asp.net mvc - Access public virtual dropdown value -

i have dropdown list , datefield. if there 1 specific value selected dropdown, datefield should required. solve that, tried use requiredif sttribute foolproof library.

but don't know how access selected value in model.

model

//this datefield [requiredif(tabstate == "discarded")] [datatype(datatype.date)] [display(name = "date discarded")] public nullable<system.datetime> datdiscarded { get; set; }  //this dropdown public virtual tabstate tabstate { get; set; } 

tabstate in controller

viewbag.intideastateid = new selectlist(db.tabstate, "intstateid", "strstate"); 

dropdown in view

<div class="form-group">     @html.labelfor(model => model.intideastateid, "state", htmlattributes: new { @class = "control-label col-md-2" })     <div class="col-md-10">         @html.dropdownlist("intideastateid", null, htmlattributes: new { @class = "form-control" })         @html.validationmessagefor(model => model.intideastateid, "", new { @class = "text-danger" })     </div> </div> 

how can check if selected value dropdown "discarded"?

thanks help.