如何创建自定义类的单例并将其注入.Net core 2.0中的其他类？ “没有注册“服务”类型的服务。”？

Question

I have a class in a .Net core 2.0 console application. The class provides data service and will be used by the Main class. So a singleton of the class will be created for DI.

public class Services
{
    private readonly DbContext _context;

    public Services(DbContext context)
    {
        _context = context;
    }

    public IEnumerable<Items> GetList(string region)
    {
        return _context.Items;
    }
}

The following code is in Program class.

var services = new ServiceCollection();
services.AddTransient<Main>(); // With constructor of public Main(Services service)
var serviceProvider = services.BuildServiceProvider();
....
services.AddDbContext<DbContext>(o => o.UseSqlServer(defaultConn));
services.AddSingleton<Services>();
var s = serviceProvider.GetRequiredService<Services>(); // Error
services.AddSingleton(s);

However, the second to the last line got the error of

'No service for type 'Services' has been registered.'

?

The Main class will need to access a singleton of Services using DI.

Answer 1

Once the service provider is built, it will not have knowledge of any additional services added to the service collection. Only build the provider once all the types have been added.

var services = new ServiceCollection();
services.AddTransient<Main>(); // With constructor of public Main(Services service)

//....
services.AddDbContext<DbContext>(o => o.UseSqlServer(defaultConn));
services.AddSingleton<Services>();

var serviceProvider = services.BuildServiceProvider();

var s = serviceProvider.GetRequiredService<Services>(); // Should work now

When Main is resolved the singleton will be injected

var main = serviceProvider.GetService<Main>(); //will include singleton Service