我们如何在C＃中更新现有的XML文件

Question

我有一个我想要更新的现有xml文件。

using (IsolatedStorageFile ISF = IsolatedStorageFile.GetUserStoreForApplication())
        {
            // Load the xml document for reading
            using (IsolatedStorageFileStream ISFS = ISF.OpenFile(FILE_NAME, FileMode.Open, FileAccess.ReadWrite))
            {
                XDocument file= XDocument.Load(ISFS);
                //Edit the file
                file.Save(ISFS);
            }
        }

问题是每当我使用file.Save(ISFS)，然后尝试再次加载它时，使用相同的方法，报告错误

  System.Xml.XmlException was unhandled by user code
  HResult=-2146232000
  Message=Unexpected XML declaration. The XML declaration must be the first node in the document, and no white space characters are allowed to appear before it. Line 25, position 14.

更新：

这是保存后的文件

<?xml version="1.0" encoding="utf-8"?>
<A>
   <AB ID="0">
       <NickName>Dummy</NickName>
   </AB>
</A><?xml version="1.0" encoding="utf-8"?>
<A>
   <AB ID="0">
       <NickName>Dummy</NickName>
   </AB>
   <AB ID="1">
       <NickName>My1</NickName>
   </AB>
</A>