Windows GetPixel typedef C++ - winapi

Question

How do I define the type for GetPixel and / or what else am I missing to use GetPixel?
#include <windows.h>
class PollPixelArray
{
public:
PollPixelArray(HDC hdcMonitor, LPRECT lprcMonitor);
unsigned long createHex(int r, int g, int b);
private:
PollPixelArray();
};
PollPixelArray::PollPixelArray(HDC hdcMonitor, LPRECT lprcMonitor)
{
GetPixel(hdcMonitor, 50, 100);
}
unsigned long createHex(int r, int g, int b){
return (((r & 0xff) << 16) + ((g & 0xff) << 8) + (b & 0xff));
}
Always returns the same unsigned long / DWORD / COLORREF no matter the X or Y co-ordinates.
while (tempX<40){
COLORREF tempREF = GetPixel(hdcMonitor, tempX, tempY); //COLORREF | unsigned long |
unsigned int dummy = GetRValue(tempREF);
std::cout << "RGB: " << ("%d", dummy);
dummy = GetGValue(tempREF);
std::cout << "," << ("%d", dummy);
dummy = GetBValue(tempREF);
std::cout << "," << ("%d", dummy);
std::cout << " at " << ("%d", tempX) << ", " << ("%d", tempY) << std::endl;
tempX++;
tempY++;
}
The loop always returns 255,255,255 for the RGB values.
The HDC Callback function:
#include <windows.h>
#include "pollpixelarray.h"
BOOL CALLBACK MonitorEnumProc(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor, LPARAM dwData)
{
PollPixelArray::PollPixelArray(hdcMonitor, lprcMonitor);
return true;
}
void main()
{
EnumDisplayMonitors(NULL, NULL, MonitorEnumProc, 0);
std::cin.get();
}

Answer 1

This has nothing at all to do with typedefs and GetPixel works correctly. There are a couple of plausible explanations for the behaviour you observe:
The device context is not valid, or
The coordinates that you pass are outside the bounds of the device.
Looking at your code, both are likely to be the case,
The documentation for the hdc parameter of EnumDisplayMonitors says:
If this parameter is NULL, the hdcMonitor parameter passed to the callback function will be NULL, and the visible region of interest is the virtual screen that encompasses all the displays on the desktop.
The documentation for GetPixel says:
If the pixel is outside of the current clipping region, the return value is CLR_INVALID (0xFFFFFFFF defined in Wingdi.h).