I had problem, that
IServiceCollection does not contain a definition for ‘AddSwaggerGen’
I turnes out, that I installed Swashbuckle.AspNetCore.Swagger nuget package instead of Swashbuckle.AspNetCore.
In .NET Core 3, there’s some issues as discussed here. The solution is to add the following to the project file, replacing the prior version.
<PackageReference Include="Swashbuckle.AspNetCore" Version="5.0.0-rc2" />
<PackageReference Include="Swashbuckle.AspNetCore.SwaggerUI" Version="5.0.0-rc2" />