asp.net mvc - HiddenFor(x => x.Id) is being populated by the UrlParameter instead of the ViewModel

Question

public ActionResult SomeAction(int Id){
    //Id is set to 2

    var model = //get some thing from db using Id(2);
    //Now model.Id is set to 9;

    return View(model);
}

----------View----------
@Html.HiddenFor(x => x.Id)

When I view source this hidden field is set to 2 not 9. How do I get it to map to the model instead of mapping to the URL routing info?

P.S. I'd prefer not to rename parameters because then I lose my nice looking url's unless i change the routing info. I have done that and it does work, but not what I want.

Answer 1

When an Action gets called the framework builds a ModelStateCollection based on the query-string values, post-data, routing values etc. And this ModelStateCollection will be passed to the View. All the HTML input helpers try to the get the values from the ModelStateCollection first, before trying to get the values from the actual model.

Because your input model is the int id but the output model is some new model the helpers will use the values from the ModelStateCollection (from the query string) because the propery names Id are match.

To make it work you have to manually clear the ModelStateCollection before returning the new model to the view:

public ActionResult SomeAction(int Id){
    //Id is set to 2

    ModelState.Clear();

    var model = //get some thing from db using Id(2);
    //Now model.Id is set to 9;

    return View(model);
}