Directory.Build.props

<Project>
 <PropertyGroup>
    <Nullable>enable</Nullable>
    <RunAnalyzersDuringBuild>true</RunAnalyzersDuringBuild>
    <RunAnalyzersDuringLiveAnalysis>true</RunAnalyzersDuringLiveAnalysis>
 </PropertyGroup>
</Project>

EnableNullableReferences.csproj

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
    <WarningLevel>3</WarningLevel>
  </PropertyGroup>

</Project>