c# - 尝试创建类型为 'EmployeeController' 的 Controller 时出错。确保 Controller 有一个无参数的公共(public)构造函数

Question

为什么我在 Employee Controller 上收到此错误，其余的都运行良好这是我的员工 Controller

public class EmployeeController : ApiController
    {
        #region Call service
        private readonly IEmployeeServices _employeeServices;
        public EmployeeController(IEmployeeServices employeeServices)
        {
            _employeeServices = employeeServices;
        }

        readonly IEmployeeServices employeeServices = new EmployeeServices();

        public EmployeeController():base()
        {
            _employeeServices = employeeServices;
        }
}

这是我完美的工作产品 Controller

public class ProductController : ApiController
    {
        #region Call service

        private readonly IProductServices _productServices;

        public ProductController(IProductServices productServices)
        {
            _productServices = productServices;
        }

        readonly IProductServices productServices = new ProductServices();

        public ProductController()
        {
            _productServices = productServices;
        }
}

这是堆栈跟踪

An error has occurred.An error occurred when trying to create a controller of type 'EmployeeController'. Make sure that the controller has a parameterless public constructor.System.InvalidOperationException 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.<SendAsync>d__1.MoveNext()An error has occurred.Resolution of the dependency failed, type = "TheWork.Controllers.EmployeeController", name = "(none)". Exception occurred while: while resolving. Exception is: InvalidOperationException - The current type, BusinessServices.IEmployeeServices, is an interface and cannot be constructed. Are you missing a type mapping? ----------------------------------------------- At the time of the exception, the container was: Resolving TheWork.Controllers.EmployeeController,(none) Resolving parameter "employeeServices" of constructor TheWork.Controllers.EmployeeController(BusinessServices.IEmployeeServices employeeServices) Resolving BusinessServices.IEmployeeServices,(none) Microsoft.Practices.Unity.ResolutionFailedException at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable1 resolverOverrides)
 at Microsoft.Practices.Unity.UnityContainer.Resolve(Type t, String name, ResolverOverride[] resolverOverrides)
 at Microsoft.Practices.Unity.UnityContainerExtensions.Resolve(IUnityContainer container, Type t, ResolverOverride[] overrides)
 at Unity.WebApi.UnityDependencyScope.GetService(Type serviceType)
 at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.GetInstanceOrActivator(HttpRequestMessage request, Type controllerType, Func1& activator) at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)An error has occurred.The current type, BusinessServices.IEmployeeServices, is an interface and cannot be constructed. Are you missing a type mapping?System.InvalidOperationException at Microsoft.Practices.ObjectBuilder2.DynamicMethodConstructorStrategy.ThrowForAttemptingToConstructInterface(IBuilderContext context) at lambda_method(Closure , IBuilderContext ) at Microsoft.Practices.ObjectBuilder2.DynamicBuildPlanGenerationContext.<>c__DisplayClass1.<GetBuildMethod>b__0(IBuilderContext context) at Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context) at Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context) at Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context) at Microsoft.Practices.ObjectBuilder2.BuilderContext.NewBuildUp(NamedTypeBuildKey newBuildKey) at Microsoft.Practices.Unity.ObjectBuilder.NamedTypeDependencyResolverPolicy.Resolve(IBuilderContext context) at lambda_method(Closure , IBuilderContext ) at Microsoft.Practices.ObjectBuilder2.DynamicBuildPlanGenerationContext.<>c__DisplayClass1.<GetBuildMethod>b__0(IBuilderContext context) at Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context) at Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context) at Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context) at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides)

更新

这是Unity配置

   public static class UnityConfig
    {
        public static void RegisterComponents()
        {
            var container = new UnityContainer();
            System.Web.Mvc.DependencyResolver.SetResolver(new UnityDependencyResolver(container));

            GlobalConfiguration.Configuration.DependencyResolver = new Unity.WebApi.UnityDependencyResolver(container);

            RegisterTypes(container);
        }

        public static void RegisterTypes(IUnityContainer container)
        {
            ComponentLoader.LoadContainer(container, ".\\bin", "TheWork.dll");
            ComponentLoader.LoadContainer(container, ".\\bin", "BusinessServices.dll");
            ComponentLoader.LoadContainer(container, ".\\bin", "DataModel.dll");
        }
}

Answer 1

隐藏在堆栈跟踪中的是问题的根本原因:

InvalidOperationException - The current type, BusinessServices.IEmployeeServices, is an interface and cannot be constructed. Are you missing a type mapping? ----------------------------------------------- At the time of the exception, the container was: Resolving TheWork.Controllers.EmployeeController,(none) Resolving parameter "employeeServices" of constructor TheWork.Controllers.EmployeeController(BusinessServices.IEmployeeServices employeeServices) Resolving BusinessServices.IEmployeeServices,(none) Microsoft.Practices.Unity.ResolutionFailedException at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable1 resolverOverrides)

问题是 EmployeeController 需要 IEmployeeServices 的实例，但 Unity 不知道要实例化什么具体类型。看起来应该通过调用 ComponentLoader.LoadContainer(container, ".\\bin", "BusinessServices.dll"); 来注册实现类，但由于某种原因它没有被注册挂号的。它可能是该代码中的错误，或者 BusinessServices.dll 可能已过时并且不包含 IEmployeeServices 定义。

如果没有看到所有代码和运行时依赖项，很难说出为什么 IEmployeeServices 没有注册(因为类型是动态加载/注册的)。