ASP.NET MVC route values and view model

Question

Here is a strange issue I'm experiencing.

The model:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

The view:

@model Person
<div>
    @Html.TextBoxFor(m => m.FirstName)
</div>
<div>
    @Html.TextBoxFor(m => m.LastName)
</div>

The action:

public ActionResult Contact(string FirstName, string LastName)
{
    // do something with FirstName and LastName passed in

    var person = new Person();
    person.FirstName = "John";
    person.LastName = "Doe";

    return View(person);
}

When call the action by:

/Home/Contact?FirstName=Jane&LastName=Smith

In the view it is Jane Smith where John Doe is expected. The view model is clearly the person.

But when the view modified to:

<div>
    @Html.TextBox("First Name", Model.FirstName)
</div>
<div>
    @Html.TextBox("Last Name", Model.LastName)
</div>

Everything is fine. Why is that? I'm on MVC 5.

score 1 · Accepted Answer · answered Aug 09 '20 at 21:27

1

That's how HTML Helper works. They will first look on the query parameters and use those values instead of using the on you specified on your Model on your controller.

For more info, you can see the same post here: Why is my MVC ViewModel member overridden by my ActionResult parameter?

Do you want to use your Model instead? Remove it on your Controller:

ModelState.Remove("FirstName");
ModelState.Remove("LastName");
var person = new Person();
person.FirstName = "John";
person.LastName = "Doe";

And it will render your desired Model values.

answered Aug 09 '20 at 21:27

Willy David Jr

8,604
6
46
57

It took me a while to get used to its behavior but now I have better understandings of how html helpers work. Thank you! – Q. Young Oct 26 '20 at 13:23
Glad you figure it out :) – Willy David Jr Oct 26 '20 at 13:31

Prince Painadath Jose · Answer 2 · 2020-07-31T08:21:50.053

0

Its because of ambiguity. Rename the function parameters FirstName to _FirstName and LastName to _LastName. Then it will work as you expected.

public ActionResult Contact(string _FirstName, string _LastName)

edited Jul 31 '20 at 08:21

answered Jul 31 '20 at 08:01

Prince Painadath Jose

54
4

Thank you for the workaround but why Model.FirstName works fine though? I tend to think it is a bug. – Q. Young Jul 31 '20 at 13:19
It's a compiler error because it would cause confusion if the names could be the same. Don't forget to accept the answer and up-vote if it solve your issue. – Prince Painadath Jose Jul 31 '20 at 14:53
I'm afraid I can't accept this as the answer. My question of what makes "m => m.FirstName" behave differently than "Model.FirstName" still stands. Thanks. – Q. Young Aug 03 '20 at 19:23

ASP.NET MVC route values and view model

2 Answers2