Different errors when using .NET framework assembly in .NET standard via nuget vs manual reference

Question

I created a ASP.NET Core web application. In this solution I also have a .NET Standard 2.0 project.

First I added a reference from .NET Standard project to a dll (.NET framework 4.6.1) manually without any issues and it works fine. I have then created a Nuget package for .NET framework dll, and tried to install this package in .NET Standard project.

I got an error

Package xxx was restored using .net framework, version = 4.6.1. instead of project target framework '.NET standard, version = v2.0'. This package may not be fully compatible with your project.

I am confused, how come i can use a .net framework assembly when referenced manually, and when it is installed via nuget i receive this error.

Further more, when i install this nuget in .NET Core 2.1. project (same solution) i get the same error.

Answer 1

This warning is specific to NuGet. When manually adding a dll, no part in the build checks the target framework of that dll (in theory, most dll files should carry a TargetFrameworkAttribute assembly metadata but it isn't required) and you are responsible for making sure that you are using a compatible assembly.

To silence this NuGet warning (and accepting that there may be issues at run-time when using a package originally authored for .NET Framework), you can add this NoWarn metadata to your package reference:

<PackageReference Include="My.Package" Version="1.2.3" NoWarn="NU1603" />