c# - Multiple forms on one MVC form, created with a loop, only the first submits data

Question

I have the following code, only the first form submits anything, the following submit null values, each model has data. If I change it to just one large form, everything submits. Why do the other individual forms post null values?

View

@model myModel[]
<ul>
    @for (int i = 0; i < Model.Length; i++)
    {
        using (Html.BeginForm("controllerAction", "Controller", FormMethod.Post,
                               new { id="Form"+i }))
        {
            <li>
                @Html.TextBoxFor(a => a[i].property1)
                @Html.CheckBoxFor(a => a[i].property2)
                @Html.HiddenFor(a => a[i].property3)
                <input type="submit" />
            </li>
        }
    }
</ul>

Controller

[HttpPost]
public ActionResult controllerAction(myModel[] models)
{
    ...do stuff...
}

Answer 1

The reason is that your creating form controls with indexers in your for loop, and your POST method parameter is myModel[] models.

By default, the DefaultModelBinder requires collection to be zero based and consecutive, so if you attempt to submit the second form, your posting back [1].property1: someValue etc. Because the indexer starts at 1, binding fails and the model is null.

You can solve this by adding a hidden input for an Index property used by the model binder to match up non consecutive indexers

<li>
    @Html.TextBoxFor(a => a[i].property1)
    @Html.CheckBoxFor(a => a[i].property2)
    @Html.HiddenFor(a => a[i].property3)
    <input type="hidden" name="Index" value="@i" /> // add this
    <input type="submit" />
</li>