how to fix this error ?

An unhandled exception of type 'System.InvalidCastException' occurred in SLA.exe

Additional information: Unable to cast object of type 'System.String' to type 'System.Byte[]'.

byte[] FileData = (byte[])DS.Tables["FilelibF"].Rows[SelectedRow]["FileData"];

                String newfile = @"D:\Temp";
                DirectoryInfo l_dDirInfo = new DirectoryInfo(newfile);
                if (l_dDirInfo.Exists == false)
                {
                    Directory.CreateDirectory(newfile);
                }