C++ how to get pixel color from outside my application window ?

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C++

WinAPI

I am trying to get the pixel color of a specified coordinate point outside my application window. So, since hwnd specifies my window to be targeted, then all i should do is use getdc function and pass my hwnd as parameter.

But what if I want a coordinate point from outside my application window? I already put the code but all I need is to know.

What parameter value to pass to getdc to make it target the screen from outside my application window, and my whole program is just to handle one window, with the area outside it, I am not using multiple windows in the same application.

What I have tried:

HDC targetArea = GetDC (???);// what should I //specify here as parameter ?

COLORREF colorValue;
colorValue = GetPixel(targetArea,500,400);