What am I misunderstanding about how Html.TextBoxFor works

I’m just trying to get back into .NET MVC with the new release and I can’t get my head round the way may view is binding to the DataModel.

I have a model with a property “first_name” and within an HTML form I have the following

<%= Html.TextBox("first_name", Model.first_name)%>
<%= Html.TextBoxFor(model => model.first_name) %> 
<input type="text" name="first_name" id="first_name" class="myLabel" 
       value="<%=Model.first_name %>" />

In an action on a controller if I set the first_name property on my model and do

mymodelObject.first_name = "Test";
return View(mymodelObject);

What is the reason only the third textbox picks up on this first_name value and the other two don’t?

Edit:

I probably haven’t explained this well enough, sorry. Imagine I have 2 controller methods –

public ActionResult Register()
{
    Registration model = new Registration();
    model.first_name = "test";
    return View(model);
}

With this one either binding works.

After this has been displayed I then click a button on the form and try and run this:

[HttpPost]
public ActionResult Register(Registration_ViewData model)
{
    model.first_name = "Steve";
    return View(model);
}

What I’m asking is why does the 3rd but not the first 2 bind to “Steve” as the new name.

Answers:

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Method 1

You need to clear your model state so your code would look something like:

[HttpPost]
public ActionResult Register(Registration model)
{
    ModelState.Clear();
    model.first_name = "Steve";
    return View(model);
}

Method 2

Because HTML helpers read the value from the ModelState and not from the model. In order the change that behavior you’ll need to work with the ModelState as well.
(see: Changing model’s properties on postback)

Method 3

This should work for the first two:

<%= Html.TextBox("first_name", x => x.first_name)%>
<%= Html.TextBoxFor(model => model.first_name) %>

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0