@Html.CheckBoxListFor(modelItem => item.Value.postedItems.IDs,
                      modelItem => item.Value.availItems,
                      i => i.Id,
                      i => i.Name,
                      modelItem => item.Value.selectedItems,
                      Position.Vertical)

public static MvcHtmlString CheckBoxListFor<TModel, TProperty, TItem, TValue, TKey>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> listNameExpr, Expression<Func<TModel, IEnumerable<TItem>>> sourceDataExpr, Expression<Func<TItem, TValue>> valueExpr, Expression<Func<TItem, TKey>> textToDisplayExpr, Expression<Func<TModel, IEnumerable<TItem>>> selectedValuesExpr, HtmlListInfo wrapInfo);



public static MvcHtmlString CheckBoxListFor<TModel, TProperty, TItem, TValue, TKey>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> listNameExpr, Expression<Func<TModel, IEnumerable<TItem>>> sourceDataExpr, Expression<Func<TItem, TValue>> valueExpr, Expression<Func<TItem, TKey>> textToDisplayExpr, Expression<Func<TModel, IEnumerable<TItem>>> selectedValuesExpr, Position position);