Can't get value of Ninject ConstructorArgument (passed in as parameter to kernel.Get)

Question

Having trouble getting the value of a ConstructorArgument parameter passed to kernel.Get(). I want to use the parameter's value to determine which of two string values will be passed into the constructor. The parameter is in fact there when expected, I just can't get at its value. I end up with a null ref exception after calling the parameter's GetValue method:

namespace MyNS.DBProviders {
    public abstract class DBProviderBase {
        private ObjectContext _db;

        public DBProviderBase(ObjectContext db) {
            _db = db;
        }

        ...
        //abstract methods here
        ...
    }

    public class DBProviderB : DBProviderBase {
        public DBProviderB(ObjectContext db) : base(db) {}
        //implementation details... applies to two DBs w/ same schema
    }

    public class DBProviderModule : NinjectModule {
        public override void Load() {
            //Bind DB providers
            Bind<DBProviderBase>().To<DBProviderB>();

            //Bind LINQ contexts to DB providers
            Bind<ObjectContext>().To<EF_DB_b>()
                .When(UseEF_DB_b_conn1).WithConstructorArgument(
                    "connectionString"
                    , ConfigurationManager.ConnectionStrings["EF_DB_b_conn1"]
                        .ToString()
                )
            ;
            Bind<ObjectContext>().To<EF_DB_b>()
                .When(UseEF_DB_b_conn2).WithConstructorArgument(
                    "connectionString"
                    , ConfigurationManager.ConnectionStrings["EF_DB_b_conn2"]
                        .ToString()
                )
            ;
        }

        public static bool UseEF_DB_b_conn1(IRequest req) {
            string idNum = (string)req.Parameters.First(
                p => p.Name == "idNum"
            ).GetValue(null, null);
            //NULL REF EXCEPTION

            return x != null ?  idNum.Substring(0, 1) == "2" : false;
        }

        public static bool UseEF_DB_b_conn2(IRequest req) {
            return !UseEF_DB_b_conn1(req);
        }
    }
}

namespace MyNS {
    public static class DBProviderFactory {

        public static DBProviderBase GetDBProvider(string idNum) {
            var kernel = new Bootstrapper().Kernel;

            return kernel.Get<DBProviderBase>(
                new ConstructorArgument("idNum", idNum)
            );
        }
    }
}

...//idNum received from elsewhere
var db = DBProviderFactory.GetDBProvider(idNum);
var something = db.GetSomethingFromOneOfThreeDBs();

Answer 1

Edit: After your comments and edits, I've reformulated the answer. The suggestion below is just one way to go about this. I'm implementing a Provider<ObjectContextFactory> that will inspect the parameter sent to the request when using kernel.Get<ObjectContextFactory>(). This parameter has nothing to do with constructor, it is only used as a "context variable" so you can decide what to do with it.

Here is the code:

public class ObjectContextFactory
{
    private readonly string _idNum;
    private readonly IResolutionRoot _container;

    public ObjectContextFactory(IResolutionRoot container, string idNum)
    {
        _container = container;
        _idNum = idNum;
    }

    public ObjectContext CreateObjectContext()
    {
        //Use _idNum here as you wish, here is a sample implementation
        ConstructorArgument constructorArgument;
        if (_idNum.Substring(0,1) == "2")
        {
            constructorArgument = new ConstructorArgument("connectionString", "your conn string 1");
        }
        else
        {
            constructorArgument = new ConstructorArgument("connectionString", "your conn string 2");
        }

        return _container.Get<ObjectContext>(constructorArgument);
    }
}

public class ObjectContextFactoryProvider : Provider<ObjectContextFactory>
{
    protected override ObjectContextFactory CreateInstance(IContext context)
    {
        var idNum = context.Parameters.FirstOrDefault(p => p.Name == "idNum");
        var idNumValue = idNum.GetValue(context, null) as string;

        var factory = new ObjectContextFactory(context.Kernel, idNumValue);
        return factory;
    }
}

And the bindings:

public override void Load()
{
    Bind<DBProviderBase>().To<DBProviderB>();
    Bind<ObjectContextFactory>().ToProvider<ObjectContextFactoryProvider>();
}

When retrieving the factory:

var factory = kernel.Get<ObjectContextFactory>(new Parameter("idNum", "1", true));

Another way to achieve this would be via Named Bindings. Just do a kind of translator from your idNum to a string (or use it directly) and call it when getting an instance.

Answer 2

I needed to use ToMethod() to get access to the appropriate context.

Bind<DBProviderBase>().ToMethod(
    ctx => {
        var idNum = ctx.Parameters.First(p => p.Name == "idNum")
            .GetValue(ctx, null) as string
        ;

        if (idNum == 2) {
            return new DBProviderB(new EF_DBEntities(
                ConfigurationManager.ConnectionStrings["EF_DB_b_conn1"]
            .ToString()));
        } else {
            return new DBProviderB(new EF_DBEntities(
                ConfigurationManager.ConnectionStrings["EF_DB_b_conn2"]
            .ToString()));
        }
    };
);

//later...
var db = kernel.Get<DBProviderBase>(
    new Ninject.Parameters.Parameter("idNum", idNum, true)
);