我在 XmlWriter 和 Linq2Xml 的帮助下制作了一些巨大的 XML 文件(几 GB)。 此文件的类型:

<Table recCount="" recLength="">
<Rec recId="1">..</Rec>
<Rec recId="2">..</Rec>
..
<Rec recId="n">..</Rec>
</Table>

我不知道 Table 的 recCount 和 recLength 属性的值，直到我写下所有内部 Rec节点，所以我必须在最后为这些属性写入值。

现在我正在将所有内部 Rec 节点写入一个临时文件，计算 Table 的属性值并按照上面显示的方式写入所有内容到结果文件。 (从具有所有 Rec 节点的临时文件中复制所有内容)

我想知道是否有一种方法可以修改这些属性的值而无需将内容写入另一个文件(就像我现在这样做)或将整个文档加载到内存中(这显然是不可能的，因为这些文件的大小)？

最佳答案

大量注释的代码。基本思想是在第一遍中我们写:

<?xml version="1.0" encoding="utf-8"?>
<Table recCount="$1" recLength="$2">
<!--Reserved space:++++++++++++++++-->
<Rec...

然后我们回到文件开头重写前三行:

<?xml version="1.0" encoding="utf-8"?>
<Table recCount="1000" recLength="150">
<!--Reserved space:#############-->

这里重要的“技巧”是你不能“插入”到一个文件中，你只能覆盖它。所以我们为数字“预留”了一些空间(Reserved space:#############. 注释。我们可以通过多种方式完成它...例如，在第一遍中我们可以:

<Table recCount="              " recLength="          ">

然后(xml 合法但丑陋):

<Table recCount="1000          " recLength="150       ">

或者我们可以在 > 之后 添加空格表的:

<Table recCount="" recLength="">                   

(> 之后 有 20 个空格)

然后:

<Table recCount="1000" recLength="150">            

(现在在 > 之后 有 13 个空格)

或者我们可以简单地添加空格而不使用 <!-- -->在一条新线上...

代码:

int maxRecCountLength = 10; // int.MaxValue.ToString().Length
int maxRecLengthLength = 10; // int.MaxValue.ToString().Length
int tokenLength = 4; // 4 == $1 + $2, see below what $1 and $2 are
// Note that the reserved space will be in the form +++++++++++++++++++

string reservedSpace = new string('+', maxRecCountLength + maxRecLengthLength - tokenLength); 

// You have to manually open the FileStream
using (var fs = new FileStream("out.xml", FileMode.Create))

// and add a StreamWriter on top of it
using (var sw = new StreamWriter(fs, Encoding.UTF8, 4096, true))
{
    // Here you write on your StreamWriter however you want.
    // Note that recCount and recLength have a placeholder $1 and $2.
    int recCount = 0;
    int maxRecLength = 0;

    using (var xw = XmlWriter.Create(sw))
    {
        xw.WriteWhitespace("\r\n");
        xw.WriteStartElement("Table");
        xw.WriteAttributeString("recCount", "$1");
        xw.WriteAttributeString("recLength", "$2");

        // You have to add some white space that will be 
        // partially replaced by the recCount and recLength value
        xw.WriteWhitespace("\r\n");
        xw.WriteComment("Reserved space:" + reservedSpace);

        // <--------- BEGIN YOUR CODE
        for (int i = 0; i < 100; i++)
        {
            xw.WriteWhitespace("\r\n");
            xw.WriteStartElement("Rec");

            string str = string.Format("Some number: {0}", i);
            if (str.Length > maxRecLength)
            {
                maxRecLength = str.Length;
            }
            xw.WriteValue(str);

            recCount++;

            xw.WriteEndElement();
        }
        // <--------- END YOUR CODE

        xw.WriteWhitespace("\r\n");
        xw.WriteEndElement();
    }

    sw.Flush();

    // Now we read the first lines to modify them (normally we will
    // read three lines, the xml header, the <Table element and the
    // <-- Reserved space:
    fs.Position = 0;

    var lines = new List<string>();

    using (var sr = new StreamReader(fs, sw.Encoding, false, 4096, true))
    {
        while (true)
        {
            string str = sr.ReadLine();
            lines.Add(str);

            if (str.StartsWith("<Table"))
            {
                // We read the next line, the comment line
                str = sr.ReadLine();
                lines.Add(str);
                break;
            }
        }
    }

    string strCount = XmlConvert.ToString(recCount);
    string strMaxRecLength = XmlConvert.ToString(maxRecLength);

