Make sure that the controller has a parameterless public constructor error

Question

I have followed this tutorial which has worked great  until I modified my DbContext to have an additional constructor  I am now having issues with the resolution and not sure what to do to fix this  Is there an easy way to force it to grab the parameterless constructor or I am approaching this incorrectly    DbContext with two constructors   public class DashboardDbContext   DbContext       public DashboardDbContext     base  DefaultConnection            public DashboardDbContext DbConnection dbConnection  bool owns            base dbConnection  owns          SiteController constructor   private readonly IDashboardRepository  repo   public SiteController IDashboardRepository repo         repo   repo      Repository   DashboardDbContext  context   public DashboardRepository DashboardDbContext context         context   context      UnityResolver code   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 lt object gt  GetServices Type serviceType                try                       return  container ResolveAll serviceType                     catch  ResolutionFailedException                        return new List lt object gt                          public IDependencyScope BeginScope                 var child    container CreateChildContainer            return new UnityResolver child              public void Dispose                  container Dispose              WebApiConfig   var container   new UnityContainer    container RegisterType lt IDashboardRepository  DashboardRepository gt  new HierarchicalLifetimeManager     config DependencyResolver   new UnityResolver container     Error from WebApi Call      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  lt SendAsync gt 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 amp  activator   at System Web Http Dispatcher DefaultHttpControllerActivator Create HttpRequestMessage request  HttpControllerDescriptor controllerDescriptor  Type controllerType    The tutorial was great and has been working well for me up until I added the second constructor

User · Answer

I had the same problem  I googled it for two days  At last I accidentally noticed that the problem was access modifier of the constructor of the Controller  I didn   t put the public key word behind the Controller   s constructor   public class MyController   ApiController               private readonly IMyClass  myClass           public MyController IMyClass myClass                         myClass   myClass                    I add this experience as another answer maybe someone else made a similar mistake

User · Answer

What s happening is that you re bitten by this problem  Basically  what happened is that you didn t register your controllers explicitly in your container  Unity tries to resolve unregistered concrete types for you  but because it can t resolve it  caused by an error in your configuration   it return null  It is forced to return null  because Web API forces it to do so due to the IDependencyResolver contract  Since Unity returns null  Web API will try to create the controller itself  but since it doesn t have a default constructor it will throw the  Make sure that the controller has a parameterless public constructor  exception  This exception message is misleading and doesn t explain the real cause   You would have seen a much clearer exception message if you registered your controllers explicitly  and that s why you should always register all root types explicitly   But of course  the configuration error comes from you adding the second constructor to your DbContext  Unity always tries to pick the constructor with the most arguments  but it has no idea how to resolve this particular constructor   So the real cause is that you are trying to use Unity s auto-wiring capabilities to create the DbContext  DbContext is a special type that shouldn t be auto-wired  It is a framework type and you should therefore fallback to registering it using a factory delegate   container Register lt DashboardDbContext gt       new InjectionFactory c   gt  new DashboardDbContext

User · Answer

In my case  Unity turned out to be a red herring  My problem was a result of different projects targeting different versions of  NET  Unity was set up right and everything was registered with the container correctly  Everything compiled fine  But the type was in a class library  and the class library was set to target  NET Framework 4 0  The WebApi project using Unity was set to target  NET Framework 4 5  Changing the class library to also target 4 5 fixed the problem for me   I discovered this by commenting out the DI constructor and adding default constructor  I commented out the controller methods and had them throw NotImplementedException  I confirmed that I could reach the controller  and seeing my NotImplementedException told me it was instantiating the controller fine  Next  in the default constructor  I manually instantiated the dependency chain instead of relying on Unity  It still compiled  but when I ran it the error message came back  This confirmed for me that I still got the error even when Unity was out of the picture  Finally  I started at the bottom of the chain and worked my way up  commenting out one line at a time and retesting until I no longer got the error message  This pointed me in the direction of the offending class  and from there I figured out that it was isolated to a single assembly

User · Answer

I had the same issue and I resolved it by making changes in the UnityConfig cs file In order to resolve the dependency issue in the UnityConfig cs file you have to add   public static void RegisterComponents             var container   new UnityContainer        container RegisterType lt ITestService  TestService gt         DependencyResolver SetResolver new UnityDependencyResolver container

User · Answer

Sometimes because you are resolving your interface in ContainerBootstraper cs it s very difficult to catch the error  In my case there was an error in resolving the implementation of the interface I ve injected to the api controller  I couldn t find the error because I have resolve the interface in my bootstraperContainer like this    container RegisterType lt IInterfaceApi  MyInterfaceImplementaionHelper gt  new ContainerControlledLifetimeManager     then I ve adde the following line in my bootstrap container    container RegisterType lt MyController gt      so when I compile the project   compiler complained and stopped in above line and showed the error

User · Answer

I ve got this error when I accidentally defined a property as a specific object type  instead of the interface type I have defined in UnityContainer   For example   Defining UnityContainer   var container   new UnityContainer    container RegisterInstance typeof IDashboardRepository   DashboardRepository   config DependencyResolver   new UnityResolver container     SiteController  the wrong way - notice repo type    private readonly DashboardRepository  repo   public SiteController DashboardRepository repo         repo   repo      SiteController  the right way    private readonly IDashboardRepository  repo   public SiteController IDashboardRepository repo         repo   repo

User · Answer

If you have an interface in your controller  public myController IXInterface Xinstance      You must register them to Dependency Injection container   container Bind lt IXInterface gt    To lt XClass gt    InRequestScope

User · Answer

In my case  it was because of exception inside the constructor of my injected dependency  in your example - inside DashboardRepository constructor   The exception was caught somewhere inside MVC infrastructure  I found this after I added logs in relevant places

User · Answer

If you are using UnityConfig cs to resister your type s mappings like below    public static void RegisterTypes IUnityContainer container             container RegisterType lt IProductRepository  ProductRepository gt             You have to let the know   webApiConfig cs   about Container  config DependencyResolver   new Unity AspNet WebApi UnityDependencyResolver UnityConfig Container

[c#] Make sure that the controller has a parameterless public constructor error

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