无法显示修改后的值

Question

我正在使用LINQ编写和读取XML数据。读取部分可以，但无法正常工作。

我的xml中的发布数据正在正确加载，但是一旦用户尝试保存新更改，数据仍保持不变。请问为什么？

读取帖子数据：

public partial class EditPost : System.Web.UI.Page
{
    private void Page_Load(object sender, EventArgs e)
    {
        string new_title = newtitle.Text.ToString();
        string new_description = update_des.Value.ToString();

        string path = "C:\\Users\\user\\Source\\Repos\\FoodBlog\\FoodBlog\\Data\\blog_post.xml";
        string postid = Request.QueryString["pid"];

        if (postid != null)
        {
            FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
            XDocument xdoc = XDocument.Load(fs);
            var post = from p in xdoc.Descendants("post")
                        select new {
                            Title = p.Element("title"),
                            Subtitle = p.Element("subtitle"),
                            Desc = p.Element("description"),
                            PID = p.Element("pid")
                        };

            foreach (var ePosts in post) {
                if (ePosts.PID.Value == postid) {
                    newtitle.Text = ePosts.Title.Value;
                    Subtitle.Value = ePosts.Subtitle.Value;
                    update_des.Value = ePosts.Desc.Value;
                }
            }

        }
    }
}

撰写帖子数据：

protected void Update_btn_click(object sender, EventArgs e)
{
    string postid = Request.QueryString["pid"];
    if (postid != null)
    {
        string path = "C:\\Users\\user\\Source\\Repos\\FoodBlog\\FoodBlog\\Data\\blog_post.xml";

        FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
        XDocument xdoc = XDocument.Load(fs);
        var post = from p in xdoc.Descendants("post")
                    where (string)p.Element("pid") == postid
                    select new {
                        Title = p.Element("title"),
                        Subtitle = p.Element("subtitle"),
                        Desc = p.Element("description"),
                        PID = p.Element("pid")
                    };

        foreach (var ePosts in post)
        {
            if (ePosts.PID.Value == postid)
                ePosts.Title.ReplaceWith(newtitle.Text);

            ePosts.Subtitle.ReplaceWith(Subtitle.Value);
            ePosts.Desc.ReplaceWith(update_des.Value);
        }
    }
}

XML数据结构：

<Posts>
  <post>Something<
  <pid>p9119</pid>
  <title>Something</title>
  <description>Something</description>
  <subtitle>Something</subtitle>
  <date>Something</date>
  <author>Something</author>
</Posts>