How to create a window in secondary monitor in win10 app - winapi

Question

I am very new to the win10 dev, and I am trying to create a win10 app which will create a floating window in all the monitors the system is connected to. So far I am successful in using GDI API to create window in my primary monitor, but it is not working on my secondary monitor, I cannot get why it is not working even when I am using the API as mentioned in the microsoft page.
Below is my code
// Register the window class.
WNDCLASSEX wcex = { sizeof(WNDCLASSEX) };
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = DXGIDraw::WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = sizeof(LONG_PTR);
wcex.hInstance = HINST_THISCOMPONENT;
wcex.hbrBackground = NULL;
wcex.lpszMenuName = NULL;
wcex.hCursor = LoadCursor(NULL, IDI_APPLICATION);
wcex.lpszClassName = L"DemoApp";
RegisterClassEx(&wcex);
m_hwnd = CreateWindowEx(WS_EX_LAYERED,
L"DemoApp",
L"Demo App",
0,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
HWND_DESKTOP,
NULL,
HINST_THISCOMPONENT,
NULL
);
const int nWidth = GetSystemMetrics(SM_CXSCREEN);
const int nHeight = GetSystemMetrics(SM_CYSCREEN);
CImage img;
DISPLAY_DEVICE ddd;
ZeroMemory(&ddd, sizeof(ddd));
ddd.cb = sizeof(ddd);
for (int i = 0; EnumDisplayDevices(NULL, i, &ddd, 0); i++)
{
if (ddd.StateFlags & DISPLAY_DEVICE_ACTIVE) {
//Active monitor
}
if (ddd.StateFlags & DISPLAY_DEVICE_PRIMARY_DEVICE) {
//Primary monitor
}
else {
//other types
}
}
//The following API supposed to work for all the monitors, but this API is not working. This is where I need attention.
HDC hdcScreen = CreateDC(ddd.DeviceName, NULL, NULL, NULL);
//But if I use below API I can get it working for the primary monitor only, still trying to understand why
//HDC hdcScreen = CreateDC(L"DISPLAY", NULL, NULL, NULL);
HDC hDC = CreateCompatibleDC(hdcScreen);
HBITMAP hBmp = CreateCompatibleBitmap(hdcScreen, nWidth, nHeight);
HBITMAP hBmpOld = (HBITMAP)SelectObject(hDC, hBmp);
img.Draw(hDC, 0, 0, nWidth, nHeight, 0, 0, nWidth, nHeight);
//Add layered window
BLENDFUNCTION blend = { 0 };
blend.BlendOp = AC_SRC_OVER;
blend.SourceConstantAlpha = 255;
blend.AlphaFormat = AC_SRC_ALPHA;
POINT ptLocation = { 0, 0 };
SIZE szWnd = { nWidth, nHeight };
POINT ptSrc = { 0, 0 };
BOOL status1 = UpdateLayeredWindow(m_hwnd, hdcScreen, &ptLocation, &szWnd, hDC, &ptSrc, 0, &blend, ULW_ALPHA);
//Set window's position
BOOL status = SetWindowPos(m_hwnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
hr = m_hwnd ? S_OK : E_FAIL;
if (SUCCEEDED(hr))
{
BOOL status;
status = ShowWindow(m_hwnd, SW_MAXIMIZE);
status = UpdateWindow(m_hwnd);
}
SelectObject(hDC, hBmpOld);
DeleteObject(hBmp);
DeleteDC(hDC);
ReleaseDC(NULL, hdcScreen);
}

Answer 1

EnumDisplayDevices is the wrong API for this job. You need to use EnumDisplayMonitors which gives you the monitor coordinates directly.

Answer 2

You need to retreive the bounding rectangle for the desired monitor within the virtual screen, and then you can position a window within that rectangle as desired. Use EnumDisplayMonitors() to get the rectangle of each moinitor. Or, if you have an HMONITOR handle to a specific monitor, you can use GetMonitorInfo().