This bug is old, but still occurs. I ran into this issue when running VS 2015 Community. I realize that this question is about VS 2013, but I was not sure else where to post my answer.
I had a Xamarin Forms Xaml app and I wanted to add an F# PCL to do some calculations. I saw the warning message when trying to reference the F# PCL in my C# PCL:
Unable to add a reference to 'PortableLibrary1'. Portable Library projects can only reference other Portable Library projects and assemblies.
I tried editing the csproj file directly and even tried upgrading to VS 2017 RC but nothing was working.
The fix is a bit of a hack.
- First build everything (including the Portable F# project)
- Add the F# reference to the C# project by browsing to the output DLL. i.e. Right Click on CS Project, Add -> Reference... -> Browse -> Click "Browse..." button and select the F# project DLL (mine was in my debug folder), then click OK.
- The F# PCL now shows in the references list in solution explorer.
- Add the F# project reference to the C# project again but by selecting the project i.e. Right Click on CS Project, Add -> Reference... -> Projects -> Check the PortableLibrary project and click OK.
- This gives me no error message box about adding the reference, and I can get to all my F# goodness in my C# project.