Loop through page controls associated with nested master page

Hi Everyone,
I need to find controls in page that is associated with nested master page. I think you all know that Master pages can be nested, with one master page referencing another as its master.
I tried using following  codes but failed
ContentPlaceHolder content = (ContentPlaceHolder)this.Master.Master.FindControl("contentID");
ContentPlaceHolder content = (ContentPlaceHolder)this.Master.FindControl("contentID");
Got null in content field.
Can anyone help me to reslove this.
Thanks



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