The problem: running unit tests on a Windows Store app that connects to a port on localhost results in the error “A connection attempted failed because the connection party did not properly respond after a period of time …”.

Connecting to a loopback address (127.0.0.1 or localhost) is not supported in a Windows Store application.  Period.

However, connecting to localhost while debugging an application in Visual Studio IS supported.  For example, suppose you have an app that connects to a web service to consume data, and to test it, you want to host the web service on your development machine.  This does work.  How to get it working is a little bit obscure.

To enable connections to localhost, you need to do two things:

• Add the Internet (Client) network capability to your package manifest Package.appxmanifest:
  

  Add network capabilities

  This should be a familiar step.  This capability (and possibly Internet (Client & Server) ) is required for all Windows Store apps that use the internet, even outside of development.  More details here.

• Enable the loopback exemption for the application. This is the obscure part.  There is a list of apps that are allowed to use loopback (but only in development).  There’s a special tool to manage this: CheckNetIsolation.exe.  Open up the Developer Command Prompt for Visual Studio 2012 and enter

  checknetisolation LoopbackExempt -s

  This shows you the list of exempt apps.  My list looks something like this:

  List Loopback Exempted AppContainers
  [1] -----------------------------------------------------------
   Name: 71019b7c-6915-4a32-a9e6-4b87a5b51d02_x1xh075qegwdj
   SID: S-1-15-2-3309481533-1492278452-4137804630-1388503216-2127054748-690736844-819066020
  [2] -----------------------------------------------------------
   Name: kexp90.3fmseattle.kexpartistdiscovery_svgdnat1p2vd6
   SID: S-1-15-2-2012563013-332449103-1712730079-3179316490-2234986268-3338726070-3088550585
  [3] -----------------------------------------------------------
   Name: AppContainer NOT FOUND
   SID: S-1-15-2-3997858894-1722826965-2906774761-292339451-3072000487-3297839464-3180616200
  [4] -----------------------------------------------------------
   Name: AppContainer NOT FOUND
   SID: S-1-15-2-2670002398-4120870820-383953950-3982430728-1889674389-1530876232-793725804
  OK.

  To add your app to the list, use this command:

  checknetisolation LoopbackExempt -a -n=package-name

  where package-name is your application’s package name.  You can find this on the Packaging tab of the app manifest editor:

  

  Windows Store app package name

You can read more about the checknetisolation tool here.

Now you should be able to use StreamSocket to connect to localhost – but only when debugging.