Unable to resolve service for type with our own class - asp.net-web-api

Question

We are getting this exception when calling a web api controller:
InvalidOperationException: Unable to resolve service for type 'SDS.Lambda.Interfaces.ISecretManager' while attempting to activate 'SDS.Lambda.Controllers.SapController'.\r\n <p class="location">Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, bool isDefaultParameterRequired)
StartUp.cs contains the following:
public void ConfigureServices(IServiceCollection services)
{
services.AddSingleton<ISecretManager, SecretManager>();
}
The controller has this constructor:
public class SapController : Controller
{
public SapController(ISecretManager secretManager)
{
_secretManager = secretManager;
}
}
We have the same issue with other types being injected into the constructor, but the IConfiguration instance can be injected, for example that parameter does not cause an exception:
public SapController(IConfiguration configuration, ISecretManager secretManager)
The ISecretManager interface looks like this (yes, it really does):
namespace SDS.Lambda.Interfaces
{
public interface ISecretManager
{
}
}
And the class (yes, really - I reduced it down to avoid complexity):
namespace SDS.Lambda.Interfaces
{
public class SecretManager : ISecretManager
{
}
}
Are we providing the interface/concrete type incorrectly?
Is there a way to retrieve the concrete type to test whether it has been provided properly?
When execution reaches the bottom of ConfigureServices, if we look at the services instance result enumeration, in the debugger view, the types we are injecting are listed, so we can't see why they are failing to be instantiated.
UPDATE
To elaborate and explain the issue with another class/dependency in the same solution:
Controller:
namespace SDS.Lambda.Controllers
{
[Route("api/[controller]")]
public class SapController : Controller
{
readonly IHelper helper;
public SapController(IHelper helpme)
{
helper = helpme;
}
...
}
Interface:
namespace SDS.Lambda.Interfaces
{
public interface IHelper
{
}
}
Class:
namespace SDS.Lambda.Helpers
{
public class Helper : IHelper
{
public Helper()
{
}
}
}
StartUp:
namespace SDS.Lambda
{
public class Startup
{
public static IConfiguration Configuration { get; private set; }
private readonly AppSettings _appSettings;
public Startup(IConfiguration configuration)
{
Configuration = configuration;
_appSettings = configuration.GetSection("AppSettings").Get<AppSettings>();
}
public void ConfigureServices(IServiceCollection services)
{
services.AddLogging(logger => logger.AddLambdaLogger());
services.AddSingleton<IHelper, Helper>();
services.AddControllers();
}
...
}