c# - 确保 Controller 有一个无参数的公共(public)构造函数错误

Question

我已经关注了这个 tutorial效果很好，直到我修改了我的 DbContext 以获得额外的构造函数。我现在在解决问题时遇到了问题，不知道该怎么做才能解决这个问题。有没有一种简单的方法可以强制它获取无参数构造函数，或者我处理得不正确？

DbContext 有两个构造函数:

public class DashboardDbContext : DbContext
{
    public DashboardDbContext() : base("DefaultConnection") { }

    public DashboardDbContext(DbConnection dbConnection, bool owns)
        : base(dbConnection, owns) { }
}

SiteController 构造函数:

private readonly IDashboardRepository _repo;

public SiteController(IDashboardRepository repo)
{
    _repo = repo;
}

存储库:

DashboardDbContext _context;

public DashboardRepository(DashboardDbContext context)
{
    _context = context;
}

UnityResolver代码:

public class UnityResolver : IDependencyResolver
{
    private readonly IUnityContainer _container;

    public UnityResolver(IUnityContainer container)
    {
        _container = container;
    }

    public object GetService(Type serviceType)
    {
        try
        {
            return _container.Resolve(serviceType);
        }
        catch (ResolutionFailedException)
        {
            return null;
        }
    }

    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();
    }
}

WebApi配置:

var container = new UnityContainer();
container.RegisterType<IDashboardRepository, DashboardRepository>(new HierarchicalLifetimeManager());
config.DependencyResolver = new UnityResolver(container);

WebApi 调用错误:

System.InvalidOperationException: An error occurred when trying to create a controller of type 'SiteController'. Make sure that the controller has a parameterless public constructor.

at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType) 
at System.Web.Http.Controllers.HttpControllerDescriptor.CreateController(HttpRequestMessage request) 
at System.Web.Http.Dispatcher.HttpControllerDispatcher.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken) 
at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__0.MoveNext()

InnerException: System.ArgumentException: Type 'Dashboard.Web.Controllers.SiteController' does not have a default constructor.

at System.Linq.Expressions.Expression.New(Type type) 
at System.Web.Http.Internal.TypeActivator.Create[TBase](Type instanceType) 
at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.GetInstanceOrActivator(HttpRequestMessage request, Type controllerType, Func`1& activator) 
at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)

教程很棒，在我添加第二个构造函数之前一直对我有用。

Answer 1

发生的事情是你被 this problem 咬了.基本上，发生的事情是您没有在您的容器中明确注册您的 Controller 。 Unity 尝试为您解析未注册的具体类型，但由于它无法解析它(由您的配置错误引起)，它返回 null。它被迫返回 null，因为 Web API 根据 IDependencyResolver 协定强制它这样做。由于 Unity 返回 null，Web API 将尝试自己创建 Controller ，但由于它没有默认构造函数，因此会抛出“确保 Controller 具有无参数公共(public)构造函数”异常。此异常消息具有误导性，无法解释真正的原因。

如果您显式注册您的 Controller ，您会看到更清晰的异常消息，这就是为什么您应该始终显式注册所有根类型。

当然，配置错误来自于您将第二个构造函数添加到您的 DbContext。 Unity 总是尝试选择参数最多的构造函数，但它不知道如何解析这个特定的构造函数。

所以真正的原因是您正在尝试使用 Unity 的 Autowiring 功能来创建 DbContext。 DbContext 是一种特殊类型，不应自动连接。它是一种框架类型，你应该 therefore fallback to registering it using a factory delegate :

container.Register<DashboardDbContext>(
    new InjectionFactory(c => new DashboardDbContext()));