title: What causes Windows Firewall to block an application?
body: <p>I have a Windows Forms application that runs locally on the user's desktop. The only way it accesses the Internet is by doing System.Diagnostics.Process.Start(url) to launch the user's default browser and point it to various URLs (to check for updates, contact us, etc.). And none of this happens without the user explicitly requesting it by clicking a menu item or button.</p>
  
  <p>On my machine I have been occasionally getting a Windows Firewall warning message upon starting up the program, saying that Windows Firewall has "blocked some features" of the program to protect the machine. I also occasionally get this warning when running my program within Visual Studio (and the warning dialog says that vshost has been blocked from the network). It doesn't happen all the time.</p>
  
  <p>I have not heard from any of my customers that this has been happening on their PCs, but that doesn't mean it's not. And it's a somewhat scary warning to a less-technically savvy user, so I'd like to figure out how to eliminate it if possible.</p>
  
  <p>What could my program possibly be doing to trigger this warning?</p>
  
  <p>Edit: The only somewhat unusual thing my program is doing at startup is that it uses the Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase class to enforce a single instance application. I know this does some threading magic behind the scenes to detect new instances and redirect them. Is it possible it's listening on the network for some reason?</p>