enable_language(CSharp)
if(NOT CMAKE_CSharp_COMPILER)
    return()
endif()

set(CMAKE_DOTNET_TARGET_FRAMEWORK "netcoreapp3.1")
set(CMAKE_DOTNET_TARGET_FRAMEWORK_VERSION "net461")
add_library(foo SHARED foo.cs)

set(CMAKE_DOTNET_TARGET_FRAMEWORK "netcoreapp3.1;net461")
add_library(bar SHARED foo.cs)