Single Instance of a .Net App

Many times, we want to allow our user to run just a single instance of our application. Such is the case when we want to:

1. Avoid resource sharing collisions (your application opens and writes to a file, a second instance would encounter a "file is being use by another process" exception).
2. Pollute a resource with outputs from 2 instances (same file now contains outputs from 2 different applications that look the same).
3. Our server side is not build to deal with 2 clients from the same location.

Here is an easy way to ensure a single instance of a .Net application per session.
This is C# code but can easily be replicated in VB.Net. If you go to your Program.cs file, the one the project wizard creates for you, you may see the following code:

static void Main()
{
   Application.EnableVisualStyles();
   Application.SetCompatibleTextRenderingDefault(false);
   Application.Run(new Form1());
}

Add a using System.Threading; to the head of the file, and channge the main function to look like this:

static Mutex mutex;  // <-- corrected 8/24/2007!
static void Main()
{
   bool isFirstInstanceOfApp;
   mutex = new Mutex(true, "Global\\" + "MyApplicationName",
          out isFirstInstanceOfApp);
   if (isFirstInstanceOfApp)
   {
       Application.EnableVisualStyles();
       Application.SetCompatibleTextRenderingDefault(false);
       Application.Run(new Form1());
   }
   else
   {
       MessageBox.Show("An instance of MyApplicationName is already running",
            "Error", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
   }
}

This adds a global mutex with your application name, since only one such mutex can exist, the next time we try to create it, we'd fail. Since the mutex is a local variable to the main function, it would exist until we exit the program.

Of course, change MyApplicationName to your application name. If you then run a second instance of the application you'd get:

The code to this demo app can be found here.