Ninject - how and when to inject - dependency-injection

Question

I'm a newbie when it comes to DI and ninject and I'm struggling a bit
about when the actual injection should happen and how to start the
binding.
I'm using it already in my web application and it working fine there,
but now I want to use injection in a class library.
Say I have a class like this:
public class TestClass
{
[Inject]
public IRoleRepository RoleRepository { get; set; }
[Inject]
public ISiteRepository SiteRepository { get; set; }
[Inject]
public IUserRepository UserRepository { get; set; }
private readonly string _fileName;
public TestClass(string fileName)
{
_fileName = fileName;
}
public void ImportData()
{
var user = UserRepository.GetByUserName("myname");
var role = RoleRepository.GetByRoleName("myname");
var site = SiteRepository.GetByID(15);
// Use file etc
}
}
I want to use property injection here because I need to pass in a
filename in my constructor. Am I correct in saying that if I need to
pass in a constructor parameter, I cannot use constructor injection?
If I can use constructor injection with additional parameters, how do
I pass those parameters in?
I have a console app that consumes by Test class that looks as
follows:
class Program
{
static void Main(string[] args)
{
// NinjectRepositoryModule Binds my IRoleRepository etc to concrete
// types and works fine as I'm using it in my web app without any
// problems
IKernel kernel = new StandardKernel(new NinjectRepositoryModule());
var test = new TestClass("filename");
test.ImportData();
}
}
My problem is that when I call test.ImportData() my repositories are null - nothing has been injected into them. I have tried creating another module and calling
Bind<TestClass>().ToSelf();
as I thought this might resolve all injection properties in TestClass but I'm getting nowhere.
I'm sure this is a trivial problem, but I just can't seem to find out
how to go about this.

Answer 1

You are directly newing TestClass, which Ninject has no way of intercepting - remember there's no magic like code transformation intercepting your news etc.
You should be doing kernel.Get<TestClass> instead.
Failing that, you can inject it after you new it with a kernel.Inject( test);
I think there's an article in the wiki that talks about Inject vs Get etc.
Note that in general, direct Get or Inject calls are a Doing It Wrong smell of Service Location, which is an antipattern. In the case of your web app, the NinjectHttpModule and PageBase are the hook that intercepts object creation - there are similar interceptors / logical places to intercept in other styles of app.
Re your Bind<TestClass>().ToSelf(), generally a StandardKernel has ImplicitSelfBinding = true which would make that unnecessary (unless you want to influence its Scope to be something other than .InTransientScope()).
A final style point:- you're using property injection. There are rarely good reasons for this, so you should be using constructor injection instead.
And do go buy Dependency Injection in .NET by #Mark Seemann, who has stacks of excellent posts around here which cover lots of important but subtle considerations in and around the Dependency Injection area.

Answer 2

OK,
I've found out how to do what I need, thanks in part to your comments Ruben. I've created a new module that basically holds the configuration that I use in the class library. Within this module I can either Bind using a placeholder Interface or I can add a constructor parameter to the CustomerLoader.
Below is the code from a dummy console app to demonstrating both ways.
This might help someone else getting started with Ninject!
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Ninject.Core;
using Ninject.Core.Behavior;
namespace NinjectTest
{
public class Program
{
public static void Main(string[] args)
{
var kernel = new StandardKernel(new RepositoryModule(), new ProgramModule());
var loader = kernel.Get<CustomerLoader>();
loader.LoadCustomer();
Console.ReadKey();
}
}
public class ProgramModule : StandardModule
{
public override void Load()
{
// To get ninject to add the constructor parameter uncomment the line below
//Bind<CustomerLoader>().ToSelf().WithArgument("fileName", "string argument file name");
Bind<LiveFileName>().To<LiveFileName>();
}
}
public class RepositoryModule : StandardModule
{
public override void Load()
{
Bind<ICustomerRepository>().To<CustomerRepository>().Using<SingletonBehavior>();
}
}
public interface IFileNameContainer
{
string FileName { get; }
}
public class LiveFileName : IFileNameContainer
{
public string FileName
{
get { return "live file name"; }
}
}
public class CustomerLoader
{
[Inject]
public ICustomerRepository CustomerRepository { get; set; }
private string _fileName;
// To get ninject to add the constructor parameter uncomment the line below
//public CustomerLoader(string fileName)
//{
// _fileName = fileName;
//}
public CustomerLoader(IFileNameContainer fileNameContainer)
{
_fileName = fileNameContainer.FileName;
}
public void LoadCustomer()
{
Customer c = CustomerRepository.GetCustomer();
Console.WriteLine(string.Format("Name:{0}\nAge:{1}\nFile name is:{2}", c.Name, c.Age, _fileName));
}
}
public interface ICustomerRepository
{
Customer GetCustomer();
}
public class CustomerRepository : ICustomerRepository
{
public Customer GetCustomer()
{
return new Customer() { Name = "Ciaran", Age = 29 };
}
}
public class Customer
{
public string Name { get; set; }
public int Age { get; set; }
}
}