I use the DirectX to capture or BitBlt. I see it just captures screen without popup menu.

Could you please advise me what the reason is?

Thanks.