asp.net mvc default model binding problem

Question

I have some problems with ASP.NET MVC’s default model binder. The View contains HTML like this:

<input name="SubDTO[0].Id" value="1" type="checkbox">
<input name="SubDTO[1].Id" value="2" type="checkbox">

This is my simplified ‘model’:

public class SubDTO
{
    public virtual string Id { get; set; }
}

public class DTO
{
    public List<SubDTO> SubDTOs { get; set; }

    public DTO()
{
    SubDTOs = new List< SubDTO>();
}
}

All this works fine if the user selects at least the first checkbox (SubDTO[0].Id). The controller ‘receives’ a nicely initialised/bound DTO. However, if the first check box is not selected but only, for example, SubDTO[1].Id the object SubDTOs is null. Can someone please explain this ‘strange’ behaviour and how to overcome it? Thanks.

Best wishes,

Christian

PS:

The controller looks like this:

     [Transaction]
        [AcceptVerbs(HttpVerbs.Post)]
        public RedirectToRouteResult Create(DTO DTO)
        {
...
}

PPS:

My problem is that if I select checkbox SubDTO[0].Id, SubDTO[1].Id, SubDTO[2].Id SubDTOs is initialised. But if I just select checkbox SubDTO[1].Id, SubDTO[2].Id (NOT the first one!!!) SubDTOs remains null. I inspected the posted values (using firebug) and they are posted!!! This must be a bug in the default model binder or might be missing something.

Answer 1

This behavior is "by design" in html. If a check-box is checked its value is sent to the server, if it is not checked nothing is sent. That's why you get null in your action and you'll not find value in the posted form either. The way to workaround this is to add a hidden field with the same name and some value AFTER the check-box like this:

<input name="SubDTO[0].Id" value="true" type="checkbox">
<input name="SubDTO[0].Id" value="false" type="hidden">
<input name="SubDTO[1].Id" value="true" type="checkbox">    
<input name="SubDTO[1].Id" value="false" type="hidden">

In this way if you check the check-box both values will be sent but the model binder will take only the first. If the check-box is not checked only the hidden field value will be sent and you\ll get it in the action instead of null.

Answer 2

I think this post on Scott Hanselman's blog will explain why. The relevant line is:

The index must be zero-based and unbroken. In the above example, because there was no people[2], we stop after Abraham Lincoln and don’t continue to Thomas Jefferson.

So, in your case because the first element is not returned (as explained by others as the default behaviour for checkboxes) the entire collection is not being initialized.