如何更新项目文件夹中的xml文件

我正在尝试添加数据到现有的xml文件。

     static void Main(string[] args)
    {
        AddData("123Texttbox value");
    }
    public static void AddData(string message)
    {
        string file = Properties.Settings.Default.XMLFileFullName;
        var doc = XDocument.Load(file);
        doc.Root.Add(
                new XElement("Request",
                new XElement("ID", message.Substring(0, 3)),
                new XElement("Message", message))
            );

        Properties.Settings.Default.XMLFileFullName = doc.ToString();
        Properties.Settings.Default.Save();
    }

setting.setting

    <userSettings>
    <Test.Properties.Settings>
        <setting name="XMLFileFullName" serializeAs="String">
            <value>@"Request'request.xml"</value>
        </setting>
    </Test.Properties.Settings>
  </userSettings>

实际xml:

    <?xml version="1.0" encoding="utf-8" ?>
    <requests>
    </requests>

预期xml:

   <?xml version="1.0" encoding="utf-8" ?>
        <requests> 
           <request>123</request>
           <request> 
              <request>123Texttbox value</request>
           </request>
        </requests> 

但是这并没有给我期望的xml输出。我错过什么了吗?由于

public static void AddData(string message)
{
    string file = Properties.Settings.Default.XMLFileFullName;
    var doc = XDocument.Load(file);
    doc.Root.Add(
        new XElement("Request",
        new XElement("ID", message.Substring(0, 3)),
        new XElement("Message", message))
    );
    doc.Save(file);
}