Question

I have an IConfig object that contains settings used throughout my application. At the moment, I inject the entire object into the constructor of each object that needs it, as follows:

public interface IConfig 
{
    string Username { get; }
    string Password { get; }
    //... other settings
}

public class Foo : IFoo
{
    private readonly string username;
    private readonly string password;

    public Foo(IConfig config)
    {
        this.username = config.Username;
        this.password = config.Password;
    }
}

The downside is that IConfig contains a large number of settings because it’s deserialised from an overall config file, so injecting the entire object is unnecessary. What I’d like to do is change the constructor to Foo(string username, string password) so that it only receives the settings it needs. This also makes creating Foo objects for testing much easier (not having to set up IConfig just to create Foo). I’d like to bind the constructor arguments directly in my NinjectModule, something like the following:

public class MyModule : NinjectModule
{
    public override void Load()
    {
        Bind<IConfig>().To<JsonConfig>()
            .InSingletonScope();

        Bind<IFoo>().To<Foo>()
            .WithConstructorArgument("username", IConfig.Username)
            .WithConstructorArgument("password", IConfig.Password);
    }
}

Obviously this code doesn’t work, but how would I go about doing what I wanted?

My original idea was to use the NinjectModule.Kernel to get the IKernel then get an instance of my IConfig object and inject the properties as required, but the object returned by NinjectModule.Kernel has no Get<T>() method.

Answer 1

You are on the right track:

The Kernel.Get<T>() method is an extension method defined on the ResolutionExtensions in the Ninject namepsace so with adding the using Ninject; it is available in your module as well.

But instead of the Module.Kernel you should use the IContext provided in the second overload of WithConstructorArgument to get the Kernel:

Bind<IFoo>().To<Foo>()
    .WithConstructorArgument("username", 
                             context => context.Kernel.Get<IConfig>().Username)
    .WithConstructorArgument("password", 
                             context => context.Kernel.Get<IConfig>().Password);