Calling children inside getChildByName / getChild At?

I found out that I can't target a object inside the main MC if I use getChildAt / getChildByName. It will return me

Error #1119: Access of possibly undefined property someProperty through a reference with static t开发者_Go百科ype flash.display:DisplayObject.

I was trying to use something like

this.getChildAt(0).getChildByName("objectName")....

getChildByName and other get child methods return an object of type DisplayObject. You must cast it appropriately before properties or methods not belonging to display objects. Also bear in mind that these get child methods belong to DisplayObjectContainer class - so you cannot chain like the way you're trying to do.

var container:DisplayObjectContainer = DisplayObjectContainer(getChildAt(3));
var mc:MovieClip = MovieClip(container.getChildByName("intro_movie"));
mc.gotoAndStop(4);

//or

var container:DisplayObjectContainer = getChildAt(3) as DisplayObjectContainer;
var mc:MovieClip = container.getChildByName("intro_movie") as MovieClip;
mc.gotoAndStop(4);

//or
MovieClip(DisplayObjectContainer(getChildAt(3)).getChildByName("intro_movie")).gotoAndStop(4);

Casting with ClassName(obj) syntax will throw an error if cast fails; casting with obj as ClassName returns null without any errors - this might lead to confusions later as it can lead to error 1009 (null reference) at unexpected locations.