How to do it...

Once the WPF project has been created, follow these steps to create a single instance of the WPF application:

1. Run the application by pressing the CTRL + F5 key combination. This will launch one instance of the application.

2. Press CTRL + F5 multiple times to launch multiple instances of the application. Now it's time to make the application a single instance application:
   
3. Close all the running processes and then follow the next steps to implement the single instance behavior.
4. Open the MainWindow.xaml and add the window title to Single Instance Demo. Here you can find the entire XAML code:

<Window x:Class="CH01.SingleInstanceDemo.MainWindow" 
  xmlns=
    "http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
     Title="Single Instance Demo"  
     Height="250" Width="400"> 
    <Grid> 
          
    </Grid> 
</Window>

5. Open the App.xaml.cs file and override the base implementation of the OnStartup method.
6. Change the code of the OnStartup method so that it looks like the following code:

protected override void OnStartup(StartupEventArgs e) 
{ 
  base.OnStartup(e); 
 
  var mutex = new Mutex(true, "SingleInstanceDemo",  
  out bool isNewInstance); 
  if (!isNewInstance) 
  { 
    MessageBox.Show("Application instance is  
     already running!"); 
    Shutdown(); 
  } 
} 

7. Add the System.Threading namespace declaration, so that the Mutex can be discoverable. The Mutex resides in the aforesaid namespace.
8. Now compile the project to make sure that there are no compiler errors.
9. Press CTRL + F5, which will run the first instance of the application.
10. Now return to the Visual Studio, without closing the application, and then hit CTRL + F5. This time, instead of launching the application UI, an Application instance is already running! message will pop up on the screen. Clicking OK will close the message.
11. Press CTRL + F5 again. Observe that no second instance of the UI is visible on the screen.