XML, LINQ and XDocument.Save problems

<?xml version=1.0" encoding="utf-8"?>
<content>
      <weapon id="1234" name="blahblah">
         <note info="blah blah" />
      </weapon>
      <weapon id="5678" name="blahblah2">
         <note info="blah blah" />
      </weapon>
</content>

FileOpenPicker openPicker = new FileOpenPicker();
openPicker.SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.DocumentsLibrary;
openPicker.FileTypeFilter.Add(".xml");
StorageFile gfile = await openPicker.PickSingleFileAsync()
fileContent = await FileIO.ReadTextAsync(gfile, Windows.Storage.Streams.UnicodeEncoding.Utf8);
Xdocument  xml = XDocument.Parse(fileContent);
xml.Descendants("weapon").Where(c => c.Attribute("id").Value.Equals("5678")).FirstorDefault().Remove();
IRandomAccessStream writeStream = await gfile.OpenAsync(FileAccessMode.ReadWrite);
Stream stream = writeStream.AsStreamForWrite();
xml.Save(stream);

<?xml version=1.0" encoding="utf-8"?>
<content>
      <weapon id="1234" name="blahblah">
         <note info="blah blah" />
</content>apon>
      <weapon id="5678" name="blahblah2">
         <note info="blah blah" />
      </weapon>
</content>

事实证明，这个问题比乍一看要复杂得多。

我们需要解决的问题包括

• 异步写入文件
• 高效写入，即使用缓冲 IO
• 覆盖整个文件，必要时修剪现有文件
• 使我们的写入操作可等待

经过大量实验，我选择的解决方案是将XML写入System.IO.MemoryStream，然后将该内存流复制到存储文件中。我确实了解这需要内存来临时复制数据。但它可以工作，速度快（缓冲 IO，少量本机 Windows 调用，只有三个等待），正确修剪，并且等待操作实际上可以正常工作。请注意，我尝试过的其他一些方法并非如此。这是解决方案：

MemoryStream ms = new MemoryStream()
xml.Save(ms, SaveOptions.DisableFormatting);
await ms.CopyToAsync(gfile);

CopyToAsync 扩展方法：

using System.IO;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Storage;
using Windows.Storage.Streams;
internal static class Extensions
{
    public static async Task CopyToAsync(this MemoryStream source, StorageFile file)
    {
        using (IRandomAccessStream raStream = await file.OpenAsync(FileAccessMode.ReadWrite))
        {
            using (IOutputStream stream = raStream.GetOutputStreamAt(0))
            {
                await stream.WriteAsync(source.GetWindowsRuntimeBuffer());
                await stream.FlushAsync();
            }
            raStream.Size = raStream.Position;
        }
    }
}

为什么不直接使用 System.IO.File.WriteAllText？

XDocument xml = XDocument.Load(xmlFilePath);
System.IO.File.WriteAllText(xmlFilePath, string.Format(@"<?xml version=""1.0""?>{0}{1}", Environment.NewLine, xml));

我让它工作了，但我需要对你的代码行为做更多的研究。 试试这个...

var local = Windows.Storage.ApplicationData.Current.LocalFolder;
var file = await local.GetFileAsync("test.xml");
var data = await FileIO.ReadTextAsync(file);
var xml = XDocument.Parse(data);
xml.Descendants("weapon").Where(c => c.Attribute("id").Value.Equals("5678")).FirstOrDefault().Remove();
file = await local.CreateFileAsync("test.xml", CreationCollisionOption.ReplaceExisting);
var writeStream = await file.OpenStreamForWriteAsync() as Stream;
xml.Save(writeStream);
writeStream.Flush();

其中 test.xml 是本地文件夹中包含原始 XML 的文件。

嗨，

今天我必须编写一个XML文件，如果我提供有效的流，基本上它运行良好。

WinRT 可能很棘手，因为它对文件系统的访问权限有限。

/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class MainPage : Page
{
    public List<DataStructure> DataList { get; set; }
    public MainPage()
    {
        this.InitializeComponent();
        DataList = Enumerable.Range(0, 25).Select(i => new DataStructure() { Id = i, Data = string.Format("Number : {0}", i) }).ToList();
        this.Loaded += MainPage_Loaded;
    }
    async void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
        this.Loaded -= MainPage_Loaded;
        //var xmlDocument =
        //    new XDocument(
        //        new XElement("DataList",
        //            DataList.Select(dataItem =>
        //                new XElement("DataItem",
        //                    new XAttribute("id", dataItem.Id), new XAttribute("data", dataItem.Data)))));
        var rootNode = new XElement("DataList");
        var xmlDocument = new XDocument(rootNode);
        foreach (var dataItem in DataList)
        {
            rootNode.Add(new XElement("DataItem",
                            new XAttribute("id", dataItem.Id), new XAttribute("data", dataItem.Data)));
        }
        FileSavePicker savePicker = new FileSavePicker();
        savePicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;
        // Dropdown of file types the user can save the file as
        savePicker.FileTypeChoices.Add("XML Document", new List<string>() { ".xml" });
        // Default file name if the user does not type one in or select a file to replace
        savePicker.SuggestedFileName = "New Xml Document";
        StorageFile file = await savePicker.PickSaveFileAsync();
        // Process picked file
        if (file != null)
        {
            // Store file for future access
            var fileToken = Windows.Storage.AccessCache.StorageApplicationPermissions.FutureAccessList.Add(file);
            var writterStream = await file.OpenStreamForWriteAsync();
            xmlDocument.Save(writterStream);
        }
    }

我在VS2012（RC）上创建了一个WinRT默认项目。然后我修改了 MainPage.cs 文件，使其看起来像上一个文件。该文件包含一个硬编码的数据结构列表，我们基于列表创建一个XDocument。最后，我们要求用户提供一个可写的流来保存我们的XDocument。（我们还可以从独立存储中获取可写流）。最后，我们只是使用正确的 Stream 调用 write 方法。

我正在写入一个刷新到磁盘的文件流。

public static async Task SaveXMLAsync(XDocument linqXml, StorageFolder localFolder, string filename)
{
  // Create a storage file
  StorageFile file = await localFolder.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
  // Write the XML to a File Stream.
  using (FileStream sourceStream = new FileStream(file.Path, FileMode.Create, FileAccess.Write, FileShare.None, bufferSize: 4096, useAsync: true))
  {
    linqXml.Save(sourceStream);
    // Async flush to disk.
    await sourceStream.FlushAsync();
  };
}