UWP 覆盖现有文件

Question

我尝试在 UWP 中创建和写入文件 application.On 第一次执行它工作正常，第二次它覆盖第一个内容并且文件只有第二次执行的内容。

         Windows.Storage.StorageFolder Location = ApplicationData.Current.LocalFolder;
    var lstfiles = await Location.GetFilesAsync(CommonFileQuery.OrderByName);
      var foundfile = lstfiles.Where(x => x.Name==Test.dat").FirstOrDefault();

        if (foundfile==null)
        {
            StorageFile fil = await Windows.Storage.ApplicationData.Current.LocalFolder.CreateFileAsync("Test.dat");
            //StorageFile file = await Windows.Storage.ApplicationData.Current.LocalFolder.GetFileAsync("Test.dat");
            using (IRandomAccessStream writeStream = await fil.OpenAsync(FileAccessMode.ReadWrite))
            {
                System.IO.Stream s = writeStream.AsStreamForWrite();
                System.Xml.XmlWriterSettings settings = new System.Xml.XmlWriterSettings();
                settings.Async = true;
                using (System.Xml.XmlWriter writer = System.Xml.XmlWriter.Create(s, settings))
                {

                    writer.WriteStartElement("Order");
                    writer.WriteElementString("OrderID", "y1");
                    writer.WriteElementString("OrderTotal", "y2");
                    writer.WriteElementString("Customer", "y3");
                    writer.WriteElementString("Phone", "y4");
                    writer.Flush();
                    await writer.FlushAsync();
                }
            }
        }
        else
        {
            StorageFile file = await Windows.Storage.ApplicationData.Current.LocalFolder.GetFileAsync("Test.dat");
            using (IRandomAccessStream writeStream = await file.OpenAsync(FileAccessMode.ReadWrite))
            {
                System.IO.Stream s = writeStream.AsStreamForWrite();
                System.Xml.XmlWriterSettings settings = new System.Xml.XmlWriterSettings();
                settings.Async = true;
                using (System.Xml.XmlWriter writer = System.Xml.XmlWriter.Create(s, settings))
                {
                    writer.WriteStartElement("Order");
                    writer.WriteElementString("OrderID", "v51");
                    writer.WriteElementString("OrderTotal", "v2");
                    writer.WriteElementString("Customer", "v3");
                    writer.WriteElementString("Phone", "v4");
                   // await FileIO.AppendTextAsync(file, writer.ToString());
                   writer.Flush();
                    await writer.FlushAsync();
                }
            }
        }

Answer 1

您可以使用 Linq Xml

Windows.Storage.StorageFolder Location = ApplicationData.Current.LocalFolder;
var lstfiles = await Location.GetFilesAsync(CommonFileQuery.OrderByName);
var foundfile = lstfiles.Where(x => x.Name == "Test.dat").FirstOrDefault();

if (foundfile == null)
{
    StorageFile fil = await Windows.Storage.ApplicationData.Current.LocalFolder.CreateFileAsync("Test.dat");
    XElement root = new XElement("Root");
    root.Add(
        new XElement("Order",
        new XElement("OrderID", "y1"),
        new XElement("OrderTotal", "y2"),
        new XElement("Customer", "y3"),
        new XElement("Phone", "y4")));
    await FileIO.WriteTextAsync(fil, root.ToString());

}
else
{
    StorageFile file = await Windows.Storage.ApplicationData.Current.LocalFolder.GetFileAsync("Test.dat");
    var doc = await FileIO.ReadTextAsync(file);
    XDocument xDocument = XDocument.Parse(doc);
    XElement root = xDocument.Element("Root");
    IEnumerable<XElement> rows = root.Descendants("Order");
    XElement firstRow = rows.First();
    firstRow.AddBeforeSelf(
        new XElement("Order",
        new XElement("OrderID", "y1"),
        new XElement("OrderTotal", "y2"),
        new XElement("Customer", "y3"),
        new XElement("Phone", "y4")));
    await FileIO.WriteTextAsync(file, xDocument.ToString());
}

Answer 2

虽然您必须阅读整个 Xml 才能添加新节点，但无论性能如何，更简洁的方法是使用

StorageFile file = await Windows.Storage.ApplicationData.Current.LocalFolder.CreateFileAsync("Test.dat", CreationCollisionOption.OpenIfExists);
var info = await file.GetBasicPropertiesAsync();
XDocument xDocument = null;
if(info.Size > 0)
    /* read existing file here */
else
    /* create an empty document here */
XElement root = xDocument.Element("Root");
IEnumerable<XElement> rows = root.Descendants("Order");
XElement firstRow = rows.First();
firstRow.AddBeforeSelf(
    new XElement("Order",
    new XElement("OrderID", "y1"),
    new XElement("OrderTotal", "y2"),
    new XElement("Customer", "y3"),
    new XElement("Phone", "y4")));
await FileIO.WriteTextAsync(file, xDocument.ToString());

这样你就没有代码重复了。

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