i have recently find the solution that how i can retrieve the fileupload id when page is linked with masterpage in codebehind as

ContentPlaceHolder content = Page.Master.FindControl("ContentPlaceHolder1") as ContentPlaceHolder;
Fileupload f=content.FindControl("FileUpload1") as FileUpload;

in my contentplaceholeder1 i have a dropdownlist with id dropdownlist1 now i m trying to use it in one of the cs file of dll file as

if (previousFields.ContainsKey("dropdownlist1"))
                    {
                        prefix = previousFields["dropdownlist1"];

                    }

where dropdownload list is the previos field of fileupload so it checks the previos field and assign the prefix to the corresponding value.nw i want to know how can i access the dropdownlist id within contentplaceholder1 id