Package third party dll in context of multi target framework project, .Net Framework & .Net Core - visual-studio

Question

I'm using next configuration within csproj :
<ItemGroup Condition="$(TargetFramework.StartsWith('net4'))">
<Reference Include="amqmdnet">
<HintPath>..\bin\amqmdnet.dll</HintPath>
</Reference>
<Content Include="..\bin\amqmdnet.dll">
<CopyToOutputDirectory>Always</CopyToOutputDirectory>
<Pack>true</Pack>
<PackagePath>lib\$(TargetFramework)</PackagePath>
</Content>
</ItemGroup>
<ItemGroup Condition="$(TargetFramework.StartsWith('netstandard2')) or $(TargetFramework.StartsWith('netcoreapp3'))">
<Reference Include="amqmdnetstd">
<HintPath>..\bin\amqmdnetstd.dll</HintPath>
</Reference>
<Content Include="..\bin\amqmdnetstd.dll">
<CopyToOutputDirectory>Always</CopyToOutputDirectory>
<Pack>true</Pack>
<PackagePath>lib\$(TargetFramework)</PackagePath>
</Content>
</ItemGroup>
each part of it working fine when the project is set with a concrete framework, for example :
<TargetFramework>netcoreapp3.1</TargetFramework>
but when the project is multi framework noting happened, dlls are not included
<TargetFrameworks>netcoreapp3.1;net451</TargetFrameworks>
I'm getting this message in multi framework scenario, this is the only context:
How I proceeded
For some reason nuget spec doesn't fill metadata; I test it with nuget version 5.5.1.6542.
I build the project with VS, how it is.
Change extension of {project folder}\bin\Release\xxx.nupkg to .zip
Extract xxx.nuspec file from xxx.zip. It will contain also dependencies metadata.
Edit xxx.nuspec with NuGet Package Explorer
Build nuget xxx.nuspec
.nuspec documentation
Answer
See this link. NuGet doc
I will post it and here:
<PropertyGroup>
<TargetsForTfmSpecificBuildOutput>$(TargetsForTfmSpecificBuildOutput);GetMyPackageFiles</TargetsForTfmSpecificBuildOutput>
</PropertyGroup>
<Target Name="GetMyPackageFiles">
<ItemGroup Condition="$(TargetFramework.StartsWith('net4'))">
<BuildOutputInPackage Include="amqmdnet.dll">
<FinalOutputPath>..\bin\amqmdnet.dll</FinalOutputPath>
</BuildOutputInPackage>
</ItemGroup>
<ItemGroup Condition="$(TargetFramework.StartsWith('netstandard2')) or $(TargetFramework.StartsWith('netcoreapp3'))">
<BuildOutputInPackage Include="amqmdnetstd.dll">
<FinalOutputPath>..\bin\amqmdnetstd.dll</FinalOutputPath>
</BuildOutputInPackage>
</ItemGroup>
</Target>

Answer 1

Package third party dll in context of multi target framework project,
.Net Framework & .Net Core
I tried your sample and face the same issue in my side. When I use TargetFrameworks to set such dll into multi target framework projects and face the same situation.
And the <pack>true</pack> does not work due to your condition. But when I looked into output folder in such project, <CopyToOutputDirectory>Always</CopyToOutputDirectory> works. And according to the conditions, copy the two types of dll to the corresponding target framework folder.
However, pack does not work,still quite be strange.
So l report this issue to our DC Forum. See this link. You can vote this issue and add any comments if I did not describe the issue in detailed. And anyone who is interested in this issue will vote for you so that it will get more attention from the staff.
Suggestion
As a suggestion, you can use nuspec file with nuget.exe cli to pack your project which I have tested successfully.
1) download nuget.exe from this link and config its path into System Environment Variable PATH.
2) call Developer Command Prompt for VS or CMD and then cd your project path(which xxx.csproj exists)
Then call nuget spec and get xxx.nuspec file
3) open xxx.nuspec file and modify like these:
<?xml version="1.0" encoding="utf-8"?>
<package >
<metadata>
......
</metadata>
<files>
<file src="bin\Debug\net451\amqmdnet.dll" target="lib\net451" />
<file src="bin\Debug\net451\PRCB.IBM.MQ.dll" target="lib\net451"/>
<file src="bin\Debug\netcoreapp3.1\amqmdnetstd.dll" target="lib\netcoreapp3.1" />
<file src="bin\Debug\netcoreapp3.1\PRCB.IBM.MQ.dll" target="lib\netcoreapp3.1" />
</files>
</package>
4) Finally, type nuget pack xxx.nuspec and you will get the xxx.nupkg file.