hiddenFor helper is pulling id from action parameter, not the viewModel

I've stumbled upon a really weird situation with ASP.Net MVC, passing an "id" as an action parameter and an "id" hidden form element. I have an action that has an "id" parameter. This id value represents a project. I have a controller action where I'm creating a data entry form for assigning an employee to the passed in project (I create a drop down list where administrator selects an employee to be assigned to passed project). By assigning an employee to a project, I create a ProjectEmployee record (project id, employee id, and an id that represents the combination which is an identity column in the database). The identity column (which is named "id") is also a hidden form element. This is necessary because I need to be able to edit the project/employee assignment at a later date.

Anyways, when creating a new employee assignment to a project, the project id (which is the "id" being passed to the action) is being applied to the "id" hidden form element.

I'm passing 117 into the action. 117 is the projectId. It's being set to the "id" hidden field which should be 0 because 0 represents a new project/employee assignemnt.

View Model

id  - unique id that represents the Project/Employee combination
projectId - the "Id" being passed to action
EmployeeId - what the admin is selecting from drop down
rate
startdate
endDate
...

Data Entry Form

@Html.HiddenFor(m => m.Id) 
@Html.HiddenFor(m => m.ProjectId)
...
Id: @Model.Id <br />
ProjectId: @Model.ProjectId<br />

So, @Html.HiddenFor(m => m.Id) renders a hidden form element with a value of 117. @Model.Id renders 0 to the UI. Stepping through the code I can visually see the value of the Id property to be 0. How come HiddenFor is getting it's wires crossed and pulling the value of 117?

This bug has made it's way into production so I have a mess on my hands with data getting messed up because instead of creating a new record in the database table, I'm actually UPDATING existing records because the "Id" property is erroneously getting set from 0 (which represents a new record) to 117 (which is the projectId) and therefore am updating a different record.

How come HiddenFor is getting it's wires crossed and pulling the value of 117?

That's by design. All HTML helpers such as TextBoxFor and HiddenFor first use the value of ModelState when binding and then the value of the model. I presume your controller action looks like this:

public ActionResult Foo(int id)
{
    SomeModel model = ...
    // At this stage model.Id = 0
    return View(model);
}

The thing is that the default model binder adds a value into the ModelState with the key id which is used by the helper and the model property value is ignored. This is not a bug. It is how the HTML helpers are designed. It's a bit confusing in the beginning when you don't know it, but once you get accustomed to this behavior you don't fall in the trap a second time.

One possibility to fix the problem is to remove this value from ModelState:

public ActionResult Foo(int id)
{
    ModelState.Remove("id");
    SomeModel model = ...
    // At this stage model.Id = 0
    return View(model);
}

Or rename the Id property inside your model to something else to avoid the conflict.