Monday, May 21, 2012

Minor Issues: Constructors and MVC Controllers

Recently I've been getting into F# a little.  It's a really cool language which has been broadening my perspective on problem solving with code.  It's a .NET language, and is mostly compatible with C#, but it does do some things differently.  For example, it has more rigorous rules around constructors:
type Point(x : float, y : float) =

  member this.X = x
  member this.Y = y

  new() = new Point(0.0, 0.0)

  new(text : string) =
    let parts = text.Split([|','|])
    let x = Double.Parse(parts.[0])
    let y = Double.Parse(parts.[1])
    new Point(x, y)
It may not immediately jump out at you, but there are some really cool things here that C# doesn't do:
  1. This actually defines 3 constructors, the "main" constructor is implicitly defined to take in floats x and y
  2. Constructors can call other constructors!  Note the empty constructor, new().
  3. All constructors ARE REQUIRED to call the "main" constructor
I fell in love with this immediately.  This requirement forces me to be keenly aware of what my class's core data really is.  And it communicates that knowledge very clearly to any consumers of the class as well.  This was especially refreshing because I have found that since C# added object initialization syntax (new Point { X = 1.0, Y = 2.0 }) I've started writing far fewer constructors.  Constructors are boilerplate and annoying to type, so I largely stopped typing them.  But now that I have done that for awhile and I have a few real world examples of classes without constructors, I find that I miss the constructors.  They communicate something nice about the core, and most important data, of the class that a whole lot of properties doesn't communicate.

So, that sounds pretty straight forward, and I should start writing constructors on my classes again.  And if I want to be really strict (which I kind of do), I shouldn't provide a default constructor either.  Then I'll be living in the rigorous world of class constructors, just like F#.

But this is where MVC Controllers finally make their first appearance in this post.  Because these controllers exert there own pressure on my classes and down right require a default (parameter-less) constructor.  At least that's the case with the way my team writes our controllers.  Why?  Here's an example.

Let's talk about CRUD.  Typically there's a controller action we call "New", it returns an empty form so you can create a new record.  This form posts to a controller action we call "Create", which binds the form data to a model, and calls .Save().  We're using MVC's standard form builder helpers, which generate form field names and IDs based on the model expression you provide as a lambda.  This is how it knows how to bind the form back to your model on "Create".  But this means you have to pass an empty model out the door in the "New" to generate the empty form.  An empty model requires a default constructor!  So the code looks like this:
public ActionResult New()
  ViewData.Model = new Point();
  return View();

public ActionResult Create(Point point)
  return View();
Obviously, real code is more complicated than that, but you get the idea.

And so I find myself with a minor issue on my hands.  On the one hand, I want to create F# inspired rigorous classes.  But on the other hand I want simple controllers that can just send the Model out the door to the view.  Alas, I can't have both, something has to give.

Obviously I could give up on the Constructors.  Or, I could give up on passing my model directly to the view.  There are other approaches well documented in this View Model Patterns post.  The quick description is I could NOT pass my model, and instead pass a View Model that looks just like my model.  And then I'd have to map that View Model back onto my Model somehow...  But that comes with it's own minor issues.

So how about it?  How do you deal with this issue?


  1. How about the null object pattern? Then you don't have to worry about creating the blank object at all outside of that class.

  2. @Toby: That's a really interesting idea! Thanks!


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