Calling class library I get the error system.data.sqlclient is not supported on this platform - asp.net-core

Question

Here is the setup when I am getting this error:
Project "main" based on Core 3.1 and of type console application. Does not have a reference to System.Data.SqlClient and does not need one.
It calls project "process" also based on Core 3.1 of type class library. Does not have a reference to System.Data.SqlClient and does not need one.
"Process" calls project "service" based on Core 3.1 of type class library. This project needs System.Data.SqlClient and has a reference to a package "System.Data.SqlClient (4.5.1)".
When debugging "main", when it gets to "service", the first time it reference SqlConnection, it errors with message "system.data.sqlclient is not supported on this platform"
Why am I getting this error and how can I fix it.
Note: When I add a refernce to System.Data.SqlClient in project "main" - no error, but of course as "main" does not need System.Data.SqlClient, it should not have one.
When I test "service" using xUnit, no errors, it works OK
Thanks

Answer 1

According to the document:
The Microsoft.Data.SqlClient NuGet package includes a number of DLLs
supporting different .NET targets and different runtime platforms. If
you are getting a PlatformNotSupported Exception when you don't think
you should be, it ultimately means your application is not loading the
appropriate DLL. There could be a number of reasons for this. The
NuGet package structure and infrastructure around referencing and
loading referenced NuGet packages includes logic that allows a package
to contain multiple DLLs which implement support for different .NET
and platform targets. Meaning a different DLL for .NET Framework, .NET
Core, .NET Standard, Windows, Linux, etc. The NuGet infrastructure
will automatically reference and load the appropriate DLL based on
your application's needs.
If your application loads a DLL from a NuGet package directly, it
bypasses all this logic and probably loads the incorrect DLL. The DLL
in the NuGet package under
lib/netstandard2.0/Microsoft.Data.SqlClient.dll is basically the
fallback DLL for any unsupported target and simply throws the
PlatformNotSupported exception for any call. This is a nicer exception
than what you would otherwise get when running on a platform that does
not have a DLL built for it. Ultimately, you want to use the NuGet
package reference infrastructure or you would have to implement all
this target framework and platform support logic yourself when
determining which DLL to load.
Additionally, the NuGet package contains all the dependency
information for the SqlClient library and facilitates the downloading
and referencing of dependencies. If you reference and load an
individual DLL manually, it is up to you to ensure all dependencies
are also available to the SqlClient library.
I suggest you could try to use Microsoft.Data.SqlClient instead. More details about cheat sheet for porting from System.Data.SqlClient to Microsoft.Data.SqlClient, you could refer to this article.