Question

I just started working with dependency injection for the first time and I am using as Ninject 2.0 as my IoC container in an ASP.NET MVC 2 website and I’m hitting an activation error that I am not sure how to react to. I am sure it’s simple so hopefully someone can point me in the right direction without too much thought.

I have a property on my class BaseController which takes an IWebsiteSettings and is flagged with the [Inject] attribute. In my StandardKernel I load a module with the following code:

public class WebModule : Module
{
    public override void Load()
    {

        Bind<IWebsiteSettings>()
            .ToProvider(new WebsiteSettingsProvider(WebConfigurationManager.AppSettings))
            .InSingletonScope();

    }
}

public class WebsiteSettingsProvider : Provider<WebsiteSettings>
{
    private const string WebsiteNameKey = "Website.Name";
    private const string ContactFormEmailSubjectKey = "ContactForm.EmailSubject";
    private const string ProductImageDirectoryKey = "Products.ImageDirectory";
    private const string UploadTempDirectoryKey = "Uploads.TempDirectory";

    protected NameValueCollection Settings { get; set; }

    public WebsiteSettingsProvider(NameValueCollection settings)
    {
        Settings = settings;
    }

    protected override WebsiteSettings CreateInstance(IContext context)
    {
        return new WebsiteSettings
                   {
                       WebsiteName = Settings[WebsiteNameKey] ?? string.Empty,
                       ContactFormEmailSubject = Settings[ContactFormEmailSubjectKey] ?? string.Empty,
                       ProductImageDirectory = Settings[ProductImageDirectoryKey] ?? string.Empty,
                       UploadsTemporaryDirectory = Settings[UploadTempDirectoryKey] ?? string.Empty
                   };
    }
}

This is fairly straightforward- I’m trying to load some data from the web.config file and store it in a singleton object for use across my controllers. The call to Bind seems to function exactly as it should and the Settings property in my provider is correctly initialized with the AppSettings collection in the config file. Still, when the application loads the first time:

Server Error in '/' Application.
Error activating SByte* using implicit self-binding of SByte*
No constructor was available to create an instance of the implementation type.

Activation path:
 4) Injection of dependency SByte* into parameter value of constructor of type string
 3) Injection of dependency string into property WebsiteName of type WebsiteSettings
 2) Injection of dependency IWebsiteSettings into property WebsiteSettings of type HomeController
 1) Request for HomeController

Suggestions:
 1) Ensure that the implementation type has a public constructor.
 2) If you have implemented the Singleton pattern, use a binding with InSingletonScope() instead.

Interestingly, if I refresh the page I don’t get the exception and a call to Kernel.Get() returns the correct object.

Any advice?

Answer 1

(We talked about this on IRC, but I’m putting it here in case someone else runs into this problem as well.)

WebsiteSettings has [Inject] attributes on its properties, so Ninject is trying to resolve a binding from System.String to inject a value into the properties. Since you’re using a custom provider to activate WebsiteSettings instances, you don’t need [Inject] attributes on its properties.