Question

我一直将.png文件存储到XML元素之一，如下所示。我一直在将图像文件转换为base64，然后将该字符串复制到XML。我也能读取。将这个XML文件加载到我的窗体中。我面临的问题是随着XML文件越来越多，节点越来越多，XML文件大小变得越来越大，现在它已经变成了300 MB。当Windows窗体试图读取这个大型XML文件时，我得到OutOfMemoryExceptions。下面是我的xml文件的片段。

<TestResult>
    <ResultsID>49</ResultsID>
    <DateExecuted>2018-02-20T09:36:12.787</DateExecuted>
    <UserExecuted>xxx</UserExecuted>
    <CorrectedMean>1966.32245</CorrectedMean>
    <CorrectedVariance>19525.6632019949</CorrectedVariance>
    <TestPassed>true</TestPassed>
    <TestResultImage>Qk2.......</TestResultImage>
</TestResult>

我一直在尝试使用以下代码

将XML加载到.net中

XDocument xmlResultsDoc = XDocument.Load("MeanData.xml");

并存储到我的模型类中，如下所示。

List<MeanVarianceTestResultDataList = 
(xmlResultsDoc.Descendants("TestResult").Select(m => new 
MeanVarianceTestResultsData()
                    {
                        ResultsID = 
Convert.ToInt32(m.Element("ResultsID").Value),
                        DateExecuted = 
Convert.ToDateTime(m.Element("DateExecuted").Value),
                        UserExecuted = 
Convert.ToString(m.Element("UserExecuted").Value),
                        CorrectedMean = 
Convert.ToString(m.Element("CorrectedMean").Value),
                        CorrectedVariance = 
Convert.ToString(m.Element("CorrectedVariance").Value),
                        TestPassed = 
Convert.ToBoolean(m.Element("TestPassed").Value),
                        TestResultImage =  

Convert.FromBase64String(
Convert.ToString(m.Element("TestResultImage").Value))
                    })).ToList();

Answer 1

对于大型xml文件，您必须使用XmlReader来防止内存不足错误。请尝试下面的代码，它使用xml linq和XmlReader

的组合

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        const string FILENAME = @"c:\temp\test.xml";
        static void Main(string[] args)
        {
            XmlReader reader = XmlReader.Create(FILENAME);

            while (!reader.EOF)
            {
                if (reader.Name != "TestResult")
                {
                    reader.ReadToFollowing("TestResult");
                }
                if (!reader.EOF)
                {
                    XElement testResult = (XElement)XElement.ReadFrom(reader);
                    string image = (string)testResult.Element("TestResultImage"); 
                }
            }
        }
    }
}

Answer 2

如果您的XML文件太大而无法一次加载到内存中，则可以使用XmlReader来流式传输文件，并一次只加载一小部分。此外，对于包含非常大的Base64编码二进制数据的<TestResultImage>元素，您可以使用XmlReader.ReadElementContentAsBase64(Byte[], Int32, Int32)以增量方式读取数据块并将其复制到某些Stream。

以下代码显示了如何完成此任务：

//https://stackoverflow.com/questions/49159697/deserializing-json-child-object-values-into-parent-object-using-jsonconvert-dese

public class MeanVarianceTestResultsData
{
    public int ResultsID { get; set; }
    public DateTime DateExecuted { get; set; }
    public string UserExecuted { get; set; }
    public string CorrectedMean { get; set; }
    public string CorrectedVariance { get; set; }
    public bool TestPassed { get; set; }

    public string TestResultImageFile { get; set; }
    public Stream TestResultImage { get; set; }
}

public static class MeanVarianceTestResultsDataExtensions
{
    public static List<MeanVarianceTestResultsData> ReadResultListFrom(XmlReader reader, Func<MeanVarianceTestResultsData, Stream> openStream, Func<Stream, Stream> closeStream)
    {
        return reader.ReadSubtrees("TestResult").Select(r => ReadResultFrom(r, openStream, closeStream)).ToList();
    }

    public static MeanVarianceTestResultsData ReadResultFrom(XmlReader reader, Func<MeanVarianceTestResultsData, Stream> openStream, Func<Stream, Stream> closeStream)
    {
        if (reader == null || openStream == null)
            throw new ArgumentNullException();
        reader.MoveToContent();
        var result = new MeanVarianceTestResultsData();
        var isEmpty = reader.IsEmptyElement;
        // Read the root
        reader.Read();
        if (isEmpty)
            return result;
        while (!reader.EOF)
        {
            if (reader.NodeType == XmlNodeType.EndElement)
            {
                reader.Read();
                break;
            }
            else if (reader.NodeType != XmlNodeType.Element)
                // Comment, text, CDATA, etc.
                reader.Skip();
            else if (reader.Name == "ResultsID")
                result.ResultsID = reader.ReadElementContentAsInt();
            else if (reader.Name == "DateExecuted")
                result.DateExecuted = reader.ReadElementContentAsDateTime();
            else if (reader.Name == "UserExecuted")
                result.UserExecuted = reader.ReadElementContentAsString();
            else if (reader.Name == "CorrectedMean")
                result.CorrectedMean = reader.ReadElementContentAsString();
            else if (reader.Name == "TestPassed")
                result.TestPassed = reader.ReadElementContentAsBoolean();
            else if (reader.Name == "TestResultImage")
                result.TestResultImage = reader.ReadElementContentAsStream(() => openStream(result), closeStream);
            else
                reader.Skip();
        }
        return result;
    }
}

public static class XmlReaderExtensions
{
    public static Stream ReadElementContentAsStream(this XmlReader reader, Func<Stream> openStream, Func<Stream, Stream> closeStream)
    {
        if (reader == null || openStream == null)
            throw new ArgumentNullException();
        Stream stream = null;
        try
        {
            stream = openStream();
            byte[] buffer = new byte[4096];
            int readBytes = 0;
            while ((readBytes = reader.ReadElementContentAsBase64(buffer, 0, buffer.Length)) > 0)
            {
                stream.Write(buffer, 0, readBytes);
            }
        }
        finally
        {
            if (closeStream != null && stream != null)
                stream = closeStream(stream);
        }
        return stream;
    }

    public static IEnumerable<XmlReader> ReadSubtrees(this XmlReader reader, string name)
    {
        while (reader.ReadToFollowing(name))
        {
            using (var subReader = reader.ReadSubtree())
                yield return subReader;
        }
    }
}

然后，您可以按如下方式使用它将每个TestResultImage图片读入MemoryStream：

List<MeanVarianceTestResultsData> results;
using (var reader = XmlReader.Create(fileName))
{
    results = MeanVarianceTestResultsDataExtensions.ReadResultListFrom(reader, m => new MemoryStream(), s => { s.Position = 0; return s; });
}

这将通过完全跳过图像的中间Base64字符串表示来节省大量内存 - 但它仍然会为每个MemoryStream使用相当多的内存。或者，您可以将图像流式传输到一些临时文件中供以后使用，例如：通过执行以下操作：

List<MeanVarianceTestResultsData> results;
using (var reader = XmlReader.Create(fileName))
{
    results = MeanVarianceTestResultsDataExtensions.ReadResultListFrom(
        reader,
        m => { m.TestResultImageFile = Path.GetTempFileName(); return File.Open(m.TestResultImageFile, FileMode.Create); },
        s => { s.Dispose(); return null; });
}

在这种情况下，每个流在写入图像后处理，文件名存储在MeanVarianceTestResultsData中。（当然，如果您计划在反序列化后立即处理它们，您可以将流保持打开状态。）

示例fiddle。

在C＃窗体中读取大型XML文件时出现OutofMemoryException

2 个答案: