Error occurred when trying to create a controller of type 'EmployeeController'. Make sure controller has a parameterless public constructor

Question

Why I'm I getting this error on Employee Controller rest of them are working perfectly Here is my Employee 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;
        }
}

AND this is my perfectly Working Product 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;
        }
}

Here is the stack trace

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)

Update

Here is the Unity Configuration

   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

Buried in the stack trace is the root cause of the issue:

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)

The issue is that the EmployeeController requires an instance of IEmployeeServices but Unity does not know what concrete type to instantiate. It looks like the implementation class is supposed to be registered by the call to ComponentLoader.LoadContainer(container, ".\\bin", "BusinessServices.dll"); but for some reason it is not being registered. It could be a bug in that code or perhaps the BusinessServices.dll is out of date and does not contain the IEmployeeServices definition.

It's hard to tell why IEmployeeServices is not registered without seeing all the code and runtime dependencies (because types are being dynamically loaded/registered).