Example IModelBinderProvider implementation for ModelBinder constructor injection in MVC 3

Question

I need to wire my custom ModelBinder up to my DI container in MVC 3, but I can't get it working.

So. This is what I have: A ModelBinder with a constructor injected service.

public class ProductModelBinder : IModelBinder{
  public ProductModelBinder(IProductService productService){/*sets field*/}
  // the rest don't matter. It works.
}

My binder works fine if I add it like this:

ModelBinders.Binders.Add(typeof(Product),
     new ProductModelBinder(IoC.Resolve<IProductService>()));

But that is the old way of doing it, and I don't want that.

What I need is help on how to hook that modelbinder up to the IDependencyResolver I've registered.

According to Brad Wilson the secret is using a IModelBinderProvider implementation, but its very unclear as to how to wire that up. (in this post)

Does anyone have an example?

Tillfourd

IModelBinderProvider would be your own implementation. It just so happens I wrote a blog post about just this thing buildstarted.com/2010/12/02/… Hope this helps – Circumnutate

Yeah, that's working well. I just replace the CreateInstance()stuff with var instance = (IModelBinder) DependencyResolver.Current.GetService(type); Thanks! . – Tillfourd

I still think it should be possible to make a more clean implementation using generics. I need to sleep on that, I think ;) – Tillfourd

Answer 1

I faced the same situation when coding my MVC 3 app. I ended up with something like this:

public class ModelBinderProvider : IModelBinderProvider
{
    private static Type IfSubClassOrSame(Type subClass, Type baseClass, Type binder)
    {
        if (subClass == baseClass || subClass.IsSubclassOf(baseClass))
            return binder;
        else
            return null;
    }

    public IModelBinder GetBinder(Type modelType)
    {
        var binderType = 
            IfSubClassOrSame(modelType, typeof(xCommand), typeof(xCommandBinder)) ??
            IfSubClassOrSame(modelType, typeof(yCommand), typeof(yCommandBinder)) ?? null;

        return binderType != null ? (IModelBinder) IoC.Resolve(binderType) : null;
    }
}

Then I registered this in my IoC container (Unity in my case):

_container.RegisterType<IModelBinderProvider, ModelBinderProvider>("ModelBinderProvider", singleton());

This works for me.

Fustian

Answer 2

You need to write your own IModelBinderProvider and register it with the ModelBinderProviders.BinderProviders collection:

public class YourModelBinderProvider : IModelBinderProvider {
    public IModelBinder GetBinder(Type modelType) {
         if(modelType == typeof(Product)) {
             return new ProductModelBinder(...);
         }
         return null;
    }
}

In Global.asax:

ModelBinderProviders.BinderProviders.Add(new YourModelBinderProvider());

Callean

Is it better to register the IModelBinderProvider in IoC so that MVC automatically requests it or register it explicitly using BinderProviders.Add(...) ? Also, if we are trying to handle multiple binders (based on the model type), is there a pattern you would recommend? – Fustian

You could do it either way. I guess I just prefer the traditional approach. – Callean

How are you trying to handle multiple binders? The firs binder provider that returns a non-null binder wins. So it's just a matter of ordering things appropriately. – Callean

Have a look at my answer below... I am currently using this is a development project. – Fustian