How can a Metro app in Windows 8 communicate with a backend desktop app on the same machine?

120
votes

In a situation where you have the UI frontend built using the new Metro style of apps for windows 8, and would like it to communicate with a .NET application running on the desktop on the same local machine (e.g. a windows service app).

What forms of interprocess communication are available between the metro app and the desktop app?

Thanks to Pavel Minaev of the Visual Studio team, who has provided some initial info here in a comment, quoted:

According to Martyn Lovell, there isn't any deliberate mechanism for that, and some that could be used for it are intentionally restricted. Named pipes aren't there, for example, nor are memory mapped files. There are sockets (including server sockets), but when connecting to localhost, you can only connect to the same app. You could use normal files in one of the shared "known folders" (Documents, Pictures etc), but that is a fairly crude hack that necessitates polling and is visible to the user. -- Pavel Minaev commenting on this issue

So failing normal approaches I was thinking of using web services or reading/writing to a database in order to get some form of communication happening, both of which seem like overkill when the processes are running on the same machine.

Is what I'm attempting here making sense? I can see a need for a metro app to be the frontend UI for an existing service which is running on the desktop. Or is it better to just use WPF for the frontend UI running on the desktop (i.e. a non-metro app).

8
c#wpfwinapiwindows-8windows-runtime
What about a local WCF service? – Gleno
@Gleno that would be covered of "thinking of using web services" in the question. That said, I do wonder if it'll even work - if the implementation of WCF client library that is provided in .NET Core is built on top of WinRT sockets, then presumably the same "no localhost" restriction would apply. This needs to be checked. – Pavel Minaev
It looks like WCF's NetNamedPipeBinding and NetTcpBinding (over localhost) wouldn't be available anyway due to the restrictions in metro. That would leave web services or MSMQ bindings? Am not sure if WCF itself is available in metro to be honest. – dodgy_coder
Let me turn your question around and ask you: What happens if the desktop service with which you're communicating isn't present? Remember that your application can only be installed from the store and thus it can't rely on the presence of the desktop service. – ReinstateMonica Larry Osterman
It appears that enterprises can sideload custom apps and bypass the Windows Store. If so, it would make sense that you could assume some applications were running in the enterprise ennvironment. That said, I think the original poster should use a desktop WPF frontend for his purposes. – Ankur Goel

8 Answers

54
votes

I'm porting my existing project to Win8 right now. It consists of windows service and tray application which are talking to each other via NamedPipes WCF. As you may already know Metro doesn't support named pipes. I ended up using TcpBinding for full duplex connection.

This post describes what functionality is supported.

Sample of my WCF server that Metro client can consume is here.

Also keep in mind that you can't use synchronous WCF in Metro. You'll have to use Task-based wrapper which is only asynchronous.

And thank you for you question. I was good starting point for me :)

38
votes

There were a number of questions like this at the end of a //build/ session I attended. Aleš Holeček, the exec who did one of the big picture sessions, came up out of the audience to handle them. Even if you're not a C++ developer, download that session and watch the Q & A. http://channel9.msdn.com/Events/BUILD/BUILD2011/TOOL-789C

Metro apps can't count on desktop apps or services being installed on the machine. And desktop apps can't count on Metro apps running since they can be suspended any time. You need to start thinking differently. Listen to Aleš on this one.

11
votes

Take note that with Windows 8.1 Update, communication between Windows Store apps and desktop components written in C# for .NET 4.5+ is now officially supported for side-loaded applications in Enterprise scenarios:

Brokered Windows Runtime Components for side-loaded Windows Store apps

To quote:

Recognizing that critical business functions and rules are embodied in existing software assets and that enterprises have a wide variety of scenarios for which the new application style will be highly productive, the Windows 8.1 Update includes a new feature called Brokered Windows Runtime Components for side-loaded applications. We use the term IPC (inter-process communication) to describe the ability to run existing desktop software assets in one process (desktop component) while interacting with this code in a Windows Store app. This is a familiar model to enterprise developers as data base applications and applications utilizing NT Services in Windows share a similar multi-process architecture.

Although implementing this approach is a bit on the complicated side initially, it allows for deep integration across Windows Store and desktop components. Just keep in mind that for the time being, it won't pass public Windows Store certification.

5
votes

There is an article on InfoQ about how to build loosely coupled Metro apps with protocol handlers. This is something which has been supported by Windows for a long time and one could foresee an desktop application register itself as a protocol handler and maybe the metro application can communicate through this mechanism.

I have no idea if this is possible, but it might be interesting to check out.

3
votes

Christophe Nasarre has blogged about a rather hacky way to do it using local files. The result is communication between desktop app/windows store app (referred to as DA/WSA in the blog), without having to switch between the UI of the two apps. He also blogged about another less hacky technique involving protocol handlers.

Note that having a WSA which communicates with a DA is explicitly forbidden by the store App certification requirements

Windows Store apps must not communicate with local desktop applications or services via local mechanisms, including via files and registry keys.

... but it restricts "local mechanisms" only. So I guess one can build a web service for routing the communications.

3
votes

If you think that you can make an additional manual cmd operation, you can try :

X:/> CheckNetIsolation.exe LoopbackExempt –a –n=<packageID>;

CheckNetIsolation.exe is included in winRT install, so there is nothing extra to be installed.

I tried it: it works, even after package updating.

As shown on: http://msdn.microsoft.com/en-us/library/windows/apps/Hh780593.aspx

Here it is explained how to find out the packageID for your app: http://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/82bad7d4-d52b-4731-a396-13ab9004c1cc/how-to-get-the-appid-of-a-metro-style-app-

2
votes

It is possible to communicate on the same machine from Metro app to desktop app using local service. I've implemented some time ago simple "proof of concept", how to bypass the WinRT sandbox using local service. It still needs some kind of "social engineering" or direct guide for installing the service, but anyway, it is possible.
I'm not sure though about the certification rules about "local service" communication when adding such app to Windows Store.

Sample here

By design Metro application cannot access underlying PC directly, only using WinRT API and available capabilities. But when you create back-end service for accessing the PC and all data there, it's basically no longer running in sandbox.

The only "problem" is that user must manually install this back-end service, but that won't be a problem using some "social engineering": User downloads "PC browser" Metro app, user can browse all pictures, music and videos, using WinRT API, but the app also shows message at the bottom: "Download our PC browser powerpack and browse your entire PC, for FREE"

User is redirected to web page, from where user can download classic desktop installer containing "PC browser" back-end service for accessing files on users entire PC. Once this desktop service is installed, the Metro app can detect it and use it for browsing the entire PC. User is happy, but the WinRT sandbox is compromised.

Of course this won't work on Windows 8 ARM tablets. Using this workaround it could be even possible to build Metro app clients for classic desktop apps like antiviruses, torrent/P2P clients, etc.

0
votes

Maybe I missed the point but when activating the Private networks capability I can connect to a local running (http) server using the local IP address (not localhost). This enables my scenario where a winrt app communicates with a wpf desktop app