Csharp 如何注册实现同一接口的多个服务并注入到控制器构造函数中

我有两个服务实现了相同的接口.在回答How to register multiple implementations of the same interface in Asp.Net Core?之后，我简化了代码，

Interface:

public interface ICheckService
{
    Method1();
    Task Method2();
}

CheckerService:

public class CheckerService : ICheckService
{
    private readonly DapperContext _context;
    private readonly ILogger<CheckerService> _logger;

    public CheckerService(DapperContext context,ILogger<CheckerService> logger)
    {
        _logger = logger;
        _context = context;
    }
}

DesignerService:

public class DesignerService : ICheckService
{
    private readonly DapperContext _context;
    private readonly ILogger<DesignerService> _logger;

    public DesignerService(DapperContext context,ILogger<DesignerService> logger)
    {
        _logger = logger;
        _context = context;
    }
}

在我的Startup.cs文件中:

public delegate ICheckService ServiceResolver(string key);
...
services.AddScoped<ICheckService, DesignerService>();
services.AddScoped<ICheckService, CheckerService>();
services.AddScope<ServiceResolver>(serviceProvider => key =>
{
switch (key)
{
    case "DesignerService":
        return serviceProvider.GetService<DesignerService>();

    case "CheckerService":
        return serviceProvider.GetService<CheckerService>();

    default:
        throw new KeyNotFoundException();
}
});

在Controller:强中

private readonly ICheckService _checklistService;

public DesignerController(ServiceResolver serviceAccessor)
{
    _checklistService = serviceAccessor("DesignerService");
}

但在启动代码后，我发现返回的服务是null.但是如果我删除switch 代码并将控制器构造函数更改为

public DesignerController(ICheckService checkService)
{
    _checklistService = checkService;
}

一切都很好.

有人能告诉我我哪里做错了吗？谢谢