Making a singleton application, i.e., preventing users from opening multiple instances of your app, can be easily implemented using a Mutex.

A Mutex is similar to a C# lock, except it can work across multiple processes, i.e. it is a computer-wide lock. Its name comes from the fact that it is useful in coordinating mutually exclusive access to a shared resource.

Let’s take a simple Console application as an example:

    class Program
    {
        static void Main()
        {
            // main application entry point
            Console.WriteLine("Hello World!");
            Console.ReadKey();
        }
    }

Using a Mutex, we can change the above code to allow only a single instance to print Hello World! and the subsequent instances to exit immediately:

static void Main()
{
    // Named Mutexes are available computer-wide. Use a unique name.
    using (var mutex = new Mutex(false, "saebamini.com SingletonApp"))
    {
        // TimeSpan.Zero to test the mutex's signal state and
        // return immediately without blocking
        bool isAnotherInstanceOpen = !mutex.WaitOne(TimeSpan.Zero);
        if (isAnotherInstanceOpen)
        {
            Console.WriteLine("Only one instance of this app is allowed.");
            return;
        }

        // main application entry point
        Console.WriteLine("Hello World!");
        Console.ReadKey();
        mutex.ReleaseMutex();
    }
}

Note that we’ve passed false for the initiallyOwned parameter, because we want to create the mutex in a signaled/ownerless state. The WaitOne call later will try to put the mutex in a non-signaled/owned state.

Once an instance of the application is running, the saebamini.com SingletonApp Mutex will be owned by that instance, causing further WaitOne calls to evaluate to false until the running instance relinquishes ownership of the mutex by calling ReleaseMutex.

Keep in mind that only one thread can own a Mutex object at a time, and just as with the lock statement, it can be released only from the same thread that has obtained it.