swift - iOS text field not resigning first responder -

i can't seem resign first responder text field. please , have looked @ other solutions on stackoverflow can't find solution. here error thrown unrecognized selector sent instance

 class viewcontroller: uiviewcontroller, uitextfielddelegate {      @iboutlet var textinputscreen: uitextfield!      override func viewdidload() {        super.viewdidload()        textinputscreen.delegate = self     }      func textfieldshouldreturn(_ textfield: uitextfield) -> bool {        // textfield.canresignfirstresponder true         textfield.resignfirstresponder() // throws error        // view.endediting(true) // throws same error        // textinputscreen.resignfirstresponder() // throws same error        return true     } 

the common case textfield has @ least 1 referencing outlet not link anything. if can take again @ storyboard, might see this.