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Limiting your .NET apps to a single instance

, 3 Nov 2001 190.7K 18
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Shows how to use the Mutex class to limit your app to a single instance using a named mutex

Introduction

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
{
private:
    //our Mutex member variable
    Mutex *m_mutex;	
public:
    CSingleInstance(String *mutexname)
    {
        m_mutex=new Mutex(false,mutexname);
    }
    ~CSingleInstance()
    {
        //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

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 running...so 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.

License

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.

A list of licenses authors might use can be found here

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About the Author

Nish Nishant
United States United States
Nish Nishant is a Software Architect/Consultant based out of Columbus, Ohio. He has over 15 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish is a recipient of the annual Microsoft Visual C++ MVP Award since 2002 (13 consecutive awards as of 2014).

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 CodeProject.com 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 : You can reach Nish on his google email id voidnish.

Website and Blog

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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 
I know this is a little late, but I just found this article and I think I figured out a workaround.

My guess is that in Release mode (with optimizations), the optimizer incorrectly destroys the Mutex prematurely. My workaround is to reference the SingleInstance variable after you need it:
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 running...so exiting!");
    else
        Application::Run(new MainForm());
 
    bool running = si->IsRunning();  // I added this line

    return 0;
}
This way, my understanding is that now the optimizer will not prematurely dispose of the si object, and it seems to work for me.

--
Marcus Kwok

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 
GeneralRe: A better way Pin
Jay Giganti27-Sep-05 3:47
memberJay Giganti27-Sep-05 3:47 
Generalmutex name Pin
Kannan Kalyanaraman9-Nov-01 23:02
memberKannan Kalyanaraman9-Nov-01 23:02 
GeneralRe: mutex name Pin
Nish [BusterBoy]9-Nov-01 23:05
memberNish [BusterBoy]9-Nov-01 23:05 
GeneralNothing new, see details MS Knowledge base Q243953 Pin
MN4-Nov-01 15:46
memberMN4-Nov-01 15:46 
GeneralRe: Nothing new, see details MS Knowledge base Q243953 Pin
Nish [BusterBoy]4-Nov-01 16:09
memberNish [BusterBoy]4-Nov-01 16:09 
GeneralRe: Nothing new, see details MS Knowledge base Q243953 Pin
DenaliChief26-Dec-01 4:33
memberDenaliChief26-Dec-01 4:33 
GeneralRe: Nothing new, see details MS Knowledge base Q243953 Pin
Nish [BusterBoy]26-Dec-01 16:04
memberNish [BusterBoy]26-Dec-01 16:04 
GeneralRe: Nothing new, see details MS Knowledge base Q243953 Pin
Tom Archer26-Feb-02 17:30
memberTom Archer26-Feb-02 17:30 
GeneralRe: Nothing new, see details MS Knowledge base Q243953 Pin
Anonymous7-Oct-02 18:38
sussAnonymous7-Oct-02 18:38 
GeneralWell Done Pin
Colin Davies3-Nov-01 22:45
memberColin Davies3-Nov-01 22:45 
GeneralRe: Well Done Pin
Nish [BusterBoy]3-Nov-01 22:49
memberNish [BusterBoy]3-Nov-01 22:49 
GeneralRe: Well Done Pin
Zac Howland4-Jun-02 5:41
memberZac Howland4-Jun-02 5:41 
GeneralRe: Well Done Pin
Nish - Native CPian4-Jun-02 6:03
memberNish - Native CPian4-Jun-02 6:03 
GeneralRe: Well Done Pin
Zac Howland4-Jun-02 6:15
memberZac Howland4-Jun-02 6:15 
GeneralRe: Well Done Pin
Nish - Native CPian4-Jun-02 6:43
memberNish - Native CPian4-Jun-02 6:43 

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