Parameter name: root cannot be Null exception Ninject

Question

I am receiving an exception from Ninject following a code refactor. therefore I know what is causing the error but cannot determine why. The code compiles file but I get a runtime error from Ninject:

Cannot be null Parameter name: root

I am trying to inject a constant instance of a class:

var subscriptionApiClientHelper = new SubscriptionApiClientHelper(AppSettings.Get<string>("AdminAPIUrl"),
            AppSettings.Get<string>("APIV2Key"), Kernel.Get<ICache>());

kernel.Bind<SubscriptionApiClientHelper>().ToConstant(subscriptionApiClientHelper);

Ninject throws the exception on the first line.

My class SubscriptionApiClientHelper inherits from another class (referenced in a Nuget Package) thus:

public SubscriptionApiClientHelper(string baseUrl, string apiKey, ICache cache) : base(baseUrl, apiKey)
{
     _cache = cache;
}

The constructor of which is this:

 public SubscriptionApiClient(string baseUrl, string apiKey)
 {
     _baseUrl = baseUrl;
     _apiKey = apiKey;
 }

A couple of methods in this base class are overridden and caching implemented, hence the reason for overriding as I do not need to implement caching on all the methods of the base class. I am injecting some other classes from the same Nuget package without issue though, these do not have the inheritance that I suspect is causing the issue. Note I am successfully injecting and using the base class I am inheriting also (SubscriptionApiClient), it is just the helper class that is causing the issue.

This is the full stack trace:

at Ninject.ResolutionExtensions.GetResolutionIterator(IResolutionRoot root, Type service, Func`2 constraint, IEnumerable`1 parameters, Boolean isOptional, Boolean isUnique)
at Ninject.ResolutionExtensions.Get[T](IResolutionRoot root, IParameter[] parameters)
at AdminPortal.NinjectWebCommon.RegisterServices(IKernel kernel) in C:\Projects\AdminPortal\AdminPortal\App_Start\NinjectWebCommon.cs:line 106
at AdminPortal.NinjectWebCommon.CreateKernel() in C:\Projects\AdminPortal\AdminPortal\App_Start\NinjectWebCommon.cs:line 72
at Ninject.Web.Common.Bootstrapper.Initialize(Func`1 createKernelCallback)
at AdminPortal.NinjectWebCommon.Start() in C:\Projects\AdminPortal\AdminPortal\App_Start\NinjectWebCommon.cs:line 39

Does anyone know what is going wrong?

Answer 1

Your Kernel variable is assigned to null reference. Because Get is an extension method, you won't get NullReferenceException, but ArgumentNullException instead. I'm getting the same exception when running this test:

StandardKernel kernel = null;
kernel.Get<ICache>();

Answer 2

this was down to my misuse of Ninject when trying to construct the instance of the class I wanted to inject. I solved it thus:

kernel.Bind<SubscriptionApiClientHelper>().To<SubscriptionApiClientHelper>()
            .WithConstructorArgument("baseUrl", AppSettings.Get<string>("AdminAPIUrl"))
            .WithConstructorArgument("apiKey", AppSettings.Get<string>("APIV2Key"))
            .WithConstructorArgument("cache", kernel.Get<ICache>());

I can now inject this class successfully.