How to use a Mutex (Part 1)

Since you've already learned all about the APIs behind the mutex class, this should be fairly straightforward. We're going to create a mutex with a set name, and try to enter the mutex in App.Open. If we're able to enter the mutex, that means no other mutex has entered it, so we must be the first instance of the application. However, if we fail to enter the mutex, another instance of the application is already running.

The code looks like this:
[rbcode]
Class App Inherits Application
Private Dim mMutex as Mutex

Sub Open()
// Create the mutex with your application's name
mMutex = new Mutex( "My Spiffy Application" )

// Now try to enter the mutex
if not mMutex.TryEnter then
// Since we failed, someone else has a lock on the
// mutex.
MsgBox "Please only use one instance of My Spiffy Application at a time"
Quit
end if

// At this point, we own the lock on the mutex -- we're the first app
End Sub

Sub Close()
// Release our lock on the mutex
mMutex.Leave
End Sub
End Class[/rbcode]

So this will only allow one instance of your application at a time. However, what if you want the user to be able to open multiple versions of the same application? Say you have v1.0 and v2.0 and you want the user to be able to have both open at the same time, but not two copies of the same version. It's easy! Just change the mutex's name so that it includes version information, like this:
[rbcode]
mMutex = new Mutex( "My Spiffy Application v1.0" )
[/rbcode]
Since the version information is specified, it changes the mutex name with each version and so multiple versions can be run at the same time.