if obj as! NSObject == NSNull(){         return ""     }            if obj is NSNull {         return ""     }

这两种写法在swift中有什么区别吗？我感觉实现起来效果一样，具体例子如下！

//从字典中获取字符串 extension NSDictionary {          func stringAttributeForKey(_ key:String)->String{                  let obj = self[key]          //        if obj as! NSObject == NSNull(){ //            return "" //        }                  if obj is NSNull {             return ""         }                  if obj is NSNumber {             let num = obj as! NSNumber             return num.stringValue         }                  return obj as! String     } }  let dic: NSDictionary!  dic = ["name" : "Tom", "qq" : "13344", "nil" : NSNull()]  print(dic)  print(dic.stringAttributeForKey("nil"))