From a windows .net application is it possible to detect if the application is already running?

Yes, this is possible. You have to use System.Threading.Mutex. This is a system wide handle for an application. A mutex has a system wide mutex name i.e. "MYAPPLICATION". You can check for the mutex and if it exists, you know that your application is already running and you can react accordingly. Here is an example from a book:

using System.Threading;

class Program
{
static void Main(string[] args)
{
Mutex oneMutex = null;
const string MutexName = "RUNMEONLYONCE";

try // Try and open the Mutex
{
oneMutex = Mutex.OpenExisting(MutexName);
}
catch (WaitHandleCannotBeOpenedException)
{
// Cannot open the mutex because it doesn't exist
}

// Create it if it doesn't exist
if (oneMutex == null)
{
oneMutex = new Mutex(true, MutexName);
}
else
{
// Close the mutex and exit the application
// because we can only have one instance
oneMutex.Close();
return;
}

Console.WriteLine("Our Application");
Console.Read();
}
}

Let me know if this helped you!

Cheers

Peter

Thanks, Peter. this is perfect!

For VB.NET take Project Properties --> Enable Application Framework --> Windows Application Framework Properties --> Check in Make Single Instance Application