Limiting your .NET apps to a single instance

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.