Hidden Window Syndrom

Question

Hello,

I have been coded a screen capture program with C# and GDI+ functions.
One thing that I'm wondering is that capturing hidden window does not work.
What is actually behind this problem? I mean actually technical perspective point of view.

If Window is first created and then made hidden why can't it be captured?
There must be structures and other code that belongs to that window but why can't it
be captured?

I read from Internet articles that WM_Print or PrintWindow would help you with this problem.
I did not find any working solutions to that using WM_PRINT.

So could someone please, tell me what is the truth behind capturing hidden windows and
if there is a solution, what it actually is? Searching this topic from Newgroups shows that
a lot's of programmers are finding solution to this problem.

Br

Mikko, Scandinavia Finland Europe