C#刷新StreamWriter和一个MemoryStream

//converts an xsd object to the corresponding xml string,using the UTF8 encoding
    public string Serialize(T t)
    {
        using (var memoryStream = new MemoryStream())
        {
            var encoding = new UTF8Encoding(false);

            using (var writer = new StreamWriter(memoryStream,encoding))
            {
                var serializer = new XmlSerializer(typeof (T));
                serializer.Serialize(writer,t);
                writer.Flush();
            }

            memoryStream.Flush();

            return encoding.GetString(memoryStream.ToArray());
        }
    }

首先,因为代码在using块中,我认为自动调用dispose方法可能会为我做这个.这是真的,还是正在形成一个完全不同的概念？

根据stackoverflow本身：

Flush meaning clears all buffers for a stream and causes any buffered data to be written to the underlying device.

在上面的代码中,这意味着什么？

其次,MemoryStream does nothing according to the api的flush方法,那有什么用呢？为什么我们称之为无效的方法？