How do you properly create a MultiSelect <select> using the DropdownList helper?

(sorry, there are several item here but none seems to allow me to get this working.)

I want to create a DropDownList which allows multiple selection. I am able to populate the list but I can't get the currently selected values to seem to work.

I have the following in my controller:

ViewBag.PropertyGroups = from g in db.eFinGroups
                              where g.GroupType.Contents == "P"
                              select new
                              {
                                  Key = g.Key,
                                  Value = g.Description,
                                  Selected = true
                              };


ViewBag.SelectedPropertyGroups = from g in company.Entities
                   .First().Properties.First().PropertyGroups 
                   select new { 
                                g.eFinGroup.Key, 
                                Value = g.eFinGroup.Description };

In the view I have:

@Html.DropDownListFor(model => model.PropertyGroupsX, 
   new MultiSelectList(ViewBag.PropertyGroups
             , "Key", "Value"
             , ViewBag.SelectedPropertyGroups), 
new { @class = "chzn-select", data_placeholder = "Choose a Property Group", multiple = "multiple", style = "width:350px;" })

PropertyGroupX is a string[] in the model.

I have tried all types of iterations with the selected properties... passing just the value, just the key, both, etc.

Also, what type is PropertyGroupX supposed to be? Is string array correct? Or should it be a dictionary that contains the current propertygroups? I really am having a hard time finding doc on this.

Someone suggested I should be using ListBoxFor. I have changed to that and still have the same issue. The selected values are not being set as selected when the option tags are rendered. Here is what I have tried:

@Html.ListBoxFor(model => model.P