Regular.csproj

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

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

</Project>

RegularWithVersion.csproj

<Project Sdk="Microsoft.NET.Sdk/2.1.0,min=2.0.0">

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

</Project>

SdkAsElement.csproj

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

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

</Project>

SdkAsExplicitImport.csproj

<Project>

  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />

</Project>