Dynamic Methods in View Data

In ASP.NET MVC 3 Preview 1, we introduced some syntactic sugar for creating and accessing view data using new dynamic properties.

Within a controller action, the ViewModel property of Controller allows setting and accessing view data via property accessors that are resolved dynamically at runtime. From within a view, the View property provides the same thing (see the addendum at the bottom of this post for why these property names do not match).

Disclaimer

This blog post talks about ASP.NET MVC 3 Preview 1, which is a pre-release version. Specific technical details may change before the final release of MVC 3. This release is designed to elicit feedback on features with enough time to make meaningful changes before MVC 3 ships, so please comment on this blog post if you have comments.

Let’s take a look at the old way and the new way of doing this:

The old way

The following is some controller code that adds a string to the view data.

public ActionResult Index() {
  ViewData["Message"] = "Some Message";
  return View();
}

The following is code within a view that accesses the view data we supplied in the controller action.

<h1><%: ViewData["Message"] %></h1>

The new way

This time around, we use the ViewModel property which is typed as dynamic. We use it like we would any property.

public ActionResult Index() {
  ViewModel.Message = "Some Message";
  return View();
}

And we reference it in a Razor view. Note that this also works in a WebForms View too.

<h1>@View.Message</h1>

Note that View.Message is equivalent to View["Message"].

Going beyond properties

However, what might not be clear to everyone is that you can also store and callmethods using the same approach. Just for fun, I wrote an example of doing this.

In the controller, I defined a lambda expression that takes in an index and two strings. It returns the first string if the index is even, and the second string if the index is odd. It’s very simple.

The next thing I do is assign that lambda to the Cycle property of ViewModel, which is created on the spot since ViewModel is dynamic.

public ActionResult Index() {
  ViewModel.Message = "Welcome to ASP.NET MVC!";

  Func<int, string, string, string> cycleMethod = 
    (index, even, odd) => index % 2 == 0 ? even : odd;
  ViewModel.Cycle = cycleMethod;

  return View();
}

Now, I can dynamically call that method from my view.

<table>
@for (var i = 0; i < 10; i++) {
    <tr class="@View.Cycle(i, "even-css", "odd-css")">
        <td>@i</td>
    </tr>
}
</table>

As a fan of dynamic languages, I find this technique to be pretty slick. :)

The point of this blog post was to show that this is possible, but it raises the question, “why would anyone want to do this over writing a custom helper method?”

Very good question! Right now, it’s mostly a curiosity to me, but I can imagine cases where this might come in handy. However, if you re-use such view functionality or really need Intellisense, I’d highly recommend making it a helper method. I think this approach works well for rapid prototyping and maybe for one time use helper functions.

Perhaps you’ll find even better uses I didn’t think of at all.

Addendum: The Property name mismatch

Earlier in this post I mentioned the mismatch between property names,ViewModel vs View. I also talked about this in a video I recorded for MvcConf on MVC 3 Preview 1. Originally, we wanted to pick a nice terse name for this property so when referencing it in the view, there is minimal noise. We liked the property View for this purpose and implemented it for our view page first.

But when we went to port this property over to the Controller, we realized it wouldn’t work. Anyone care to guess why? Yep, that’s right. Controller already has a method named View so it can’t also have a property named the same thing. So we called it ViewModel for the time being and figured we’d change it once we came up with a better name.

So far, we haven’t come up with a better name that’s both short and descriptive. And before you suggest it, the acronym of “View Data” is not an option.

If you have a better name, do suggest it. :)

Addendum 2: Unit Testing

Someone on Twitter asked me how you would unit test this action method. Here’s an example of a unit tests that shows you can simply call this dynamic method directly from within a unit test (see the act section below).

[TestMethod]
public void CanCallCycle() {
  // arrange
  var controller = new HomeController();
  controller.Index();

  // act
  string even = controller.ViewModel.Cycle(0, "even", "odd");

  // assert
  Assert.AreEqual("even", even);
}