Copy native libs in a local C# project

CSharpNativeLibs.md

I want to copy OS-specific native runtime libraries to the bin directory. This will copy them, but will copy ALL runtimes (win-x64, linux-x64, osx, etc).

<ItemGroup>
  <Content Include="libs/**/*.*" CopyToOutputDirectory="PreserveNewest" Link="%(Filename)%(Extension)" />
</ItemGroup>

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OK so we can just check the current platform, and only copy those:

<ItemGroup Condition="$([MSBuild]::IsOSPlatform('Windows'))">
  <Content Include="libs/win-x64/**" CopyToOutputDirectory="PreserveNewest" Link="%(Filename)%(Extension)" />
</ItemGroup>

Except this will now copy the host OS runtimes when you specify a runtime identifier (ex. dotnet build -r linux-x64 will still copy Windows libs)...

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OK, so we can just use the runtime identifer instead:

<ItemGroup Condition="$(RuntimeIdentifier) == 'win-x64'">
  <Content Include="libs/win-x64/**" CopyToOutputDirectory="PreserveNewest" Link="%(Filename)%(Extension)" />
</ItemGroup>

Except now when you do a standard dotnet build, the runtime identifier is never set, so this breaks simple builds...

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OK, so we can check if the runtime identifier is empty and fallback to the current OS:

<ItemGroup Condition="($(RuntimeIdentifier) == '' and $([MSBuild]::IsOSPlatform('Windows'))) or $(RuntimeIdentifier) == 'win-x64'">
  <Content Include="libs/win-x64/**" CopyToOutputDirectory="PreserveNewest" Link="%(Filename)%(Extension)" />
</ItemGroup>

Except now, when you do ex. dotnet publish -r linux-x64, msbuild runs in multiple steps, and during some of those steps $(RuntimeIdentifer) will not be assigned, so you will end up with both the host OS runtimes and the target runtimes in the bin directory...

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So now what? I genuinely do not know how to simply copy native libs to the bin directory such that dotnet build and dotnet publish -r [target] both work. I could just always copy all the native libraries, regardless of platform, and use NativeLibrary.SetDllImportResolver to find the correct ones at runtime, but that seems kind of ridiculous.

Hey thanks for the detailed answer! :)

try setting a default RID

This sort of works but has the issue I mentioned in the original post where I believe during a dotnet publish -r [target] it will copy content for both the host runtimes and the target runtimes ...

For me, a target running before BeforeBuild always has a RuntimeIdentifier set

Unfortunately I don't seem to be able to get this to work... RuntimeIdentifier seems to always be empty, even in BeforeBuild and AfterBuild, unless I add <RuntimeIdentifiers> to the PropertyGroup, in which case it will perform both an empty one and then one with the correct value. I guess this makes sense as it does a generic build and then since it has a matching runtime identifier, it performs that build.

assuming the native asset is coming from a nuget package with the expected relative path

I'm not using a nuget package in this case as this is just a local csproj (imagine I wrote a C dll and have it for various runtimes, and want my C# project to use it). I know nuget handles this more gracefully as I have set up public-facing nuget packages that do this before. If that's the intended route I could create a local nuget package I guess, but that also seems weird?

please capture a binlog of your build misbehaving

I'll do this and DM you shortly!

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I've been investigating a bit more and I do think my project has some weirdness. Alongside this local project with custom libs, I am importing nuget packages with runtimes libs (ImGui), and so even when I don't include -r in my build, it still puts everything into an RID folder - but all my reporting ($(OutputPath), $(PublishDir), $(OutDir), $(TargetDir)) don't include that in their path. I was briefly thinking I could just detect the runtime from the path but

OK, after a lot of debugging with Kate I have come to a solution:

I needed to specify that the csproj was not RID agnostic, and would use the current RID so that dotnet build still worked:

<IsRidAgnostic>false</IsRidAgnostic>
<UseCurrentRuntimeIdentifier>true</UseCurrentRuntimeIdentifier>

This would ensure that an RID always exists, regardless of whether it's passed in through -r or not, for both dotnet build and dotnet publish.

After that, it was fairly trivial to copy the libraries needed to the bin:

<Target Name="CopyNativeLibs" BeforeTargets="PrepareForBuild">
  <Message Importance="High" Text="RuntimeIdentifier=$(RuntimeIdentifier)" />
  <ItemGroup>
    <Content Include="libs/$(RuntimeIdentifier)/*" />
    <Content
      Update="libs/$(RuntimeIdentifier)/*" 
      CopyToOutputDirectory="PreserveNewest"
      TargetPath="%(Filename)%(Extension)"
      Link="%(Filename)%(Extension)"/>
  </ItemGroup>
</Target>

The only other thing that had to happen was to disable default items as otherwise a modern csproj will include all of the libraries anyway. And then it worked!

A few other notes:

• I couldn't use <Copy> because this is a Library project, included several layers deep. <Copy> wouldn't put it in the Exe bin dir, just the Library bin.
• Because it was nested through several references, I had to make sure the whole chain had <IsRidAgnostic> set to false.

Ultimately a lot of this would be avoided if you just copied all of the native libs and accepted they'd all be there, but I wanted to try and figure this out. Appreciate the help from everyone who reached out!