WindowsVersions

Program.cs

using System;
using Microsoft.Win32;

namespace TestVersion
{
    class Program
    {
        static void Main(string[] args)
        {
            var key = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\Windows NT\CurrentVersion");
            var major = key.GetValue("CurrentMajorVersionNumber") as int?;
            var minor = key.GetValue("CurrentMinorVersionNumber") as int?;
            Console.WriteLine($"{major}.{minor}");
            Console.WriteLine(Environment.OSVersion);
        }
    }
}

WindowsVersion.csproj

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Win32.Registry" Version="4.5.0"/>
  </ItemGroup>

</Project>