asp.net mvc checkbox/radiobutton matrix model-binding

Question

I have a dynamic forms with checkbox/radio-button lists & matrices:

Following code renders checkbox list:

@foreach (var sq in Model.SubQuestions)
{
    <label>
        <input type="hidden" name="answerResult.index" value="@sq.Id" />
        <input type="checkbox" name="answerResult[@sq.Id].SubQuestionId" value="@sq.Id" />
        @sq.Label.Name
    </label>
}

radio-button list:

<input type="hidden" name="answerResult.index" value="@Model.Id" />
@foreach (var sq in Model.SubQuestions)
{
    <label>
        <input type="radio" name="answerResult[@Model.Id].SubQuestionId" value="@sq.Id" />
        @sq.Label.Name
    </label>
}

My POST-action in controller:

[HttpPost]
public ActionResult PassageSurvey(int surveyId, int surveyPageIndex, IList<AnswerResult> answerResult)

where IList<AnswerResult> is an auto-bound collection from my form. I get only items that were checked/selected. Everything is going well.

Now I need to get the same collection from checkbox/radio-button matrices.

Radio-button matrix:

<table width="100%">
    <tr>
        <th></th>
        @foreach (var av in Model.AnswerVariants)
        {
            <th style="text-align: center;">
                <label>@av.Label.Name</label>
            </th>
        }
    </tr>

    @foreach (var sq in Model.SubQuestions)
    {
        <tr>
            <td>
                <label>@sq.Label.Name</label>
                <input type="hidden" name="answerResult.index" value="@sq.Id" />
                <input type="hidden" name="answerResult[@sq.Id].SubQuestionId" value="@sq.Id" />
            </td>

            @foreach (var av in Model.AnswerVariants)
            {   
                <td align="center">
                    <input type="radio" name="answerResult[@sq.Id].AnswerVariantId" value="@av.Id" />
                </td>
            }
        </tr>
    }
</table>

Checkbox matrix:

<table width="100%">
    <tr>
        <th></th>
        @foreach (var av in Model.AnswerVariants)
        {
            <th style="text-align: center;">
                <label>@av.Label.Name</label>
            </th>
        }
    </tr>

    @foreach (var sq in Model.SubQuestions)
    {
        <tr>
            <td>
                <label>@sq.Label.Name</label>
            </td>
            @foreach (var av in Model.AnswerVariants)
            {   
                <td align="center">
                    <input type="hidden" name="answerResult.index" value="@sq.Id" />
                    <input type="hidden" name="answerResult[@sq.Id].AnswerVariantId" value="@sq.Id" />
                    <input type="checkbox" name="answerResult[@sq.Id].SubQuestionId" value="@sq.Id" />
                </td>
            }
        </tr>
    }
</table>

POST-action in controller always the same. Now from radio-button matrix (in current sample 3x3) IList<AnswerResult> gets always 3 items, depending on items that were selected in rows and columns. But from Checkbox matrix (3x3) IList<AnswerResult> gets always all 9 items (regardless items were checked, hidden-inputs always have values)

But I want to get only items, that were checked. How could I change my checkbox-matrix template to solve this problem?

Answer 1

Finally I decided to handle this problem in my post-action instead. Removing items (checkboxes) that were not selected, and have nulls in answerResult[##].SubQuestionId fields.