ASP MVC nested arrays model binding

Question

I have the followingsimple models:

public class MainModel
{
    public int Id {get;set;}
    public List<Question> Questions {get;set;}
}

public class Question
{
    public string Text {get;set;}
    public List<Answer> Answers {get;set;}
}

public class Answer
{
    public byte No {get;set;}
    public string Text {get;set;}
    public bool Correct {get;set;}
}

I have strongly typed view to MainModel which allows users to add custom number of questions and answers for each question (and remove them as well).

I'm using the method with adding hidden indexing field and it works ok for the Question level (dynamically added questions are being included on POST). However, when it comes to dynamically added answers, it's not working properly. This is the HTML I'm having rendered as a result:

<div class="answer">
<input type="hidden" value="1" name="Questions[2].Answers.Index">
<input type="checkbox" data-checkbox-for="Questions[2].Answers[1].Correct" checked="checked">
<input type="hidden" value="1" name="Questions[2].Answers[1].No">
<input type="text" value="2.1" name="Questions[2].Answers[1].Text">
<input type="hidden" value="true" name="Questions[2].Answers[1].Correct">
<span class="remove-answer link">Remove</span>
</div>

What am I doing wrong here?

iBoonZ · Accepted Answer · 2012-10-11 20:17:16Z

1

Do you start with zero index?

 <div class="answer">
<input type="hidden" value="1" name="Questions[2].Answers.Index">
<input type="checkbox" data-checkbox-for="Questions[2].Answers[0].Correct" checked="checked">
<input type="hidden" value="1" name="Questions[2].Answers[0].No">
<input type="text" value="2.1" name="Questions[2].Answers[0].Text">
<input type="hidden" value="true" name="Questions[2].Answers[0].Correct">
<span class="remove-answer link">Remove</span>

if you don't start counting from 0, you will get no post values, because mvc wants index 0 before index 1

and following property is not in your model, is it?

<input type="hidden" value="1" name="Questions[2].Answers.Index">

answered Oct 11, 2012 at 20:17

iBoonZ

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2 Comments

Admir Tuzović Over a year ago

That property is used to bypass requirement for sequential counting. Meaning I can have indexes 1, 5, 90... as long as I have that row, they will get bound on server-side as 1,2,3. I believe I've found issue, seems what I wrote above is actually working - error was somewhere else.

rossisdead Over a year ago

Barisa: Are you using a custom binder of some sort? Because, at least in MVC3, the default model binders will ignore any array indexes the moment it finds a gap in the numbering(ie, if you have [0, 1, 3], you'll never see 3).

Leniel Maccaferri · Accepted Answer · 2014-12-29 18:05:50Z

0

Here's the way to go over this:

Model Binding To List Of Objects In ASP.NET MVC

Make sure you add a hidden index with some sort of randomness. This way you can have "gaps" in between various index values.

Something like this:

var random = new Random(10);
var rnd = random.Next();

@Html.Hidden("Questions[" + (i) + "].Answers.Index", i + j + rnd)
@Html.TextBox("Questions[" + (i) + "].Answers[" + (i + j + rnd) + "].Text", Model.Questions[i].Answers[j].Text, new { @class = "form-control" })

answered Dec 29, 2014 at 18:05

Leniel Maccaferri

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