Question

How to programmatically get the Device Instance ID (unique ID) of a USB mass storage device that a user just plugged in?

Answer 1

Catch WM_DEVICECHANGE from any window handle by registering for device change notifications. As such:

DEV_BROADCAST_DEVICEINTERFACE dbd = { sizeof(dbd) };
dbd.dbcc_devicetype = DBT_DEVTYP_DEVICEINTERFACE;
dbd.dbcc_classguid = GUID_DEVINTERFACE_USB_DEVICE;
RegisterDeviceNotification(hwnd, &dbd, DEVICE_NOTIFY_WINDOW_HANDLE);

The lParam of the WM_DEVICECHANGE can be cast to DBT_DEVTYP_DEVICEINTERFACE. Note – when plug in a device you may get multiple WM_DEVICECHANGE notifications. Just filter on the arrival event and ignore duplicates.

LRESULT WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch(hwnd)
    {
        case WM_DEVICE_CHANGE:
        {
            PDEV_BROADCAST_HDR pHdr = NULL;
            PDEV_BROADCAST_DEVICEINTERFACE pDev = NULL;
            pHdr = (PDEV_BROADCAST_HDR)lParam;
            bool fDeviceArrival = (wParam == DBT_DEVICEARRIVAL);
            if (fDeviceArrival)
            {
                if (pHdr && (pHdr->dbch_devicetype==DBT_DEVTYP_DEVICEINTERFACE))
                {
                    pDev = (PDEV_BROADCAST_DEVICEINTERFACE)lParam;
                }
                if (pDev && (pDev->dbcc_classguid == GUID_DEVINTERFACE_USB_DEVICE))
                {
                    // the PNP string of the device just plugged is in dbcc_name
                    OutputDebugString(pDev->dbcc_name);
                    OutputDebugString("\r\n");
                }
            }
        ....