Web Api Start up Exceptions with IDependencyResolver implementation

Question

I am developing a Web Api and I decided to use custom DependencyResolver. I refer this [Dependency Injection for Web API Controllers] article. Everything is working well so far in the terms of dependency injection into controllers. Code snippet of my configuration from my Owin startup class

private void RegisterIoC(HttpConfiguration config) {     _unityContainer = new UnityContainer();     _unityContainer.RegisterType<IAccountService, AccountService>();     .........     .........     config.DependencyResolver = new UnityResolver(_unityContainer); } 

But at the time when Api starts for the very first time some ResolutionFailedException thrown (but catched) inside the UnityResolver's GetService method. Here is the exception message

"Exception occurred while: while resolving.  Exception is: InvalidOperationException -  The current type, System.Web.Http.Hosting.IHostBufferPolicySelector,  **is an interface and cannot be constructed. Are you missing a type mapping?**" 

Above same exception thrown following types

System.Web.Http.Hosting.IHostBufferPolicySelector System.Web.Http.Tracing.ITraceWriter System.Web.Http.Metadata.ModelMetadataProvider System.Web.Http.Tracing.ITraceManager System.Web.Http.Dispatcher.IHttpControllerSelector System.Web.Http.Dispatcher.IAssembliesResolver System.Web.Http.Dispatcher.IHttpControllerTypeResolver System.Web.Http.Controllers.IHttpActionSelector System.Web.Http.Controllers.IActionValueBinder System.Web.Http.Validation.IBodyModelValidator System.Net.Http.Formatting.IContentNegotiator 

I know that these ResolutionFailedException are thrown because I did not provide mappings in my unity configuration for above types.

Now here is my question :-, If I implement custom unity DependencyResolver I need to define mappings of above types and if need to define what will be their corresponding default implementation types OR is there some alternative way to implement DependencyResolver. I am really concerned even though application is running fine now, failing to resolve above type can cause serious issue later. Please help.

One final Addition:- For following types, same ResolutionFailedException thrown when I make request for any action into the my web api

System.Web.Http.Dispatcher.IHttpControllerActivator System.Web.Http.Validation.IModelValidatorCache System.Web.Http.Controllers.IHttpActionInvoker 

Answer 1

I was running in to the same issue using Unity with WebApi and OWIN/Katana.

The solution for me was to use the UnityDependencyResolver defined in the Unity.WebApi Nuget package instead of my own custom implementation (like @Omar Alani above)

Install-Package Unity.WebAPI 

Note that the package will try and add a file named UnityConfig.cs in App_Start (the filename I used myself).

In that UnityConfig.cs file the package will add code to register the container against the GlobalConfiguration.Configuration.DependencyResolver which is not what we want with OWIN.

So instead of using:

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

Change to use:

config.DependencyResolver = new UnityDependencyResolver(container); 

For completeness:

My UnityConfig.cs

public static class UnityConfig {     public static void Register(HttpConfiguration config)     {         var container = new UnityContainer();          // Your mappings here          config.DependencyResolver = new UnityDependencyResolver(container);     } } 

My Startup.cs

[assembly: OwinStartup(typeof(UnityTest.BusinessLayer.Api.ApiStartup))] namespace UnityTest.BusinessLayer.Api {     public partial class ApiStartup     {         public void Configuration(IAppBuilder app)         {              app.UseCors(Microsoft.Owin.Cors.CorsOptions.AllowAll);              HttpConfiguration httpConfig = new HttpConfiguration();              UnityConfig.Register(httpConfig);              ConfigureAuth(app); //In App_Start ->Startup.Auth              WebApiConfig.Register(httpConfig);              app.UseWebApi(httpConfig);     }   } } 

Answer 2

In case any of the above solutions still don't work for people, here's how I solved it.

After spending a day chasing down this error, it turned out to be some sort of VS caching issue. Out of desperation, I deleted all .suo files and force-get-latest, which seems to have resolved the issue.