c# using是try-finally语法糖

            //using
            var list = new List<Object>();
            using (var items = list.GetEnumerator()) {
            }

            //try-finally(这也是foreach内部实现)
            var items = list.GetEnumerator();
            try {
                while (items.MoveNext()) {
                    var current = items.Current;
                }
            }
            finally {
                items.Dispose();
            }