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>

---

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)...

---

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.

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!