Why does setting the Name attribute explicitly on a Partial View input cause the Value to change after POST?

I ran across a problem today where the Code value of a nested object in one of our forms was being changed to an incorrect value. After some digging, I discovered that it is being assigned the value of the Parent object Code, only after POSTing, and only when I try to set the Name attribute explicitly with Html.TextBoxFor's second object parameter.

I setup a simple MVC(Version 5.2.2.0) project to isolate the issue. Here is the code for that.

Models

public class Parent
{
    public string Code { get; set; }
    public Child Child { get; set; } 
}

public class Child
{
    public string Code { get; set; }
}

Controllers

public class ParentController : Controller
{
    public ActionResult Show()
    {
        var child = new Child() { Code = "999"};
        var parent = new Parent() { Code = "1", Child = child };

        return View("Show", parent);
    }

    public ActionResult Update(Parent parent)
    {
        return View("Show", parent);
    }
}

Views/Parent/Show

@model TextBoxForBugTest.Models.Parent

@using (Html.BeginForm("Update", "Parent"))
{
    @Html.TextBoxFor(o => o.Code)
    @Html.Partial("~/Views/Child/Show.cshtml", Model.Child)
    <button type="submit">Submit</button>
}

Views/Child/Show

@model TextBoxForBugTest.Models.Child

@Html.TextBoxFor(o => o.Code, new { Name = "Child.Code" })

When I first load /Parent/Show, I see the correct values in the inputs: 1(Code), and 999(Child.Code).

Before POST

However, after returning from the Update Action Method after submitting the form, Child.Code has been assigned the Value "1" - the Parent Code.

After POST

I've found that I can fix the issue by setting the HtmlFieldPrefix.

@model TextBoxForBugTest.Models.Child

@{ Html.ViewContext.ViewData.TemplateInfo.HtmlFieldPrefix = "Child"; }

@Html.TextBoxFor(o => o.Code)

or by using a local variable

@model TextBoxForBugTest.Models.Child

@{ var theCode = Model.Code; }

@Html.TextBoxFor(o => theCode, new { Name = "Child.Code" })

but I'd like to understand why. What is going on here? Why is Child.Code being assigned the value of Parent.Code after POSTing?

I also found some related questions that get into using extensions, but they seem to be answering different questions

ASP.NET MVC partial views: input name prefixes

ASP.MVC 3 Razor Add Model Prefix in the Html.PartialView extension

***Edit - It's clear from the answers that I did a poor job of stating my actual question, so I'll attempt to clarify a bit more here.

The problem I was seeing that was leading to an end user identified bug was that

@Html.TextBoxFor(o => o.Code, new { Name = "Child.Code" })

was generating html with a different "value" the second time it was called(after POSTing).

I was able to solve that problem by setting the Html.ViewContext.ViewData.TemplateInfo.HtmlFieldPrefix. Stephen Muecke also pointed out another - probably better - solution to that problem in Editor Templates.

What I was trying to ask though was this:

Why does

@Html.TextBoxFor(o => o.Code, new { Name = "Child.Code" })

generate

<input name="Child.Code" id="Code" type="text" value="999">

the first time (/Parent/Show), but then generate

<input name="Child.Code" id="Code" type="text" value="1">

the second time (after POSTing to /Parent/Update)?

The Form Data that gets POSTed is

and the binded Model in

public ActionResult Update(Parent parent)
{
    return View("Show", parent);
}

has the expected values of Parent.Code == 1 and Child.Code == 999.

I think Stephen Muecke is probably close to the answer I'm looking for in his comment

Note also that the new { Name = "Child.Code" } hack does not change the id attribute and you have invalid html. – Stephen Muecke

Indeed, using

@Html.TextBoxFor(o => o.Code, new { Name = "Child.Code" })

, I end up with 2 inputs with id="Code", which is invalid according to the spec.

Even knowing that though, I still don't understand why the value attribute generated by TextBoxFor is different based on whether I'm GETing /Parent/Show or POSTing to /Parent/Update.