1. output.log

$ msbuild FSharpMod.sln -restore -p:Configuration=Release          
Microsoft (R) Build Engine version 16.6.0 for Mono
Copyright (C) Microsoft Corporation. All rights reserved.

Building the projects in this solution one at a time. To enable parallel build, please add the "-m" switch.
Build started 8/14/2021 1:59:20 PM.
Project "/Users/ec/Sync/Code/FSharpMod/FSharpMod.sln" on node 1 (Restore target(s)).
ValidateSolutionConfiguration:
  Building solution configuration "Release|Any CPU".
_GetAllRestoreProjectPathItems:
  Determining projects to restore...
Restore:
  Restoring packages for /Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/FSharpModLibrary.fsproj...
  Committing restore...
  Generating MSBuild file /Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/obj/FSharpModLibrary.fsproj.nuget.g.props.
  Writing assets file to disk. Path: /Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/obj/project.assets.json
  Restored /Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/FSharpModLibrary.fsproj (in 248 ms).
  
  NuGet Config files used:
      /Users/ec/.config/NuGet/NuGet.Config
  
  Feeds used:
      https://api.nuget.org/v3/index.json
Done Building Project "/Users/ec/Sync/Code/FSharpMod/FSharpMod.sln" (Restore target(s)).
Project "/Users/ec/Sync/Code/FSharpMod/FSharpMod.sln" on node 1 (default targets).
ValidateSolutionConfiguration:
  Building solution configuration "Release|Any CPU".
Project "/Users/ec/Sync/Code/FSharpMod/FSharpMod.sln" (1:2) is building "/Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/FSharpModLibrary.fsproj" (2:6) on node 1 (default targets).
GenerateFSharpTextResources:
  Creating directory "obj/Release/net472/".
CoreCompile:
  /usr/local/Cellar/mono/6.12.0.122/lib/mono/fsharp/fsc.exe -o:obj/Release/net472/FSharpModLibrary.dll
  --debug:portable
  --noframework
  --define:TRACE
  --define:RELEASE
  --define:NETFRAMEWORK
  --define:NET472
  --optimize+
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/Assembly-CSharp-firstpass.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/Assembly-CSharp.dll
  -r:/Users/ec/.nuget/packages/fsharp.core/5.0.0/lib/netstandard2.0/FSharp.Core.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/ISharpZipLib.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/Mono.Posix.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/Mono.Security.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/mscorlib.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/NAudio.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/NVorbis.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.Configuration.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.Core.dll
  -r:/usr/local/Cellar/mono/6.12.0.122/lib/mono/4.7.2-api/System.Data.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.dll
  -r:/usr/local/Cellar/mono/6.12.0.122/lib/mono/4.7.2-api/System.Drawing.dll
  -r:/usr/local/Cellar/mono/6.12.0.122/lib/mono/4.7.2-api/System.IO.Compression.FileSystem.dll
  -r:/usr/local/Cellar/mono/6.12.0.122/lib/mono/4.7.2-api/System.Numerics.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.Runtime.Serialization.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.Security.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.ServiceModel.Internals.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.Xml.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/System.Xml.Linq.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/Unity.TextMeshPro.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.AccessibilityModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.AIModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.AndroidJNIModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.AnimationModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ARModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.AssetBundleModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.AudioModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ClothModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ClusterInputModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ClusterRendererModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.CoreModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.CrashReportingModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.DirectorModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.DSPGraphModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.GameCenterModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.GridModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.HotReloadModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ImageConversionModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.IMGUIModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.InputLegacyModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.InputModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.JSONSerializeModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.LocalizationModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ParticleSystemModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.PerformanceReportingModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.Physics2DModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.PhysicsModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ProfilerModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.ScreenCaptureModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.SharedInternalsModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.SpriteMaskModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.SpriteShapeModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.StreamingModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.SubstanceModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.SubsystemsModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.TerrainModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.TerrainPhysicsModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.TextCoreModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.TextRenderingModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.TilemapModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.TLSModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UI.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UIElementsModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UIModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UmbraModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UNETModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityAnalyticsModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityConnectModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityTestProtocolModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityWebRequestAssetBundleModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityWebRequestAudioModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityWebRequestModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityWebRequestTextureModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.UnityWebRequestWWWModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.VehiclesModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.VFXModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.VideoModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.VRModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.WindModule.dll
  -r:/Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472/UnityEngine.XRModule.dll
  -r:/usr/local/Cellar/mono/6.12.0.122/lib/mono/4.7.2-api/Facades/netstandard.dll
  --target:library
  --warn:3
  --warnaserror:3239,76
  --fullpaths
  --flaterrors
  --subsystemversion:6.00
  --highentropyva+
  --targetprofile:mscorlib
  --nocopyfsharpcore
  --deterministic+
  --simpleresolution
  obj/Release/net472/.NETFramework,Version=v4.7.2.AssemblyAttributes.fs
  obj/Release/net472/FSharpModLibrary.AssemblyInfo.fs
  Library1.fs
  
