Calling F# Libraries from Metro Style Apps

This weekend, I finally got around to trying to build a polyglot Metro style app (C# front-end + F# back-end). This post will talk about the project file changes that were required and the one outstanding issue that I'm still working. Additionally, a VSIX package is provided that provides the F# Metro Library project template as it exists today.

Note: This approach has only been tested with the simplest of examples.

Getting Setup:

1. In order to create a Metro style app, you need to install Windows 8 Developer Preview with Developer Tools.
2. To also have F#, you'll then need to install Visual Studio 11 Developer Preview.
3. Now create a new C# Windows Metro style application (I chose the Application template).

Adding F#:

If you have installed the VSIX that is linked at the bottom of this post, you can simply add the F# Metro Library, write some code, wire it up, and test. If not, you'll need to add a standard F# Library project and modify the project file.

Modifying the F# Project File:

To allow a F# Library project to be called from a Metro app., the project file must be modified so that it imports the Microsoft.Windows.UI.Xaml.Common.targets. While this small adjustment allows the project to be interacted with, the reference shown in the Metro style project will provide a warning indicating that something isn't quite copacetic. This warning can be eliminated by adding the following elements to the property group:

<TargetFrameworkIdentifier>.NETCore</TargetFrameworkIdentifier><TargetFrameworkVersion>v4.5</TargetFrameworkVersion>

Unfortunately, adding these elements causes some sort of conflict with the F# targets, which I am still working to resolve. I have not yet run into an issue (other than the unsightly warning display) that is caused by leaving out these two elements, so for the moment only the common Xaml targets import has been added.   

Conclusion:

While there is still a lot of work to do, adding the import of the common Xaml targets to the F# project file will allow you to start using F# projects from Metro style apps (Note: In simple cases only--for now).  Once the remaining issues are identified and resolved, I'll add the F# Metro Library VSIX to Visual Studio Gallery. Until then, you can find the F# Metro Library VSIX (with the known issues) here.