c# - ASP.net MVC Grouping within a SelectList -

i want create grouped select list, e.g. similar travel sites:

- europe     - london     - paris     - madrid - north america     - new york     - toronto - asia     - hong kong     - bangkok 

my model looks this:

public class topic {     public virtual guid id { get; private set; }     public virtual string code { get; set; }     public virtual string description { get; set; }     public virtual string groupdescription { get; set; } } 

where code value select, description text want show, , groupdescription want group by.

i found groupby method on selectlist - i'm not sure how use achieve end (if indeed correct method use!)

i create selectlist in controller this:

    private ienumerable<selectlistitem> generatetopiclist()     {         var selectoptions =             concern in _topicsrepository.listtopics()             select new selectlistitem             {                 value = concern.code,                 text = concern.groupdescription + " : " + concern.description,             };          return new selectlist(selectoptions, "value", "text");     } 

and use in view this:

<%= html.dropdownlist("concerns[0].topic", (selectlist)viewdata["topics"])%> 

the problem drop down wide both description , group description on same line.

is possible group selectlist topics want do?

i'm using mvc 3.

cheers!