Ninject - how and when to inject

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.