CSingleInstance v1.0

Welcome to CSingleInstance, a freeware MFC class to make your app into a single instance. Limiting your app to single instance is more tricky in Win32 with the removal of the hPrevInstance parameter from WinMain and the introduction of separate address spaces.

There are numerous examples already out there of making your app single instance, but some such as the one I found on www.codeguru.com require knowledge of undocumented or heavy duty MFC.

Other methods such as using FindWindow, FindWindowEx are not 100% bullet proof due to the preemptive Win32 environment.

This class internally uses a memory mapped file (MMF) into which the handle of the main window of your application is stuffed. When a second instance of your app is run, it detects that the memory mapped file exists, gets the window handle of the first instance from the MMF and reactivates it.

Features

• Very simple API, only 2 functions exposed.
• Does not require any messing around with window styles and your mainfrm source code.
• Much more robust implementation that other methods as already mentioned.
• The classes are fully Unicode compliant and include Unicode built options in the workspace file.

Usage

• To use the class in your code simply include sinstance.cpp in your project and #include sinstance.h in your apps source file. Create your mainfrm window as normal but before you show it, simply include the following code:

CInstanceChecker instanceChecker;
if (instanceChecker.PreviousInstanceRunning())
{
  instanceChecker.ActivatePreviousInstance();
  return FALSE;
}

• That's all there is to it. Your application should now be single instance.
• For those interested the nitty gritty of the code, have a look at sinstance.cpp

History

V1.0 (29th July 1998)

• Initial public release.

API Reference

The API consists of the the two public member functions of the class CSingleInstance

CSingleInstance::PreviousInstanceRunninging

BOOL CSingleInstance::PreviousInstanceRunning()

Return Value

TRUE if a previous instance of this application is running otherwise FALSE.

Remarks

In response to TRUE being sent back the program should call ActivatePreviousInstance and abort prematurely from the current instance of the app.

CSingleInstance::ActivatePreviousInstance

BOOL CSingleInstance::ActivatePreviousInstance()

Remarks

Activates the previous instance of the app as detected in a call to PreviousInstanceRunning.

CONTACTING THE AUTHOR

PJ Naughter
Email: pjn@indigo.ie
Web: http://indigo.ie/~pjn
29th July 1998