Using Interfaces for Model in ASP.NET MVC (Impl)…

August 18, 2009

I mentioned in my first post that we were using interfaces to represent our models and WHY we did it. So here’s the HOW of doing it.

Binding

As I mentioned, we used the model attribute to decorate our models when they are in the method signature. This attribute was based on the AbstractParameterBinderAttribute from MvcContrib (if you’re using ASP.NET MVC and NOT using MvcContrib, you’re probably re-inventing more than a few wheels) which is just a CustomModelBinderAttribute with a little bit of happy sprinkled in. Here’s what our binding looked like:

public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
	ViewDataDictionary viewData = GetViewData(controllerContext.Controller);
	object result = null;
	if (viewData != null)
	{
		result = viewData.For(controllerContext, bindingContext);
		SetViewData(result, controllerContext, bindingContext, viewData);
	}
	return result;
}

The GetViewData and SetViewData are pretty self-explanatory but the real heavy lifting is in that extension method. Let’s take a look at it.

DictionaryAdapter

We know that we have a giant dictionary holding on to our data and tedious mapping using strings make me itchy, not to mention it breaking my refactoring tool, so how do we push the values without them? DictionaryAdapter from Castle. This little known, under utilized piece of work was shown to me by Craig Neuwirt, it’s creator, and I’m constantly amazed at it’s usefulness. Observe the following extension method for ViewData:

private static readonly IDictionaryAdapterFactory
	ViewDataFactory = new DictionaryAdapterFactory();

public static object For(this ViewDataDictionary viewData, ControllerContext controllerContext, ModelBindingContext bindingContext)
{
	ViewDataAdapter data = new ViewDataAdapter(viewData);
	object adapter = ViewDataFactory.GetAdapter(bindingContext.ModelType, data);

	IDictionaryAdapter meta = adapter as IDictionaryAdapter;
	meta.FetchProperties();
	
	return adapter;
}

It’s that simple. No, really. The DictionaryAdapter will match up the properties on the interface you defined with the corresponding elements in the ViewData and push their values into an instance of the model. All of that, without you even having to buy it dinner.

There’s more??

This approach worked great for us and gave us most of what we needed, however, this code doesn’t handle rehydrating objects from ActiveRecord – yet. But that’s a post for another night…

CORRECTION: Craig informed me that he did not create it originally but re-wrote it’s initial implementation to what it is now. Sorry for the misunderstanding.


David O’Hara is a Principal with Improving Enterprises in Dallas, Texas.


Improving Podcasts…

August 11, 2009

My first podcast (the fifth for the series) is up – we discussed technical presentations and had a great time doing so. I’m very fortunate to work with such great people and had a wonderful time. Thanks to Allen and Mike for moderating and putting the whole thing together.

Check it out at: Improving Podcasts


David O’Hara is a Principal with Improving Enterprises in Dallas, Texas.


Using Interfaces for Model in ASP.NET MVC…

August 5, 2009

Brad Wilson tweeted asking about using interfaces for models in an ASP.NET MVC project and I was tempted to respond but figured this, rather than the 140 character limited avenue of Twitter, would be a better medium. The short answer is that we used binders and attributes to do our heavy lifting for us so that instead of our code looking like this:

public ActionResult Login(string username, string password, bool rememberMe)
{
  ...
}

it ended up looking like this:

public ActionResult Login([Model]ILoginModel model)
{
  ...
}

with the declaration of the model looking like this:

public interface ILoginModel
{
  string Username{get;set;}
  string Password{get;set;}
  bool RememberMe{get;set;}
}

Our model attribute was responsible for pulling in the values from our ViewData and then binding them using the DictionaryAdapter (probably one of the most powerful, under-documented pieces of the whole stack) from Castle.

Advantages

Doing this allowed us to keep our dealings with the models strongly typed (yes, this is NOT unique to using interfaces vs. classes), minimized the code needed for the model since it’s just an interface but also keeps anyone from “accidentally” pushing logic down into the model that belongs elsewhere.

Next up, the guts of how we used DictionaryAdapter to do this…


David O’Hara is a Principal with Improving Enterprises in Dallas, Texas.