我已经看过几个关于如何为MVC 5项目实现统一的教程和stackoverflow问题，但我似乎无法克服这个错误：
无法构造String类型。您必须配置容器以提供此值。
我安装了NuGet包Unity.Mvc5，并在unity配置中注册了我的类型。我还在unityconfig文件中调用了register components方法。
public static class UnityConfig
{
public static void RegisterComponents()
{
var container = new UnityContainer();
// register all your components with the container here
// it is NOT necessary to register your controllers
// e.g. container.RegisterType<ITestService, TestService>();
container.RegisterType<IStateService, StateService>();
DependencyResolver.SetResolver(new UnityDependencyResolver(container));
}
}
protected void Application_Start()
{
AreaRegistration.RegisterAllAreas();
RouteConfig.RegisterRoutes(RouteTable.Routes);
UnityConfig.RegisterComponents();
}
public class StateController : Controller
{
private readonly IStateService stateService;
public StateController(IStateService stateService)
{
this.stateService = stateService;
}
// GET: /State/
public ActionResult Index()
{
return View(stateService.GetStates());
}
}
public interface IStateService : IDisposable
{
IEnumerable<State> GetStates();
}
public class StateService : IStateService
{
private MyAppDbContext context;
public StateService() : this(new MyAppDbContext()){}
public StateService(MyAppDbContext context)
{
this.context = context;
}
private IQueryable<State> All()
{
return context.States;
}
public IEnumerable<State> GetStates()
{
return this.All().ToList();
}
public void Dispose()
{
context.Dispose();
}
}
这是堆栈跟踪：
[InvalidOperationException: The type String cannot be constructed. You must configure the container to supply this value.]
Microsoft.Practices.ObjectBuilder2.DynamicMethodConstructorStrategy.GuardTypeIsNonPrimitive(IBuilderContext context) +311
Microsoft.Practices.ObjectBuilder2.DynamicMethodConstructorStrategy.PreBuildUp(IBuilderContext context) +229
Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context) +274
Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlanCreatorPolicy.CreatePlan(IBuilderContext context, NamedTypeBuildKey buildKey) +162
Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context) +245
Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context) +274
Microsoft.Practices.ObjectBuilder2.BuilderContext.NewBuildUp(NamedTypeBuildKey newBuildKey) +250
Microsoft.Practices.Unity.ObjectBuilder.NamedTypeDependencyResolverPolicy.Resolve(IBuilderContext context) +101
lambda_method(Closure , IBuilderContext ) +215
Microsoft.Practices.ObjectBuilder2.c__DisplayClass1.b__0(IBuilderContext context) +71
Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context) +42
Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context) +333
Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context) +274
Microsoft.Practices.ObjectBuilder2.BuilderContext.NewBuildUp(NamedTypeBuildKey newBuildKey) +250
Microsoft.Practices.Unity.ObjectBuilder.NamedTypeDependencyResolverPolicy.Resolve(IBuilderContext context) +101
lambda_method(Closure , IBuilderContext ) +205
Microsoft.Practices.ObjectBuilder2.c__DisplayClass1.b__0(IBuilderContext context) +71
Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context) +42
Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context) +333
Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context) +274
Microsoft.Practices.ObjectBuilder2.BuilderContext.NewBuildUp(NamedTypeBuildKey newBuildKey) +250
Microsoft.Practices.Unity.ObjectBuilder.NamedTypeDependencyResolverPolicy.Resolve(IBuilderContext context) +101
lambda_method(Closure , IBuilderContext ) +209
Microsoft.Practices.ObjectBuilder2.c__DisplayClass1.b__0(IBuilderContext context) +71
Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context) +42
Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context) +333
Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context) +274
Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides) +383
[ResolutionFailedException: Resolution of the dependency failed, type = "MyApp.MVC.Controllers.StateController", name = "(none)".
Exception occurred while: while resolving.
Exception is: InvalidOperationException - The type String cannot be constructed. You must configure the container to supply this value.
-----------------------------------------------
At the time of the exception, the container was:
Resolving MyApp.MVC.Controllers.StateController,(none)
Resolving parameter "stateService" of constructor MyApp.MVC.Controllers.StateController(MyApp.Service.Services.IStateService stateService)
Resolving MyApp.Service.Services.StateService,(none) (mapped from MyApp.Service.Services.IStateService, (none))
Resolving parameter "context" of constructor MyApp.Service.Services.StateService(MyApp.Data.DAL.MyAppDbContext context)
Resolving MyApp.Data.DAL.MyAppDbContext,(none)
Resolving parameter "connectionString" of constructor MyApp.Data.DAL.MyAppDbContext(System.String connectionString, System.Data.Entity.Infrastructure.DbCompiledModel model)
Resolving System.String,(none)
]
Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides) +446
Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, String name, IEnumerable`1 resolverOverrides) +50
Microsoft.Practices.Unity.UnityContainer.Resolve(Type t, String name, ResolverOverride[] resolverOverrides) +48
Microsoft.Practices.Unity.UnityContainerExtensions.Resolve(IUnityContainer container, Type t, ResolverOverride[] overrides) +61
Unity.Mvc5.UnityDependencyResolver.GetService(Type serviceType) +140
System.Web.Mvc.DefaultControllerActivator.Create(RequestContext requestContext, Type controllerType) +87
[InvalidOperationException: An error occurred when trying to create a controller of type 'MyApp.MVC.Controllers.StateController'. Make sure that the controller has a parameterless public constructor.]
System.Web.Mvc.DefaultControllerActivator.Create(RequestContext requestContext, Type controllerType) +247
System.Web.Mvc.DefaultControllerFactory.GetControllerInstance(RequestContext requestContext, Type controllerType) +438
System.Web.Mvc.DefaultControllerFactory.CreateController(RequestContext requestContext, String controllerName) +257
System.Web.Mvc.MvcHandler.ProcessRequestInit(HttpContextBase httpContext, IController& controller, IControllerFactory& factory) +326
System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) +157
System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) +88
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) +50
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +301
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155我可能做错了什么？
更新
我将以下代码添加到UnityConfig类中的RegisterComponents方法中，它似乎正在工作：
container.RegisterInstance<StateService>(new StateService());
没有一个教程显示需要这个，我觉得我的例子就是我使用它非常标准。 为什么我需要这条线？我还不太了解Unity，所以这就是我仍然好奇的原因。
答案 0 :(得分：5)
我在任何地方都没有看到这个答案，所以我也想添加它。对我来说，你的StateService对象上有多个构造函数。因此，Unity正在选择具有最多参数的构造函数（如注释中所述）。
您可以使用InjectionConstructor调用告诉Unity使用哪个构造函数。
下面告诉它使用无参数构造函数。
container.RegisterType<IStateService, StateService>(new InjectionConstructor());
或者你也可以告诉它使用一个类型。
container.RegisterType<IStateService, StateService>(new InjectionConstructor(<some type>))
这将强制Unity使用您想要的构造函数。
答案 1 :(得分：4)
您的StateService正在尝试解决＆＃34; context＆＃34;。我建议您将上下文保留在DI映射之外，以防止它出自您想要解析的构造函数。
Resolving parameter "context" of constructor MyApp.Service.Services.StateService(MyApp.Data.DAL.MyAppDbContext context)