c# - 无法解决与 Unity 和 WebAPI 的依赖关系

Question

尝试在 WebAPI 中运行我的 Controller 操作时，我收到了这个奇怪的错误:

An error occurred when trying to create a controller of type 'PostController'. Make sure that the controller has a parameterless public constructor.
Resolution of the dependency failed, type = "Example.Controllers.PostController", name = "(none)".

异常发生在:调用构造函数 Example.Models.PostRepository() 时。异常是:NullReferenceException - 未将对象引用设置为对象的实例。

这是有问题的代码:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        var container = new UnityContainer();
        container.RegisterType<IPostRepository, PostRepository>(new HierarchicalLifetimeManager());

        config.DependencyResolver = new UnityResolver(container);
        config.MapHttpAttributeRoutes();
        config.Routes.MapHttpRoute(
            name: "ExampleApi",
            routeTemplate: "api/{controller}"
        );
    }
}

public class PostController : ApiController
{
    IPostRepository _repository;

    public PostController(IPostRepository repository)  
    {
        _repository = repository;
    }

    public IEnumerable<Post> GetAllProducts()
    {
        return _repository.GetAll();
    }
}

public class PostRepository : IPostRepository
{
    private IDbConnection _connection = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString);

    public IEnumerable<Post> GetAll()
    {
        return _connection.Query<Post>("SELECT * FROM Posts").ToList();
    }
}

public class UnityResolver : IDependencyResolver
{
    protected IUnityContainer Container;

    public UnityResolver(IUnityContainer container)
    {
        if (container == null)
        {
            throw new ArgumentNullException("container");
        }

        this.Container = container;
    }

    public object GetService(Type serviceType)
    {
        if (!Container.IsRegistered(serviceType))
        {
            if (serviceType.IsAbstract || serviceType.IsInterface)
            {
                return null;
            }
        }

        return Container.Resolve(serviceType);
    }

    public IEnumerable<object> GetServices(Type serviceType)
    {
        try
        {
            return Container.ResolveAll(serviceType);
        }
        catch (ResolutionFailedException)
        {
            return new List<object>();
        }
    }

    public IDependencyScope BeginScope()
    {
        var child = Container.CreateChildContainer();
        return new UnityResolver(child);
    }

    public void Dispose()
    {
        Container.Dispose();
    }
}

有没有人知道可能是什么原因？我遵循了本教程:http://www.asp.net/web-api/overview/advanced/dependency-injection

Answer 1

根据我的经验，当我看到此消息时，它通常无法构建某些依赖项或缺少依赖项。您已将 IPostRepository 注册到 PostRepository，这样看起来不错。 PostRepository 中创建的 SqlConnection 呢？您能否仅针对存储库运行一些测试代码，看看它是否可以自行构建？​​

在浏览您的代码时，还会检查是否有接口(interface)阻止您解析 UnityResolver 类中的 IPostRepository？

    if (serviceType.IsAbstract || serviceType.IsInterface)
    {
        return null;
    }