System.Data.SqlClient is not supported on this platform

I ran into the same issue a couple of days ago - I'm not sure what the underlying issue is, but reverting some of the EntityFrameworkCore nuget packages back to 2.0.0 seems to have resolved the issue for me. These are the packages I downgraded:

<PackageReference Include="Microsoft.EntityFrameworkCore" Version="2.0.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="2.0.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="2.0.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="2.0.0" />

I ran into this issue recently with .net standard 2.0 classes being consumed by a regular .net framework app. (.net 4.7.x). The only thing that ultimately fixed my issue was migrating from packages.config to PackageReference on the regular .net app.

Same problem here but for me it is a failure on the part of System.Data.SqlClient to load dynamically as part of a plugin. Our plugin dlls are loaded dynamically via Autofac and a controlling service selects the correct one at run time. Unfortunately System.Data.SqlClient will not load dynamically like this, result in the above error message. So I had to load it when the controlling service starts up. This is obviously not ideal but for now it is a usable workaround as all our plugins are still under our control.

I'll be more specific, following a question in comments.

A service selects plug-ins at run time. The plug-ins register their own dependencies via Autofac and if that dependency is a Nuget package they will also include the package as a normal Nuget dependency.

The controlling service registers the plug-in dlls on start up and the first time they are used the plug-in dependencies are also loaded. When System.Data.SqlClient load is attempted following a call to the plug-in that uses SqlClient the "not supported" error results.

Setting System.Data.SqlClient as a Nuget dependency in the controlling service works OK and the library is loaded correctly without error. However, this is not ideal because the the SqlClient library always has to be loaded by the controlling service even if the plug-in selected to run it does not need it.

In other words the SqlClient library is always loaded at service start up occupying resources, etc when it may not even be needed. But at least it works.

Just in case somebody lands here who is trying to run System.Data.SqlClient on net50/netstandard on rid freebsd-x64: Microsoft.Data.SqlClient worked for me.

Maybe this works on every portable option and/or for all System.[...] ->Microsoft.[...] dll.