    // We do some replaces for the tokens
    int oldLen = lines[lines.Count - 2].Length;
    lines[lines.Count - 2] = lines[lines.Count - 2].Replace("=\"$1\"", string.Format("=\"{0}\"", strCount));
    lines[lines.Count - 2] = lines[lines.Count - 2].Replace("=\"$2\"", string.Format("=\"{0}\"", strMaxRecLength));
    int newLen = lines[lines.Count - 2].Length;

    // Remove spaces from reserved whitespace
    lines[lines.Count - 1] = lines[lines.Count - 1].Replace(":" + reservedSpace, ":" + new string('#', reservedSpace.Length - newLen + oldLen));

    // We move back to just after the UTF8/UTF16 preamble
    fs.Position = sw.Encoding.GetPreamble().Length;

    // And we rewrite the lines
    foreach (string str in lines)
    {
        sw.Write(str);
        sw.Write("\r\n");
    }
}

较慢的 .NET 3.5 方式

在 .NET 3.5 中 StreamReader/StreamWriter想关基地FileStream ，所以我不得不多次重新打开文件。这有点慢。

int maxRecCountLength = 10; // int.MaxValue.ToString().Length
int maxRecLengthLength = 10; // int.MaxValue.ToString().Length
int tokenLength = 4; // 4 == $1 + $2, see below what $1 and $2 are
                        // Note that the reserved space will be in the form +++++++++++++++++++

string reservedSpace = new string('+', maxRecCountLength + maxRecLengthLength - tokenLength);
string fileName = "out.xml";

int recCount = 0;
int maxRecLength = 0;

using (var sw = new StreamWriter(fileName))
{
    // Here you write on your StreamWriter however you want.
    // Note that recCount and recLength have a placeholder $1 and $2.
    using (var xw = XmlWriter.Create(sw))
    {
        xw.WriteWhitespace("\r\n");
        xw.WriteStartElement("Table");
        xw.WriteAttributeString("recCount", "$1");
        xw.WriteAttributeString("recLength", "$2");

        // You have to add some white space that will be 
        // partially replaced by the recCount and recLength value
        xw.WriteWhitespace("\r\n");
        xw.WriteComment("Reserved space:" + reservedSpace);

        // <--------- BEGIN YOUR CODE
        for (int i = 0; i < 100; i++)
        {
            xw.WriteWhitespace("\r\n");
            xw.WriteStartElement("Rec");

            string str = string.Format("Some number: {0}", i);
            if (str.Length > maxRecLength)
            {
                maxRecLength = str.Length;
            }
            xw.WriteValue(str);

            recCount++;

            xw.WriteEndElement();
        }
        // <--------- END YOUR CODE

        xw.WriteWhitespace("\r\n");
        xw.WriteEndElement();
    }
}

var lines = new List<string>();

using (var sr = new StreamReader(fileName))
{
    // Now we read the first lines to modify them (normally we will
    // read three lines, the xml header, the <Table element and the
    // <-- Reserved space:

    while (true)
    {
        string str = sr.ReadLine();
        lines.Add(str);

        if (str.StartsWith("<Table"))
        {
            // We read the next line, the comment line
            str = sr.ReadLine();
            lines.Add(str);
            break;
        }
    }
}

// We have to use the Stream overload of StreamWriter because
// we want to modify the text!
using (var fs = File.OpenWrite(fileName))
using (var sw = new StreamWriter(fs))
{
    string strCount = XmlConvert.ToString(recCount);
    string strMaxRecLength = XmlConvert.ToString(maxRecLength);

    // We do some replaces for the tokens
    int oldLen = lines[lines.Count - 2].Length;
    lines[lines.Count - 2] = lines[lines.Count - 2].Replace("=\"$1\"", string.Format("=\"{0}\"", strCount));
    lines[lines.Count - 2] = lines[lines.Count - 2].Replace("=\"$2\"", string.Format("=\"{0}\"", strMaxRecLength));
    int newLen = lines[lines.Count - 2].Length;

    // Remove spaces from reserved whitespace
    lines[lines.Count - 1] = lines[lines.Count - 1].Replace(":" + reservedSpace, ":" + new string('#', reservedSpace.Length - newLen + oldLen));

    // We move back to just after the UTF8/UTF16 preamble
    sw.BaseStream.Position = sw.Encoding.GetPreamble().Length;

    // And we rewrite the lines
    foreach (string str in lines)
    {
        sw.Write(str);
        sw.Write("\r\n");
    }
}

关于c# - (C#) 如何在不加载或重写整个文件的情况下修改现有 XML 文件中的属性值？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/43186834/