How to create a CheckBoxListFor extension method in ASP.NET MVC?

Here is a strongly typed HtmlHelper for CheckBoxListFor that handles selected items as an array in your viewdata model. I chose not to wrapper the Html.CheckBox or Html.CheckBoxFor methods as I don’t want the hidden “false” fields in my checkbox lists.

Please feel free to improve on this and repost 🙂

//View

<%: Html.CheckBoxListFor(model => model.FreightTypeIds, FreightTypeMultiSelectList)  %>

//Controller

    public ActionResult SomeAction(int[] FreightTypeIds)
    {
       //...

       return View();
    }


//Extension
public static MvcHtmlString CheckBoxListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, IEnumerable<TProperty>>> expression, MultiSelectList allOptions, object htmlAttributes = null)
{
    ModelMetadata modelMetadata = ModelMetadata.FromLambdaExpression<TModel, IEnumerable<TProperty>>(expression, htmlHelper.ViewData);

    // Derive property name for checkbox name
    string propertyName = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(modelMetadata.PropertyName);

    // Get currently select values from the ViewData model
    IEnumerable<TProperty> list = expression.Compile().Invoke(htmlHelper.ViewData.Model);

    // Convert selected value list to a List<string> for easy manipulation
    IList<string> selectedValues = new List<string>();

    if (list != null)
    {
        selectedValues = new List<TProperty>(list).ConvertAll<string>(delegate(TProperty i) { return i.ToString(); });
    }

    // Create div
    TagBuilder divTag = new TagBuilder("div");
    divTag.MergeAttributes(new RouteValueDictionary(htmlAttributes), true);

    // Add checkboxes
    foreach (SelectListItem item in allOptions)
    {
        divTag.InnerHtml += string.Format(
                                          "<div><input type=\"checkbox\" name=\"{0}\" id=\"{1}_{2}\" " +
                                          "value=\"{2}\" {3} /><label for=\"{1}_{2}\">{4}</label></div>",
                                          propertyName,
                                          TagBuilder.CreateSanitizedId(propertyName),
                                          item.Value,
                                          selectedValues.Contains(item.Value) ? "checked=\"checked\"" : string.Empty,
                                          item.Text);
    }

     return MvcHtmlString.Create(divTag.ToString());
}