这一行self.displayResultLable.text+=(titleas!String)抛出错误cannotconvertvaluetype"string?"toexpectedargumenttype"inoutstring"这是我的代码:ifresults.count>0{vardisplayResult:String?forbooksinresultsas![NSManagedObject]{iflettitle=books.valueForKey("title"){self.displayResultLable.text+=(titleas!String)}}}这里的

这一行self.displayResultLable.text+=(titleas!String)抛出错误cannotconvertvaluetype"string?"toexpectedargumenttype"inoutstring"这是我的代码:ifresults.count>0{vardisplayResult:String?forbooksinresultsas![NSManagedObject]{iflettitle=books.valueForKey("title"){self.displayResultLable.text+=(titleas!String)}}}这里的