The issue as described below was happening on .NET Core 2.2 until 2019-05-21.

The fix is currently available in the public versions of .NET Core SDK starting from 2.1.700 and 2.2.300. No changes in code needed, dotnet.exe just have to be the fresh one.

On older versions, it might work fine with release build, as shown below.

This information below is now outdated, referencing is working fine with public up to date versions. This repo will be kept only for historical reasons.

The script script.fsx trying to make use of the library B.dll.

The library B.dll, in turn, uses a function from A.dll (B.fsproj has a reference to A.fsproj).

Trying to run the script.fsx leads to an error:

PS C:\...\FsiOddityDemo
[6] > dotnet fsi .\script.fsx
System.IO.FileNotFoundException: Could not load file or assembly 'A, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.
File name: 'A, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'
   at B.Worker.doWork(Int32 x)
   at <StartupCode$FSI_0002>.$FSI_0002.main@()


Stopped due to error
PS C:\...\FsiOddityDemo
[7] >

Solution had been built beforehand, IDE shows autocompletion for the script without issues.

Referencing A.dll in the script explicitly before B.dll doesn't change anything.

Switching between fsi.exe from following locations doesn't change anything:

• C:\Program Files\dotnet\sdk\2.2.202\FSharp
• C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\FSharp

(Both are shown as Microsoft (R) F# Interactive version 10.4.0 for F# 4.6.)

The script script3.fsx trying to make use of the library C.dll.

The library C.dll, in turn, uses a function from A.dll. (C.fsproj has a reference to A.dll).

Trying to run the script3.fsx leads to the same as above error:

PS C:\...\FsiOddityDemo
[2] > dotnet fsi .\script3.fsx
System.IO.FileNotFoundException: Could not load file or assembly 'A, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.
File name: 'A, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'
   at C.Doer.doStuff(Int32 x)
   at <StartupCode$FSI_0001>.$FSI_0001.main@()


Stopped due to error
PS C:\...\FsiOddityDemo
[3] >

It isn't clear how to choose the release build from GUI tools available in vscode so I went with what's available.

Now let's check it specifically.

> dotnet build .\B\B.fsproj -c Release
> dotnet build .\C\C.fsproj -c Release

> dotnet fsi .\script.release.fsx
5 ^2 *2 = 50
> dotnet fsi .\script3.release.fsx
System.IO.FileNotFoundException: Could not load file or assembly 'A, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.
File name: 'A, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'
   at C.Doer.doStuff(Int32 x)
   at <StartupCode$FSI_0001>.$FSI_0001.main@()


Stopped due to error

This means the bug is absent in the release build when the reference is made by project, not dll.

The script script2.fsx works around the issue by loading all source files directly:

[13] > dotnet fsi .\script2.fsx
5 ^2 *2 = 50
[14] >

This is not a feasible way in case of bigger projects with many files. This will require manual tracking of code structure changes.

Added B.Tests project that calls a function from B that calls a function from A. The test works, dependencies are resolved.

I got another issue in the process, but that's irrelevant here.

Added B.DllTests project to do the same while referencing dll instead of fsproj. This works too, after a small fix.

I added the dll reference using DCE "Add DLL Reference" feature. The reference in the fsproj file looked like this:

    <ItemGroup>
        <Reference Include="B">
            <HintPath>FsiOddityDemo\B\bin\Debug\netcoreapp2.2\B.dll</HintPath>
        </Reference>
    </ItemGroup>

Which is wrong. Changing path to ..\B\bin\Debug\netcoreapp2.2\B.dll manually made it to work as intended. Again, this might be something to look into later, but it is irrelevant to the FSI issue above.