Adding any files from old NuGet packages projects into .NET Core SDK-based MSBuild projects

Not so long ago, while working on an open-source project, I had to solve a problem: how to integrate an old C++/CLI NuGet package (built in 2012) that's only compatible only with the full .NET Framework, and a project built using .NET Core SDK. While solving the issue, I've found several peculiarities in MSBuild and NuGet integration, and was eventually able to solve the problem.

The package in question contains several native DLLs. That means that my project couldn't have any references to these DLLs, but I need these files in my output directory (like bin/Debug) to run the project.

For example, there's a file lib/net20/Cosmo.dll that should be placed into the same directory as MyProject.exe after the build.

The NuGet package in question works correctly with "old" .NET SDK, because in the old days MSBuild was silently copying everything from the lib/net20 directory into the output directory. For the new SDK, package authors should use contentFiles (and actually they should've used that for the old SDK, too), although, obviously, it's unlikely they'll ever fix their C++/CLI package to be compatible with the .NET Core SDK: it isn't in fact compatible with .NET Core at all, so why bother?

Fortunately, MSBuild is flexible enough to allow us to fix the issue by writing a bit of additional XML. We'll have to manually create a package file list (note that wildcards are supported), and make MSBuild to copy these files to the output directory.

The following code should be included into the <Project> element in your .csproj:

<PropertyGroup>
    <ActivizNetPackage>Activiz.NET.x64</ActivizNetPackage>
    <ActivizNetVersion>5.8.0</ActivizNetVersion>
    <ActivizNetPackagePath>$(NuGetPackageRoot)\$(ActivizNetPackage)\$(ActivizNetVersion)</ActivizNetPackagePath>
    <ActivizNetContents>$(ActivizNetPackagePath)\lib\net20\*.dll</ActivizNetContents>
    <ActivizNetExclude>$(ActivizNetPackagePath)\lib\net20\msvc?90.dll</ActivizNetExclude>
</PropertyGroup>
<ItemGroup>
    <PackageReference Include="$(ActivizNetPackage)" Version="$(ActivizNetVersion)"/>
    <Content CopyToOutputDirectory="Always" Include="$(ActivizNetContents)">
        <Visible>false</Visible>
    </Content>
    <Content Remove="$(ActivizNetExclude)" />
</ItemGroup>

This sample shows how to get the package content path (via the NuGetPackageRoot builtin variable) and how to add that package content into your project (optionally excluding some items with <Content Remove="…"/>).

Using this mechanic, you can create a .NET Core SDK project that will use any old NuGet packages that used to rely on this MSBuild + NuGet quirk.