"Target F# Runtime" grayed out - why?

Question

I'm trying to change the project target runtime from F# 3.1 to F# 4.0 in VS2015 RTM, but the target runtime dropdown is grayed out:

Any reason for this? How can I fix it?

Answer 1

Check your fsproj file, it needs to specify the FSharp.Core reference templatized by $(TargetFSharpCoreVersion), e.g. <Reference Include="FSharp.Core, Version=$(TargetFSharpCoreVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />.

Some nuget packages whack the templatized FSharp.Core reference and replace it with a hard-coded package-specific version. That prevents the project system from letting you toggle the version easily.

Fix is to simply paste back in the templatized version.