Building action parameter object from XHR-request data MVC3 - asp.net-mvc

Question

I have this action in my RESTful application on MVC3:
[HttpPut]
public void Rest(ViewModel view_model, int id)
{
//doing something with view_model
}
Where ViewModel class is a class for passing data to/from client Javascript (I don`t want to pass pure DB entities):
public class ViewModel
{
public ViewModel() //parameterless constructor, needed for accepting as parameter in action
{
}
public ViewModel(Model m)
{
id = m.ID;
Title = m.Title;
}
public int? id { get; set; }
private string _title;
public string Title
{
get
{
if (String.IsNullOrWhiteSpace(_title)) throw new Exception("Empty field");
return _title;
}
set
{
_title = value;
}
}
}
BUT when I make PUT request with that data:
{ "id" : 7, "Title" : "Hello world!" }
I get that "Empty field" exception. Seems like something is trying to get Title property, even before it has been set with incoming "Hello world!" data.
Why?
And where can I get some information, how this whole operation works, i.e. looking for object ViewModel that specified as action parameter, in actual XHR-request.
Thank you for your thoughts.

Answer 1

Seems like something is trying to get Title property, even before it
has been set with incoming "Hello world!" data. Why?
It is the default model binder. And more specifically the BindProperty method. This method is called during binding and it uses reflection to call the getter. Because the model binder recurses down the object hierarchy graph it first needs to get the value of the property, build a binding context and model metadata for each property and then invoke the SetProperty method.
If you don't want the title property to be empty use the validation mechanisms and auto implemented properties:
[Required]
public string Title { get; set; }
and then in your RESTful action check if the ModelState.IsValid. It is much more easier and MVCish:
[HttpPut]
public ActionResult Rest(ViewModel view_model, int id)
{
if (!ModelState.IsValid)
{
...
}
// doing something with view_model
...
}