Some of the applications need to be restricted to run as a
single instance. This can be achieved very easily in MFC windows
applications.
When the application is started, it has to check if there is
another instance running. If there is one already running, it can display a
message and quit from the environment. The MFC function FindWindow is used
to find out if another instance of the Windows application is already
running. If such an instance is already running, the program can exit.
This check is performed inside the CWinApp derived classes in
MFC. The CWinApp::InitInstance function is the ideal place for performing
such initial check-ups.
The following sample code is a code extract from a Dialog based
MFC application. This code checks if there is an application already
running. If it is running, it displays a messagebox and quits.
BOOL
TestApp::InitInstance()
{
if
(!AfxSocketInit())
{
AfxMessageBox(IDP_SOCKETS_INIT_FAILED);
return FALSE;
}
// Standard initialization
// If you are not using these features and wish to reduce
the size
// of your final executable, you should remove from the
following
// the specific initialization routines you do not need.
#ifdef _AFXDLL
Enable3dControls(); // Call this when using MFC in a
shared DLL
#else
Enable3dControlsStatic(); // Call this when linking to MFC
statically
#endif
// Change the registry key under which our settings are
stored.
// TODO: You should modify this string to be something
appropriate
// such as the name of your company or organization.
SetRegistryKey(_T("Local AppWizard-Generated
Applications"));
// To create the main window, this code creates a new
frame window
// object and then sets it as the application's main
window object.
CMainFrame* pFrame = new CMainFrame;
m_pMainWnd = pFrame;
// create and load the frame with
its resources
if(FindWindow(NULL,"Test")==NULL)
{
pFrame->LoadFrame(IDR_MAINFRAME,
WS_OVERLAPPEDWINDOW | FWS_ADDTOTITLE, NULL,
NULL);
// The one and only window has been initialized, so
show and update it.
pFrame->ShowWindow(0);
pFrame->UpdateWindow();
}
else
{
MessageBox(pFrame->GetSafeHwnd(), "One Instance of TestApp
is already running!","Test",MB_OK|MB_ICONINFORMATION);
PostQuitMessage(0);
}
return TRUE;
}
The code marked in green color is
the one which has to be modified inside the Initinstance
function. There are some other ways to find also. But this is
one of the easier ways.
