When creating extension method for auto Migration in part 3 of the series, it is not going to work directly in the class library, we have to include a reference for AspNetCore in Entities.csproj

<ItemGroup>
 <FrameworkReference Include="Microsoft.AspNetCore.App" />
</ItemGroup>

in order for The 'WebApplication' type to work we have to add using Microsoft.AspNetCore.Builder; in MigrationManager class