c# - Returning a validation error -

i have logincontroller. loginmodel able provide validation on string lengths , required fields. displayed fine in razor page.

public class logincontroller : controller {     public actionresult index()     {         return view();     }      [httppost]     public actionresult index(loginmodel model)     {         if (modelstate.isvalid)         {             if (membership.validateuser(model.username, model.password))             {                 formsauthentication.setauthcookie(model.username, model.notpublicpc);                 var url = formsauthentication.getredirecturl(model.username, model.notpublicpc);                 return redirect(url);             }             else             {                 //here want throw own validation message or otherwise                 //give feedback login unsuccessful             }         }         return view();     } }  public class loginmodel {     [required]     [stringlength(50, minimumlength = 4)]     public string username { get; set; }      [required]     public string password { get; set; }      [display(name = "keep me logged in - not check on public computer")]     public bool notpublicpc { get; set; } } 

how throw own validation error - i.e. want show message when login fails. though appreciate validation required , length done in browser different.

i have tried throwing exception , system.componentmodel.dataannotations.validationexception

if throw exception not handled client receive http 500 error (unless throw httpexception , specify error number). if desireable thats can do. otherwise can try add error message model state:

modelstate.addmodelerror("propertyname", "error message");