(C#) 如何修改现有 XML 文件中的属性值而不加载或重写整个文件？

我正在 XmlWriter 和 Linq2Xml 的帮助下制作一些巨大的 XML 文件（几个 GB）。 该文件的类型为：

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

我不知道的价值观Table's recCount and 记录长度属性，直到我写完所有内部Rec节点，所以我必须在最后向这些属性写入值。

现在我正在写所有的内心Rec节点到临时文件，计算Table的属性值，并将所有内容按照我上面显示的方式写入结果文件。 （从临时文件中复制所有内容Rec nodes)

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（就像我现在所做的那样）或将整个文档加载到内存中（由于这些文件的大小，这显然是不可能的）？

大量注释的代码。基本思想是在第一遍中我们编写：

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

然后我们回到文件的开头并重写前三行：

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

这里重要的“技巧”是您不能“插入”文件，只能覆盖它。所以我们为数字“保留”一些空间（Reserved space:#############.评论。我们可以有很多方法来做到这一点......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       ">

或者我们可以附加空格after the >表的：

<Table recCount="" recLength="">                   

（有20个空格after the >)

Then:

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

（现在还有13个位子after the >)

或者我们可以简单地添加空格而不用<!-- -->在新线上...

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想要关闭基地FileStream，所以我必须多次重新打开该文件。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");
    }
}