写入 Filestream 并复制到 MemoryStream

Question

我想在磁盘上覆盖或创建一个 xml 文件，并从函数中返回 xml。我想我可以通过从 FileStream 复制到 MemoryStream 来做到这一点。但我最终将一个新的 xml 文档附加到同一个文件中，而不是每次都创建一个新文件。 我究竟做错了什么？如果我删除复制，一切正常。

 public static string CreateAndSave(IEnumerable<OrderPage> orderPages, string filePath)
    {
        if (orderPages == null || !orderPages.Any())
        {
            return string.Empty;
        }

        var xmlBuilder = new StringBuilder();

        var writerSettings = new XmlWriterSettings
        {
            Indent = true,
            Encoding = Encoding.GetEncoding("ISO-8859-1"),
            CheckCharacters = false,
            ConformanceLevel = ConformanceLevel.Document
        };

        using (var fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite))
        {
            try
            {
                XmlWriter xmlWriter = XmlWriter.Create(fs, writerSettings);
                xmlWriter.WriteStartElement("PRINT_JOB");
                WriteXmlAttribute(xmlWriter, "TYPE", "Order Confirmations");

                foreach (var page in orderPages)
                {
                    xmlWriter.WriteStartElement("PAGE");
                    WriteXmlAttribute(xmlWriter, "FORM_TYPE", page.OrderType);

                    var outBound = page.Orders.SingleOrDefault(x => x.FlightInfo.Direction == FlightDirection.Outbound);
                    var homeBound = page.Orders.SingleOrDefault(x => x.FlightInfo.Direction == FlightDirection.Homebound);

                    WriteXmlOrder(xmlWriter, outBound, page.ContailDetails, page.UserId, page.PrintType, FlightDirection.Outbound);
                    WriteXmlOrder(xmlWriter, homeBound, page.ContailDetails, page.UserId, page.PrintType, FlightDirection.Homebound);

                    xmlWriter.WriteEndElement();
                }

                xmlWriter.WriteFullEndElement();

                MemoryStream destination = new MemoryStream();
                fs.CopyTo(destination);

                Log.Progress("Xml string length: {0}", destination.Length);

                xmlBuilder.Append(Encoding.UTF8.GetString(destination.ToArray()));

                destination.Flush();
                destination.Close();
                xmlWriter.Flush();
                xmlWriter.Close();
            }
            catch (Exception ex)
            {
                Log.Warning(ex, "Unhandled exception occured during create of xml. {0}", ex.Message);
                throw;
            }

            fs.Flush();
            fs.Close();
        }

        return xmlBuilder.ToString();
    }

干杯 延斯

Answer 1

FileMode.OpenOrCreate 导致文件内容被覆盖而不缩短，留下之前运行的任何“尾随”数据。如果使用FileMode.Create，文件将首先被截断。但是，要读回您刚刚编写的内容，您需要使用Seek 来重置文件指针。

另外，在从底层流复制之前刷新XmlWriter。

另见问题Simultaneous Read Write a file in C Sharp (3817477)。

以下测试程序似乎可以满足您的需求（减去您自己的日志记录和订单详细信息）。

using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Threading.Tasks;

namespace ReadWriteTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string filePath = Path.Combine(
                Environment.GetFolderPath(Environment.SpecialFolder.Personal),
                "Test.xml");

            string result = CreateAndSave(new string[] { "Hello", "World", "!" }, filePath);

            Console.WriteLine("============== FIRST PASS ==============");
            Console.WriteLine(result);

            result = CreateAndSave(new string[] { "Hello", "World", "AGAIN", "!" }, filePath);
            Console.WriteLine("============== SECOND PASS ==============");
            Console.WriteLine(result);

