An ASP.NET MVC Gotcha? Frustrated - asp.net-mvc

Question

I have an issue with ASP.NET MVC html helpers like TextBoxFor(), HiddenFor(), etc. If I have a model such as Employee with the string member Name and execute Html.TextBoxFor(p => p.Name), is it wrong for me to assume that ASP.NET MVC will always use the value in the Employee's Name? Because it doesn't. ASP.NET will override that binding and use what's in the POST.
For exmaple let's say I have the following code:
Model
namespace MvcApplication2.Models
{
public class Company
{
public string Name { set; get; }
public List<Employee> Employees { set; get; }
}
public class Employee
{
public string Name { set; get; }
}
}
Controller
namespace MvcApplication2.Controllers
{
public class HomeController : Controller
{
public ActionResult Company(string Name)
{
return View(new Company {
Name = Name,
Employees = new List<Employee> {
new Employee { Name = "Ralph" },
new Employee { Name = "Joe" } }
});
}
}
}
Home/Company.cshtml
#using MvcApplication2.Models;
#model Company
<h2>Company's Name: #Model.Name</h2>
#foreach (Employee emp in Model.Employees)
{
Html.RenderPartial("Employee", emp);
}
Home/Employee.cshtml
#model MvcApplication2.Models.Employee
<b>Employee's Name: </b> #Html.TextBoxFor(p => p.Name);
When I hit the relative url "Home/Company?Name=MSFT", I expected Employee.cshtml to render "Ralph" and "Joe" in the textboxes but instead it renders MSFT for both textboxes. What do I have to do so that "Ralph" and "Joe" shows up in the textboxes? Do i have to make sure that my POST and GET variables never conflict in all layers of my view models (in this case the Company and Employee classes)? This seems silly. There's got to be an easy workaround, right?
Here's a screenshot of the result:

Answer 1

HtmlHelper methods and ModelState
An important thing to understand with MVC HtmlHelper methods:
They always look at ModelState first, value second, ViewData third.
The ModelState is important, because it contains the user-submitted values. If a page fails validation, the ModelState is used to store the previous values and error messages.
If you want to POST a form, and if everything is valid, you want to show the form again, you either have to:
Redierect from the POST to the GET (following the PRG Pattern)
Clear the ModelState: ModelState.Clear();
EditorFor vs RenderPartial
Another important thing to mention is the difference between EditorFor/DisplayFor vs RenderPartial.
When you use EditorFor/DisplayFor for an item (such as .EditorFor(m => m.Person)), it adds a sort-of "namespace" to the template of "Person" so that the sub-controls will have a unique name. For example, in the template, .TextBoxFor(p => p.Name) will render something like <input name="Person.Name" ....
However, when you use RenderPartial, no such namespace is created. Therefore, .TextBoxFor(p => p.Name) will render <input name="Name" ..., which will not be unique if you have multiple editors.

Answer 2

I understand that this behavior might be frustrating, and that it might seem illogical. However, your frustration is a result of not fully understanding how Model binding works. There is a logical chain of events that occur during binding, and if you understand how it works then it will help avoid these kinds of mistakes.
As Scott mentioned, MVC will always look in ModelState first. You can avoid this problem by making sure you don't have any querystring parameters that have the same name as any properties in your model.