Question

For instance, there is some object that will be model for strongly-typed view:

class SomeModel {
    public string SomeUsualField { get; set; }
}

Further, some action exists in controller that will work with object specified above:

public ActionResult Index(SomeModel obj)
{ 
    return View(obj);
}

So the question is why obj isn’t a null while Index action first called? It’s created new instance of SomeModel class with null SomeUsualField.

Answer 1

The ASP.NET MVC model binding infrastructure tries to fill all properties with data coming from the request object (query string, form fields, …). Therefore it creates an instance of all parameters of the controller to try to match the properties.
Because you don’t pass a SomeUsualField, it is null, but the parameter object has an empty instance.

You can initialize the property SomeUsualField when you call http://localhost/MySite/MyController/Index?SomeUsualField=test. The property SomeUsualField will automagically initialized with ‘test’

If you don’t want the parameter to be set, you can leave it away and make a 2nd action with the attribute [HttpPost]. A good tutorial is the music store.

public ActionResult Index() 
{  
    var obj = new SomeModel();
    return View(obj); 
} 

[HttpPost]
public ActionResult Index(SomeModel obj) 
{  
    // update logic
    return View(obj); 
}