MVC CheckBoxListFor not posting as expected - asp.net-mvc

Question

I have build a CheckBoxListFor extension on HtmlHelper thanks to this wonderful answer http://bit.ly/Aevcea (code below) but am finding it doesn't post as expected.
My form is based on a model Group which has (amongst other properties) string Name and int[] PersonIDs.
The CheckBoxListFor renders something like this:
<ul>
<li><input type="checkbox" name="PersonIDs" value="1" id="PersonIDs_1" /></li>
<li><input type="checkbox" name="PersonIDs" value="2" id="PersonIDs_2" /></li>
</ul>
My controller has an Edit(Group group) method to handle submission of this form. However, upon submit I'm finding group.PersonIDs is null. There is though a Request.Form["PersonIDs"] set to the selected values (e.g. "1,2" if both items above are checked). Also, if I add another parameter to my Edit method (int[] PersonIDs) then that arrives with the expected contents (the selected IDs).
Can anyone explain what I'm doing wrong? The relevant bit of my view looks like this (extra bits stripped out):
#Html.TextBoxFor(m => m.Group.Name)
#Html.CheckBoxListFor(m => m.Group.PersonIDs, Model.MultiSelectListOfAllPeople)
Note that the group parameter in my Edit method does come back with Name set according to the form.
Just for completeness, here is the full body of my CheckBoxListFor extension:
public static MvcHtmlString CheckBoxListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, IEnumerable<TProperty>>> expression, MultiSelectList multiSelectList, object htmlAttributes = null)
{
//Derive property name for checkbox name
MemberExpression body = expression.Body as MemberExpression;
string propertyName = body.Member.Name;
//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
List<string> selectedValues = new List<string>();
if (list != null)
{
selectedValues = new List<TProperty>(list).ConvertAll<string>(delegate(TProperty i) { return i.ToString(); });
}
//Create div
TagBuilder wrapper = new TagBuilder("ul");
wrapper.AddCssClass("clearfix");
wrapper.MergeAttributes(new RouteValueDictionary(htmlAttributes), true);
//Add checkboxes
foreach (SelectListItem item in multiSelectList)
{
wrapper.InnerHtml += String.Format("<li><input type=\"checkbox\" name=\"{0}\" id=\"{0}_{1}\" " +
"value=\"{1}\" {2} /><label for=\"{0}_{1}\">{3}</label></li>",
propertyName,
item.Value,
selectedValues.Contains(item.Value) ? "checked=\"checked\"" : "",
item.Text);
}
return MvcHtmlString.Create(wrapper.ToString());
}

Answer 1

OK, the problem was that the CheckBoxListFor extension needed to render the control name as Group.PersonIDs and not simply PersonIDs. The form was bound to an object that itself was a sub-property of the view model. I've quickly adapted my CheckBoxListFor method as follows, but would gratefully accept a more elegant solution! I'm passing an additional boolean parameter includeDeclaringType to tell it whether to include the name of the declaring type in the ID. Not sure if this can be inferred any other way..?
public static MvcHtmlString CheckBoxListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, IEnumerable<TProperty>>> expression, MultiSelectList multiSelectList, bool includeDeclaringType, object htmlAttributes = null)
{
//Derive property name for checkbox name
MemberExpression body = expression.Body as MemberExpression;
string declaringTypeName = body.Member.DeclaringType.Name;
string propertyName = body.Member.Name;
//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
List<string> selectedValues = new List<string>();
if (list != null)
{
selectedValues = new List<TProperty>(list).ConvertAll<string>(delegate(TProperty i) { return i.ToString(); });
}
//Create div
TagBuilder wrapper = new TagBuilder("ul");
wrapper.AddCssClass("clearfix");
wrapper.MergeAttributes(new RouteValueDictionary(htmlAttributes), true);
//Add checkboxes
foreach (SelectListItem item in multiSelectList)
{
var name = string.Concat(
includeDeclaringType ? string.Format("{0}.", declaringTypeName) : "",
propertyName
);
var id = string.Concat(
includeDeclaringType ? string.Format("{0}_", declaringTypeName) : "",
propertyName,
"_",
item.Value
);
wrapper.InnerHtml += String.Format("<li><input type=\"checkbox\" name=\"{0}\" id=\"{1}\" " +
"value=\"{2}\" {3} /><label for=\"{1}\">{4}</label></li>",
name,
id,
item.Value,
selectedValues.Contains(item.Value) ? "checked=\"checked\"" : "",
item.Text);
}
return MvcHtmlString.Create(wrapper.ToString());
}

Answer 2

You can also use ExpressionHelper.GetExpressionText method, it will generate the correct input name for you:
var expressionText = ExpressionHelper.GetExpressionText(expression);
wrapper.InnerHtml +=
string.Format("<li><input type=\"checkbox\" name=\"{0}" id=\"{0}_{1}\" " +
"value=\"{1}\" {2} /><label for=\"{0}_{1}\">{3}</label></li>",
expressionText,
item.Value,
selectedValues.Contains(item.Value) ? "checked=\"checked\"" : "",
item.Text);