Activator.CreateInstance and Ninject

Question

I have a project I'm working on where I don't know what class I will need to instantiate at compile time. I'm trying to use Activator.CreateInstance() to generate a new class for me based on the user input. The code below works well, but I had to change my constructor on my INECCQuery classes to only have a default constructor and not use any sort of dependency injection. Is there a way I can still use my injection bindings and the Activator.CreatInstance()? I'm using Ninject for injection.

    [HttpGet]
    public ActionResult Index(string item) {
      Type t = Type.GetType(string.Format("Info.Audit.Query.{0}Query, Info.Audit", item.ToUpper()));
      if (t != null) {
        INECCQuery query = (INECCQuery)Activator.CreateInstance(t);
        var results = query.Check();
        return View("Index", results);
      }
      return View("Notfound");
    }

Answer 1

Constructor injection is always preferred where possible, but a suitable backup would be to leverage property injection.

http://ninject.codeplex.com/wikipage?title=Injection%20Patterns

class SomeController {

  [Inject]
  public Object InjectedProperty { get; set; }

}

Based on the assumption that you are trying to replace the Activator.CreateInstance you can inject a Func<T, INECCQuery> or whatever factory you wish to use.

Answer 2

You can get Ninject to give you an object of type t at runtime and still get the dependency injection via the constructor.... I do something similar for one case within my application.

In the Global.asax.cs file, I have the following method:

    /// <summary>
    /// Gets the instance of Type T from the Ninject Kernel
    /// </summary>
    /// <typeparam name="T">The Type which is requested</typeparam>
    /// <returns>An instance of Type T from the Kernel</returns>
    public static T GetInstance<T>()
    {
        return (T)Kernel.Get(typeof(T));
    }

This depends on a static Kernel reference.

Then, in code, I do

var myInfrastructureObject = <YourAppNameHere>.GetInstance<MyInfrastructureType>();

So, I know the type at compile time whereas you don't, but it wouldn't be that difficult to change that.

You may also wish to look into the ServiceLocator Pattern.

Answer 3

I've actually found out you can just pass in a second option to the Activator.CreateInstance method and as long as it matches your constructors signature it will work. The only problem is if your parameters don't match you will get a runtime error.

Type t = Type.GetType(string.Format("Info.Audit.Query.{0}Query, Info.Audit", item.ToUpper()));
INECCQuery query = (INECCQuery)Activator.CreateInstance(t, repository);

Thanks for all the help.