创建一个 xml 文件以在本地存储数据 C# 并从该创建的文件读取数据

我想将从URL获得的数据存储到xml文件中，并在没有互联网时显示该数据。

我有这个：

 async Task CreateSaveFile()
    {
        // Create document
        XDocument xmlDocument = new XDocument(new XDeclaration("1.0", "utf-8", "yes"),
            new XElement("Objects", from test in TestList
                                  select new XElement("test",
                                      new XElement("name", test.Name),
                                      new XElement("number", test.Number))));
        // Storage Folder
        StorageFolder folder = Windows.Storage.ApplicationData.Current.LocalFolder;
        string fileName = "test.xml";
        StorageFile file = await folder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);
        Stream stream = await file.OpenStreamForWriteAsync();
        // Write file
        xmlDocument.Save(stream);
        stream.Flush();
    }

而这个方法从创建的测试中获取数据.xml到一个列表中：

 private List<Test> testList = new List<Test>();
 async Task<List<Test>> getTestList()
    {
        StorageFolder folder = Windows.Storage.ApplicationData.Current.LocalFolder;
        IReadOnlyList<StorageFile> files = await folder.GetFilesAsync();

        foreach (StorageFile file in files)
        {
        // Get xml files
        XDocument xmlFile = XDocument.Load(file.Path);
        // Get TestObjects
        testList = (from element in xmlFile.Root.Descendants("test")
                       select new Player(
                           element.Element("name").Value,
                           element.Element("number").Value)).ToList<Test>();
        }
        return testList;
    }

我收到此错误 System.Windows.ni 中发生了类型为"System.Reflection.TargetInvocationException"的未处理异常.dll

 public RelayCommand LoadedCommand
    {
        get
        {
            return new RelayCommand(async () =>
            {
                await CreateSaveFile();
                try
                { 
                    TestList = await getTestList();
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.ToString()); throw;
                }
            });
        }
    }