Test.cs

using System;

namespace Test {
    class Program {
        static void Main() {
            Console.WriteLine("Hello World!");
#if NETFRAMEWORK
            Console.WriteLine(".NET Framework");
            Console.WriteLine((System.Numerics.Vector<byte>)new System.Numerics.Vector<int>(42));
#elif NETCOREAPP
            Console.WriteLine(".NET Core");
            Console.WriteLine(Newtonsoft.Json.JsonConvert.SerializeObject(new { a = 1, b = 2 }));
#else
            Console.WriteLine("Unknown");
#endif
        }
    }
}

Test.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <LangVersion>Latest</LangVersion>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>
      netcoreapp2.1;
      net472;
    </TargetFrameworks>
  </PropertyGroup>

  <PropertyGroup Condition="$(Platform) == 'AnyCPU'">
    <PlatformTarget>AnyCPU</PlatformTarget>
  </PropertyGroup>

  <ItemGroup Condition="$(TargetFramework) == 'netcoreapp2.1'">
    <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
  </ItemGroup>

  <ItemGroup Condition="$(TargetFramework) == 'net472'">
    <PackageReference Include="System.Numerics.Vectors" Version="4.5.0" />
  </ItemGroup>

</Project>