C / C++ / MFC

HINSTANCE hInst = ::GetModuleHandle(NULL);

WNDCLASSEX wc;
wc.cbSize=sizeof(WNDCLASSEX);
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hbrBackground = (HBRUSH)GetStockObject( NULL_BRUSH );
wc.hCursor = LoadCursor( NULL, IDC_CROSS );
wc.hIcon = LoadIcon( NULL, IDI_APPLICATION );
wc.hInstance = hInst;
wc.lpfnWndProc = (WNDPROC) CCaptureDlg::WndProc;
wc.lpszClassName = L"Test";
wc.lpszMenuName = 0;
wc.style = CS_HREDRAW | CS_VREDRAW;
wc.hIconSm = NULL;

        m_hWnd=::CreateWindowEx(WS_EX_TRANSPARENT|WS_EX_TOPMOST,L"Test",0, WS_VISIBLE|WS_POPUP,0,0, m_ComputerResolutionX, m_ComputerResolutionY,0,0,0,this);

case WM_MOUSEMOVE:
     {
 if (pClass->m_MouseDown==1)  //to indicate mouse is pressed before drawing
 {
             HDC hdc;
 hdc = ::GetWindowDC(hwnd);
             RECT re;
 SetRect(&re, 0,0,pClass->m_ComputerResolutionX,pClass->m_ComputerResolutionY);

 HBRUSH brush =(HBRUSH)::GetStockObject(NULL_BRUSH);
 if (pClass->m_tempPoint.x!=p.x && pClass->m_tempPoint.y!=p.y)    //if mouse move into different position
 {
     static int count = 0;
     TRACE(L"Test %d\n", count++);
     FillRect(hdc,&re,brush);
     pClass->m_tempPoint=p;
 }

 CPen mypen(PS_DOT, 1, RGB(0,0,0));
 HPEN pen = (HPEN)mypen;

 SelectObject(hdc, pen);
 SelectObject(hdc, brush);
             ::Rectangle( hdc, pClass->m_InitPoint.x, pClass->m_InitPoint.y, p.x , p.y);
             }
 break;
     }