Operation not permitted on IsolatedStorageFileStream.

　　我在使用IsolatedStorageFileStream时遇到了一个很顽固的异常“Operation not permitted on IsolatedStorageFileStream.”至今没办法找到。大侠求助

异常截图

代码如下

Student.cs

   public class Student
    {
        public string EMail { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public Student(string FirstName, string LastName)
        {
            this.FirstName = FirstName;
            this.LastName = LastName;
            this.EMail = FirstName + "@" + LastName + ".com";
        }

        public Student(XElement xElement)
        {
            this.FirstName = xElement.Attribute("FirstName").Value;
            this.LastName = xElement.Attribute("LastName").Value;
            this.EMail = xElement.Attribute("EMail").Value;
        }

        public XElement Information
        {
            get
            {
                return new XElement("Student", new XAttribute("EMail", EMail), new XElement("FirstName", FirstName), new XElement("LastName", LastName));
            }
        }
    }

StudentList .cs

  public class StudentList : List<Student>
    {
        public void Load(string strXMLFile)
        {
            IsolatedStorageFile isfData = IsolatedStorageFile.GetUserStoreForApplication();
            XDocument doc = null;

            IsolatedStorageFileStream isfStream = null;

            if (isfData.FileExists(strXMLFile))
            {
                isfStream = new IsolatedStorageFileStream(strXMLFile, System.IO.FileMode.Open,FileAccess.Write, isfData);
                doc = XDocument.Load(isfStream);
                isfStream.Close();
            }

            else
            {
                doc = XDocument.Load(strXMLFile);
                isfStream = new IsolatedStorageFileStream(strXMLFile, System.IO.FileMode.CreateNew, FileAccess.Write, isfData);
                doc.Save(isfStream);
                isfStream.Close();
            }

            var vStudent = from s in doc.Descendants("Student") select new Student(s);
            this.Clear();
            AddRange(vStudent);
        }

        public void Save(string strXMLFile)
        {
            try
            {
                XElement xml = new XElement("Students", from p in this select p.Information);
                IsolatedStorageFileStream isfStream = new IsolatedStorageFileStream(
                    strXMLFile,
                    System.IO.FileMode.Open,
                    IsolatedStorageFile.GetUserStoreForApplication());
                xml.Save(isfStream);
                isfStream.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }

Students.xml

<?xml version="1.0" encoding="utf-8" ?>
<Students>
  <Student EMail="Woo@Jianqiang.com">
    <FirstName>Woo</FirstName>
    <LastName>Jianqiang</LastName>
  </Student>
  <Student EMail="Whitcomb@Donald.com">
    <FirstName>Whitcomb</FirstName>
    <LastName>Donald</LastName>
  </Student>
  <Student EMail="Kadi@Wadad.com">
    <FirstName>Kadi</FirstName>
    <LastName>Wadad</LastName>
  </Student>
</Students>

Mainpage.xaml

<ListBox x:Name="lbData"></ListBox>

Mainpage.cs

    public partial class MainPage : PhoneApplicationPage
    {
        StudentList data =null;
        // Constructor
        public MainPage()
        {
            InitializeComponent();
        }

        private void loadData() {
            data = new StudentList();
            data.Load("Model/Students.xml");
            lbData.ItemsSource = data;
        }

        private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
        {
            loadData();
        }