c# - problems with getting data for dropdownlist -

i have dropdownlist getting data data base

it contains 4 data :

my controller :

public actionresult index()     {                 viewbag.postes = _db.postes.tolist();                    return view();      }      [httppost]     public actionresult index( poste poste,string num_cin="r346399")     {          if (modelstate.isvalid)         {             if (poste.id == 3)             {             return redirecttoaction("inscription", "candidat");             }          return view(poste);             }        } 

my view

        @html.dropdownlistfor(model => model.id,         new selectlist(viewbag.postes, "id", "intitule_poste"),"choisir le poste") 

the problem if choose value dropdownlist !=3 it's give me error "that items must not null "

you view includes @html.dropdownlistfor() generated based on value of viewbag.postes. when return view (i.e. when poste.id not equal 3) must reassign value of viewbag.postes

if (modelstate.isvalid) {   if (poste.id == 3)   {     return redirecttoaction("inscription", "candidat");   }   viewbag.poste = _db.postes.tolist(); // reassign collection dropdown   return view(poste); }