Simple Single Executable separate 2.1 and 3.0

Program.cs

using System;

namespace SimpleSingleExecutable
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
        }
    }
}

SimpleSingleExecutable.csproj

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

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

  <PropertyGroup Condition="'$(PublishSingleFile)' == 'true'">
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <PublishSingleFile>true</PublishSingleFile>
    <PublishTrimmed>true</PublishTrimmed>
    <IncludeSymbolsInSingleFile>true</IncludeSymbolsInSingleFile>
  </PropertyGroup>

</Project>