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Posted 3 Nov 2001

Limiting your .NET apps to a single instance

, 3 Nov 2001
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Shows how to use the Mutex class to limit your app to a single instance using a named mutex


Sometimes you'd want to limit your application to a single instance. In Win32 we had the CreateMutex API function using which we could create a named mutex and if the call failed, we assume that the application is already running. Well the .NET SDK gives us the Mutex class for inter-thread/inter-process synchronization. Anyway in this article we are more interested in using the Mutex class to limit our apps to a single instance rather than in its use as an inter-process data synchronization object.

The Class

The code below is nothing new. It's simply a .NET version of a universal technique that has been used pretty much successfully over the years. For a thorough understanding of this technique and other techniques and issues involved when making single instance applications, you must read Joseph M Newcomer's article - Avoiding Multiple Instances of an Application

__gc class CSingleInstance
    //our Mutex member variable
    Mutex *m_mutex;	
    CSingleInstance(String *mutexname)
        m_mutex=new Mutex(false,mutexname);
        //we must release it when the CSingleInstance object is destroyed
        m_mutex->ReleaseMutex ();
    bool IsRunning()
        //you can replace 10 with 1 if you want to save 9 ms
        return !m_mutex->WaitOne(10,true);           

Using it in your program

<pre lang=mc++>int __stdcall WinMain() { //create a mutex with a unique name CSingleInstance *si= new CSingleInstance("{94374E65-7166-4fde-ABBD-4E943E70E8E8}"); if(si->IsRunning()) MessageBox::Show("Already exiting!"); else Application::Run(new MainForm()); return 0; }

Remember to put the following line on top of your program.

using namespace System::Threading;

I have used the string {94374E65-7166-4fde-ABBD-4E943E70E8E8}as my unique mutex name. You can use a name that you believe will be unique to your application. Using a GUID would be the smartest option obviously. You can put the class in a DLL and thus you can use it from all your applications.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Nish Nishant
United States United States
Nish Nishant is the Principal Software Architect/Consultant for Ganymede Software Solutions LLC, and is based out of Columbus, Ohio. He has over 17 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish was a Microsoft Visual C++ MVP between 2002 and 2015.

Nish is an industry acknowledged expert in the Microsoft technology stack. He authored C++/CLI in Action for Manning Publications in 2005, and had previously co-authored Extending MFC Applications with the .NET Framework for Addison Wesley in 2003. In addition, he has over 140 published technology articles on and another 250+ blog articles on his WordPress blog. Nish is vastly experienced in team management, mentoring teams, and directing all stages of software development.

Contact Nish : If you are interested in hiring Nish as a consultant, you can reach him via his google email id voidnish.

Company Website :

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Comments and Discussions

Generaldoesn't work in a release build Pin
Anonymous28-Jun-04 12:31
sussAnonymous28-Jun-04 12:31 
GeneralRe: doesn't work in a release build Pin
Kluch27-Oct-04 13:25
memberKluch27-Oct-04 13:25 
AnswerRe: doesn't work in a release build Pin
ricecake12-Mar-07 10:50
memberricecake12-Mar-07 10:50 
GeneralQuestion on how to add bring to foreground Pin
smesser29-Feb-04 8:14
membersmesser29-Feb-04 8:14 
GeneralException in ReleaseMutex() in destructor Pin
Serge T11-Mar-03 7:16
sussSerge T11-Mar-03 7:16 
GeneralRe: Simplest Way To Make Single Instance Application Pin
Nish [BusterBoy]13-May-02 23:38
memberNish [BusterBoy]13-May-02 23:38 
GeneralRe: Simplest Way To Make Single Instance Application Pin
Anonymous14-May-02 0:13
memberAnonymous14-May-02 0:13 
GeneralRe: Simplest Way To Make Single Instance Application Pin
A.R Gupta14-May-02 0:14
memberA.R Gupta14-May-02 0:14 
GeneralA better way Pin
Andy Smith4-Mar-02 11:06
memberAndy Smith4-Mar-02 11:06 
GeneralRe: A better way Pin
Ramu Pulipati26-Jun-02 5:08
memberRamu Pulipati26-Jun-02 5:08 

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