Call C# dll from F#

Question

Is there a simply way to load and call functions of a C# dll from F# both located in the same solution?

I've set up this very simple example:

• My solution is called FSharpInterop and contains two project
  1. One C# library called CSharpComm "CSharpComm\CSharpComm.csproj"
  2. One F# project called FSharpInterop "FSharpInterop\FSharpInterop.fsproj"

The C# library provides this simple service:

using System;

namespace CSharpComm
{
    public class CSharpService
    {
        public string Request()
        {
            return "Hello World provided by C#";
        }
    }
}

1st attempt: open command

After adding the C# project in the F# reference (right click on the References icon Add Reference... > Projects > Select CSharpComm), I tried to run this line in F#:

open CSharpComm

Error message: InteropHelloWorld.fsx(1,6): error FS0039: The namespace or module 'CSharpComm' is not defined.

2nd attempt: #r command

Tried to run:

#r "C:\src\FSharpInterop\CSharpComm\bin\Debug\CSharpComm.dll"

or

#r "C:\src\FSharpInterop\CSharpComm\bin\Debug\CSharpComm"

This ends up with the error message: Invalid directive. Expected '#r "<file-or-assembly>"'.

3rd attempt: Nuclear option

I've tried using the interop service library:

open System.Runtime.InteropServices

module InteropWithNative =
    [<DllImport(@"C:\src\FSharpInterop\CSharpComm\bin\Debug\CSharpComm.dll", CallingConvention = CallingConvention.Cdecl)>]
    extern void Request()
InteropWithNative.Request()

Error message: > System.EntryPointNotFoundException: Unable to find an entry point named 'Request' in DLL 'C:\src\FSharpInterop\CSharpComm\bin\Debug\CSharpComm.dll'. at FSI_0005.InteropWithNative.Request() at <StartupCode$FSI_0005>.$FSI_0005.main@()

Here I guess I need to instantiate a CSharpService object first but the Interop doesn't let me create a ctor in F#. Also, even if I manage to call the C# service that way, that looks a bit overkill for two technologies that supposes to cohabit easily.

Last attempt: Stack overflow

Following this question Import/open DLLs in F#, I tried to add an <ItemGroup> in my F# project but didn't find where and how I can do that (I thought Visual Studio would take care of that when adding the reference project).

Additional info on my solution

F# App.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.8" />
    </startup>
</configuration>

F# packages.config

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="FSharp.Core" version="5.0.1" targetFramework="net48" />
  <package id="System.ValueTuple" version="4.5.0" targetFramework="net48" />
</packages>

[Edit] FSharpInterop.fsproj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <SchemaVersion>2.0</SchemaVersion>
    <ProjectGuid>425bd4f6-3577-4c20-9ceb-3f95609706d8</ProjectGuid>
    <OutputType>Exe</OutputType>
    <RootNamespace>FSharpInterop</RootNamespace>
    <AssemblyName>FSharpInterop</AssemblyName>
    <TargetFrameworkVersion>v4.8</TargetFrameworkVersion>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
    <UseStandardResourceNames>true</UseStandardResourceNames>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <Tailcalls>false</Tailcalls>
    <OutputPath>bin\$(Configuration)\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <WarningLevel>3</WarningLevel>
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DocumentationFile>bin\$(Configuration)\$(AssemblyName).XML</DocumentationFile>
    <Prefer32Bit>true</Prefer32Bit>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <Tailcalls>true</Tailcalls>
    <OutputPath>bin\$(Configuration)\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <WarningLevel>3</WarningLevel>
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DocumentationFile>bin\$(Configuration)\$(AssemblyName).XML</DocumentationFile>
    <Prefer32Bit>true</Prefer32Bit>
  </PropertyGroup>
  <PropertyGroup>
    <MinimumVisualStudioVersion Condition="'$(MinimumVisualStudioVersion)' == ''">11</MinimumVisualStudioVersion>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(FSharpTargetsPath)' == '' AND Exists('$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\FSharp\Microsoft.FSharp.Targets') ">
    <FSharpTargetsPath>$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\FSharp\Microsoft.FSharp.Targets</FSharpTargetsPath>
  </PropertyGroup>
  <Import Project="$(FSharpTargetsPath)" />
  <ItemGroup>
    <Compile Include="AssemblyInfo.fs" />
    <Compile Include="Program.fs" />
    <None Include="App.config" />
    <Content Include="packages.config" />
    <None Include="InteropHelloWorld.fsx" />
  </ItemGroup>
  <ItemGroup>
    <Reference Include="FSharp.Core">
      <HintPath>..\packages\FSharp.Core.5.0.1\lib\netstandard2.0\FSharp.Core.dll</HintPath>
    </Reference>
    <Reference Include="mscorlib" />
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="System.Numerics" />
    <Reference Include="System.ValueTuple">
      <Private>True</Private>
    </Reference>
    <Reference Include="CSharpComm">
        <HintPath>..\CSharpComm\bin\Debug\CSharpComm.dll</HintPath>
    </Reference>
  </ItemGroup>
  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
       Other similar extension points exist, see Microsoft.Common.targets.
  <Target Name="BeforeBuild">
  </Target>
  <Target Name="AfterBuild">
  </Target>
  -->
</Project>

Answer 1

Disclaimer: This is my first ever attempt at F#

Add a reference to the C# project from the F# project

Unsure why this "doesn't work" (reference the project, not the dll as @user1981 stated)

C# Library Project in Solution:

using System;

namespace SampleLibInCSharp
{
    public class CsharpService
    {
        // a static function to do trivial test....
        public static string Request()
        {
            return "Hello World provided by C#";
        }
    }
}

Trivial F# Console App (from VS 2019 template), set as startup app in VS: Program.fs

open System
open SampleLibInCSharp 

// Define a function to construct a message to print
let from whom =
    sprintf "from %s" whom

[<EntryPoint>]
let main argv =
    let message = from "F#" // Call the function
    printfn "Hello world %s" message
    let cs = CsharpService.Request()
    printfn  "ola %s" cs
    0 // return an integer exit code

---

Run the F# console app:

Hello world from F#
ola Hello World provided by C#

Note: I did see some odd red squiglies denoting some syntax error with the message you posted, but it seems more like a VS artifact or bug. Rebuilding and/or running is fine (as above).

As stated, this is a trivial F#/C# test only...

Hth...