WM_DEVICECHANGE lParam is null - c++

Question

I want to detect the insertion and removing of any USB drive in my computer. I am not experienced in Windows GUI programming, and usually use Qt, but I had to use the Win32 API this time.
The problem is that lParam is always null.
I read that I need to use RegisterDeviceNotification(), but I used it and even with DEVICE_NOTIFY_ALL_INTERFACE_CLASSES, lParam is always null, however the function succeeds.
This is my code. Most of it I took from a Win32 app template from Visual Studio 2015:
#define MAX_LOADSTRING 100
HINSTANCE hInst; // current instance
WCHAR szTitle[MAX_LOADSTRING]; // The title bar text
WCHAR szWindowClass[MAX_LOADSTRING]; // the main window class name
// Forward declarations of functions included in this code module:
ATOM MyRegisterClass(HINSTANCE hInstance);
BOOL InitInstance(HINSTANCE, int);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK About(HWND, UINT, WPARAM, LPARAM);
int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
_In_opt_ HINSTANCE hPrevInstance,
_In_ LPWSTR lpCmdLine,
_In_ int nCmdShow)
{
// TODO: Place code here.
// Initialize global strings
LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
LoadStringW(hInstance, IDC_USBSPREAD, szWindowClass, MAX_LOADSTRING);
MyRegisterClass(hInstance);
// Perform application initialization:
if (!InitInstance (hInstance, nCmdShow))
{
return FALSE;
}
HACCEL hAccelTable = LoadAcceleratorsW(hInstance, MAKEINTRESOURCE(IDC_USBSPREAD));
MSG msg;
// Main message loop:
while (GetMessageW(&msg, nullptr, 0, 0))
{
if (!TranslateAcceleratorW(msg.hwnd, hAccelTable, &msg))
{
TranslateMessage(&msg);
DispatchMessageW(&msg);
}
}
return (int) msg.wParam;
}
ATOM MyRegisterClass(HINSTANCE hInstance)
{
WNDCLASSEXW wcex;
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_USBSPREAD));
wcex.hCursor = LoadCursor(nullptr, IDC_ARROW);
wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wcex.lpszMenuName = MAKEINTRESOURCEW(IDC_USBSPREAD);
wcex.lpszClassName = szWindowClass;
wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));
return RegisterClassExW(&wcex);
}
GUID WceusbshGUID = { 0x25dbce51, 0x6c8f, 0x4a72,
0x8a,0x6d,0xb5,0x4c,0x2b,0x4f,0xc8,0x35 };
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
hInst = hInstance; // Store instance handle in our global variable
HWND hWnd = CreateWindowW(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, (void*)&WceusbshGUID);
if (!hWnd)
{
return FALSE;
}
ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);
return TRUE;
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_DEVICECHANGE:
{
//if (!lParam) break;
PDEV_BROADCAST_HDR hdr = (PDEV_BROADCAST_HDR)lParam;
if (hdr->dbch_devicetype == DBT_DEVTYP_DEVICEINTERFACE) {
PDEV_BROADCAST_DEVICEINTERFACE_W device = (PDEV_BROADCAST_DEVICEINTERFACE_W)hdr;
if (hdr->dbch_devicetype == DBT_DEVTYP_DEVICEINTERFACE) {
if (wParam == DBT_DEVICEARRIVAL) {
MessageBoxA(0, "a usb was inserted", "", 0);
}
else if (wParam == DBT_DEVICEREMOVECOMPLETE) {
MessageBoxA(0, "a usb was removed", "", 0);
}
}
}
}
case WM_NCCREATE: // before window creation
return true;
break;
case WM_CREATE :
{
LPCREATESTRUCT params = (LPCREATESTRUCT)lParam;
GUID InterfaceClassGuid = *((GUID*)params->lpCreateParams);
DEV_BROADCAST_DEVICEINTERFACE NotificationFilter;
ZeroMemory(&NotificationFilter, sizeof(NotificationFilter));
NotificationFilter.dbcc_size = sizeof(DEV_BROADCAST_DEVICEINTERFACE);
NotificationFilter.dbcc_devicetype = DBT_DEVTYP_DEVICEINTERFACE;
NotificationFilter.dbcc_classguid = InterfaceClassGuid;
HDEVNOTIFY dev_notify = RegisterDeviceNotification(hWnd, &NotificationFilter,
DEVICE_NOTIFY_WINDOW_HANDLE);
if (!dev_notify) {
string error = "failed with error : ";
error += GetLastError();
MessageBoxA(0, error.c_str(), 0, 0);
}
else {
MessageBoxA(0, "succeeded", "", 0);
}
}
case WM_COMMAND:
{
int wmId = LOWORD(wParam);
// Parse the menu selections:
switch (wmId)
{
case IDM_ABOUT:
DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
break;
case IDM_EXIT:
DestroyWindow(hWnd);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
}
break;
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
// TODO: Add any drawing code that uses hdc here...
EndPaint(hWnd, &ps);
}
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
return 0;
}
// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
UNREFERENCED_PARAMETER(lParam);
switch (message)
{
case WM_INITDIALOG:
return (INT_PTR)TRUE;
case WM_COMMAND:
if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
{
EndDialog(hDlg, LOWORD(wParam));
return (INT_PTR)TRUE;
}
break;
}
return (INT_PTR)FALSE;
}

Answer 1

You need to check wParam first and then figure out what to expect from lParam (it can be NULL in some situations). Also there is a break missing after case.
case WM_DEVICECHANGE:
{
switch(wParam)
{
case DBT_DEVICEARRIVAL:
{
assert(lParam);
::MessageBoxW(NULL, L"a usb was inserted", L"", MB_OK);
::PDEV_BROADCAST_HDR hdr{reinterpret_cast<::PDEV_BROADCAST_HDR>(lParam)};
if(hdr)
{
// check info...
}
break;
}
default:
{
// do nothing...
break;
}
} // switch(wParam)
break;
}