cant invoke a fileupload

i need to call a fileupload field that is generated in c#, but my problem is that teh name that i am calling does not exist , can someone help ? i only put part of code that matters , if you need all code you can just ask and ill post it here 

            topico.Controls.Add(topico2);
            dr.Cells.Add(topico);
            TableCell descricao = new TableCell();
            FileUpload descricao2 = new FileUpload();
            descricao2.ID = "descricao_"+a;
            // Add the control to the TableCell
            descricao.Controls.Add(descricao2);
            dr.Cells.Add(descricao);
            tableResult.Rows.Add(dr);
            resultHolder.Controls.Add(tableResult);


            string filename = Path.GetFileName(descricao_ + a.PostedFile.FileName);

            Stream str = descricao_ + a.PostedFile.InputStream;
            BinaryReader br = new BinaryReader(str);