asp.net mvc 3 - Is there a reason why the default modelbinder doesn't bind to fields?

Question

I'm using ASP.NET MVC3 and i'm wondering that the default modelbinder binds to public properties but not to public fields.

Normally i just define the model classes with properties but sometimes i use some predefined classes which contains some fields. And everytime i have to debug and remember that the modelbinder just don't like fields.

The question: Whats the reason behind it?

Answer 1

but sometimes i use some predefined classes which contains some fields

While I cannot answer your question about the exact reason why the default model binder works only with properties (my guess is that it respects better encapsulation this way and avoids modifying internal state of the object which is what fields represent) I can say that what you call predefined classes should normally be view models. You should always use view models to and from your controller actions. Those view models are classes that are specifically defined to meet the requirements of the given view.

So back to the main point: fields are supposed to be modified only from within the given class. They should not be accessed directly from the outside. They represent and hold internal state of the class. Properties on the other hand is what should be exposed to the outside world. Imagine that in the property getter/setter you had some custom logic. By modifying directly the field this custom logic would be broken and potentially bring the object into an inconsistent state.