            Console.ReadLine();
        }

        public static string CreateAndSave(IEnumerable<string> orderPages, string filePath)
        {
            if (orderPages == null || !orderPages.Any())
            {
                return string.Empty;
            }

            var xmlBuilder = new StringBuilder();

            var writerSettings = new XmlWriterSettings
            {
                Indent = true,
                Encoding = Encoding.GetEncoding("ISO-8859-1"),
                CheckCharacters = false,
                ConformanceLevel = ConformanceLevel.Document
            };

            using (var fs = new FileStream(filePath, FileMode.Create, FileAccess.ReadWrite))
            {
                try
                {
                    XmlWriter xmlWriter = XmlWriter.Create(fs, writerSettings);
                    xmlWriter.WriteStartElement("PRINT_JOB");

                    foreach (var page in orderPages)
                    {
                        xmlWriter.WriteElementString("PAGE", page);
                    }

                    xmlWriter.WriteFullEndElement();

                    xmlWriter.Flush();  // Flush from xmlWriter to fs
                    xmlWriter.Close();

                    fs.Seek(0, SeekOrigin.Begin); // Go back to read from the begining

                    MemoryStream destination = new MemoryStream();
                    fs.CopyTo(destination);

                    xmlBuilder.Append(Encoding.UTF8.GetString(destination.ToArray()));

                    destination.Flush();
                    destination.Close();
                }
                catch (Exception ex)
                {
                    throw;
                }

                fs.Flush();
                fs.Close();
            }

            return xmlBuilder.ToString();
        }
    }
}

对于那里的优化器，StringBuilder 是不必要的，因为字符串是完整的，只需将 fs 包装在StreamReader 中就可以避免MemoryStream。这将使代码如下。

        public static string CreateAndSave(IEnumerable<string> orderPages, string filePath)
        {
            if (orderPages == null || !orderPages.Any())
            {
                return string.Empty;
            }

            string result;

            var writerSettings = new XmlWriterSettings
            {
                Indent = true,
                Encoding = Encoding.GetEncoding("ISO-8859-1"),
                CheckCharacters = false,
                ConformanceLevel = ConformanceLevel.Document
            };

            using (var fs = new FileStream(filePath, FileMode.Create, FileAccess.ReadWrite))
            {
                try
                {
                    XmlWriter xmlWriter = XmlWriter.Create(fs, writerSettings);
                    xmlWriter.WriteStartElement("PRINT_JOB");

                    foreach (var page in orderPages)
                    {
                        xmlWriter.WriteElementString("PAGE", page);
                    }

                    xmlWriter.WriteFullEndElement();

                    xmlWriter.Close();  // Flush from xmlWriter to fs

                    fs.Seek(0, SeekOrigin.Begin); // Go back to read from the begining

                    var reader = new StreamReader(fs, writerSettings.Encoding);
                    result = reader.ReadToEnd();
                    // reader.Close();  // This would just flush/close fs early(which would be OK)
                }
                catch (Exception ex)
                {
                    throw;
                }
            }

            return result;
        }

Answer 2

我知道我迟到了，但似乎有一个更简单的解决方案。您希望您的函数生成 xml，将其写入文件并返回生成的 xml。显然分配 string 是不可避免的（因为你希望它被返回），写入文件也是如此。但是通过简单地“交换”操作可以轻松避免从文件中读取（如在您和 SensorSmith 的解决方案中） - 生成 xml 字符串并将其写入文件。像这样：

var output = new StringBuilder();
var writerSettings = new XmlWriterSettings { /* your settings ... */ };
using (var xmlWriter = XmlWriter.Create(output, writerSettings))
{
    // Your xml generation code using the writer
    // ...
    // You don't need to flush the writer, it will be done automatically
}
// Here the output variable contains the xml, let's take it...
var xml = output.ToString();
// write it to a file...
File.WriteAllText(filePath, xml);
// and we are done :-)
return xml;

重要更新： 原来XmlWriter.Create(StringBuider, XmlWriterSettings) 重载忽略了设置中的Encoding 并且总是使用"utf-16"，所以如果您需要其他编码，请勿使用此方法。