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Class Library (Standard 2.1 or Core 3.1) : namespace name 'IWebHostEnvironment' could not be found

public class Class1
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
This fixed (correctly?) the IApplicationBuilder red squiggly :
However the library project does not compile :
"Error  CS0246  The type or namespace name 'IWebHostEnvironment' could not be found ..."    
What references should be added for these two interfaces please ?
Or are you only supposed to only ever use these interfaces within the web site project ?
Also same result with a .NET Core 3.1 Class Library.
Visual Studio 16.4.4 
Any help much appreciated.
You might want to follow this guide (for migrating 2.2 to 3.0)
https://learn.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-3.1&tabs=visual-studio
This explains what happened
https://github.com/dotnet/AspNetCore/issues/7749
Update:
Net Standard
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
<!-- Should work with either if you have sdks installed and do restore -->
<!--<TargetFramework>netstandard2.1</TargetFramework>-->
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Hosting" Version="2.2.7" />    
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="3.1.1" />
  </ItemGroup>
</Project>
With class file 
using Microsoft.AspNetCore.Builder;
using Microsoft.Extensions.Hosting;
namespace ClassLibrary2
    public class Class1
        public void Configure(IApplicationBuilder app, IHostEnvironment env)
Net Core 3.1
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>
</Project>
With class file
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
namespace ClassLibrary3
    public class Class1
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
                Reading your first link, it is explained that is is the otherway round : IHostingEnvironment is being replaced with IWebHostEnvironment. You can confirm this by adding a two new ASP.NET Core projects :  the 2.2 Startup.cs contains : Configure(IApplicationBuilder app, IHostingEnvironment env);  whereas the 3.1 project contains : Configure(IApplicationBuilder app, IWebHostEnvironment env)
– Naked Coder
                Feb 11, 2020 at 14:04
                This answer does not work if you are using <Project Sdk="Microsoft.NET.Sdk">. In order for this to work, you have to switch to <Project Sdk="Microsoft.NET.Sdk.Web"> which also forces you to have a static Program.Main() so it is not an actual class library anymore.
– BearsEars
                Oct 12, 2020 at 17:16
This seems to work in a .NET Core 3.1 Class Library project file, but not a .NET Standard 2.1 one :
I added the following to the project file :
  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>
Now the build succeeds with : 
public class Class1
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
Is there a way for a .NET Standard 2.1 Class Library project to support the new IWebHostEnvironment  ?
        
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