asp.net - How to pre-select a drop down option list based on the parent key -

i have 2 entities rack , server. each server has foreign key parent rack. assigning server rack, using drop down list follow inside server create view. server create action method looks follow:-

public actionresult create()         {             populateviewbagdata();              return view(new serverjoin() { isipunique = true,             ismacunique = true});         } 

part of server create view include drop down list select rack follow:-

@model tms.viewmodels.serverjoin <div>    <span class="f"> rack</span>      @html.dropdownlistfor(model =>model.server.rackid, ((ienumerable<tms.models.tmsrack>)viewbag.racks).select(option => new selectlistitem {         text = (option == null ? "none" : option.technology.tag),          value = option.tmsrackid.tostring(),         selected = (model.server != null) && (option.tmsrackid == model.server.rackid)     }), "choose...")     @html.validationmessagefor(model =>model.server.rackid) </div> 

what trying implement , inside rack view want add link add server, , force rack drop down list select current rack , such as:-

@html.actionlink(“create server under rack”, “create”,”server”, new {rackid = model.rackid},null) 

but not sure how force drop down list select rackid passed, baring in mind user can still create server without going rack, rackid null??

any idea how implement ? br

here how should it:

public actionresult create(int? rackid) {     var model = new serverjoin() { isipunique = true, ismacunique = true};     if(rackid.hasvalue)     {        model.rackid = rackid.value;     }     populateviewbagdata();      return view(model); } 

then, in view, can use if clause replace dropdownlist hidden input if model has rackid.

the actionlink in rack view correct.