FSC : error FS0078: Unable to find the file 'System.Numerics.dll' in any of /Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472 /Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary /usr/local/Cellar/mono/6.12.0.122/lib/mono/fsharp/ [/Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/FSharpModLibrary.fsproj]
Done Building Project "/Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/FSharpModLibrary.fsproj" (default targets) -- FAILED.
Done Building Project "/Users/ec/Sync/Code/FSharpMod/FSharpMod.sln" (default targets) -- FAILED.

Build FAILED.

"/Users/ec/Sync/Code/FSharpMod/FSharpMod.sln" (default target) (1:2) ->
"/Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/FSharpModLibrary.fsproj" (default target) (2:6) ->
(CoreCompile target) -> 
  FSC : error FS0078: Unable to find the file 'System.Numerics.dll' in any of /Users/ec/.nuget/packages/krafs.rimworld.ref/1.3.3087/ref/net472 /Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary /usr/local/Cellar/mono/6.12.0.122/lib/mono/fsharp/ [/Users/ec/Sync/Code/FSharpMod/Source/FSharpModLibrary/FSharpModLibrary.fsproj]

    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:03.12

2. FSharpMod.sln

Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio Version 16
VisualStudioVersion = 16.0.30611.23
MinimumVisualStudioVersion = 10.0.40219.1
Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "FSharpModLibrary", "Source\FSharpModLibrary\FSharpModLibrary.fsproj", "{A5D6A1AB-F391-46D4-BC74-F017F708FED0}"
EndProject
Global
	GlobalSection(SolutionConfigurationPlatforms) = preSolution
		Debug|Any CPU = Debug|Any CPU
		Release|Any CPU = Release|Any CPU
	EndGlobalSection
	GlobalSection(ProjectConfigurationPlatforms) = postSolution
		{A5D6A1AB-F391-46D4-BC74-F017F708FED0}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{A5D6A1AB-F391-46D4-BC74-F017F708FED0}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{A5D6A1AB-F391-46D4-BC74-F017F708FED0}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{A5D6A1AB-F391-46D4-BC74-F017F708FED0}.Release|Any CPU.Build.0 = Release|Any CPU
	EndGlobalSection
	GlobalSection(SolutionProperties) = preSolution
		HideSolutionNode = FALSE
	EndGlobalSection
	GlobalSection(ExtensibilityGlobals) = postSolution
		SolutionGuid = {18F7C863-BF2C-41D1-B7B7-4FE04FAD4A4E}
	EndGlobalSection
EndGlobal

3. FSharpModLibrary.fsproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Library</OutputType>
    <TargetFramework>net472</TargetFramework> 

  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
    <OutputPath>../../Assemblies</OutputPath>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Library1.fs" />
    <None Include="Script.fsx" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="Krafs.Rimworld.Ref" Version="1.3.3087" />
  </ItemGroup>
</Project>

4. Library1.fs

amespace FSharpModLibrary

type Class1() = 
    member this.X = "F#"