在 C# 中使用 XmlReader 读取 Xml

Question

I'm trying to read the following Xml document as fast as I can and let additional classes manage the reading of each sub block.

<ApplicationPool>
    <Accounts>
        <Account>
            <NameOfKin></NameOfKin>
            <StatementsAvailable>
                <Statement></Statement>
            </StatementsAvailable>
        </Account>
    </Accounts>
</ApplicationPool>

However, I'm trying to use the XmlReader object to read each Account and subsequently the "StatementsAvailable". Do you suggest using XmlReader.Read and check each element and handle it?

I've thought of seperating my classes to handle each node properly. So theres an AccountBase class that accepts a XmlReader instance that reads the NameOfKin and several other properties about the account. Then I was wanting to interate through the Statements and let another class fill itself out about the Statement (and subsequently add it to an IList).

Thus far I have the "per class" part done by doing XmlReader.ReadElementString() but I can't workout how to tell the pointer to move to the StatementsAvailable element and let me iterate through them and let another class read each of those proeprties.

Sounds easy!

Answer 1

My experience of XmlReader is that it's very easy to accidentally read too much. I know you've said you want to read it as quickly as possible, but have you tried using a DOM model instead? I've found that LINQ to XML makes XML work much much easier.

If your document is particularly huge, you can combine XmlReader and LINQ to XML by creating an XElement from an XmlReader for each of your "outer" elements in a streaming manner: this lets you do most of the conversion work in LINQ to XML, but still only need a small portion of the document in memory at any one time. Here's some sample code (adapted slightly from this blog post ):

static IEnumerable<XElement> SimpleStreamAxis(string inputUrl,
                                              string elementName)
{
  using (XmlReader reader = XmlReader.Create(inputUrl))
  {
    reader.MoveToContent();
    while (reader.Read())
    {
      if (reader.NodeType == XmlNodeType.Element)
      {
        if (reader.Name == elementName)
        {
          XElement el = XNode.ReadFrom(reader) as XElement;
          if (el != null)
          {
            yield return el;
          }
        }
      }
    }
  }
}

I've used this to convert the StackOverflow user data (which is enormous) into another format before - it works very well.

EDIT from radarbob, reformatted by Jon - although it's not quite clear which "read too far" problem is being referred to...

This should simplify the nesting and take care of the "a read too far" problem.

using (XmlReader reader = XmlReader.Create(inputUrl))
{
    reader.ReadStartElement("theRootElement");

    while (reader.Name == "TheNodeIWant")
    {
        XElement el = (XElement) XNode.ReadFrom(reader);
    }

    reader.ReadEndElement();
}

This takes care of "a read too far" problem because it implements the classic while loop pattern:

initial read;
(while "we're not at the end") {
    do stuff;
    read;
}

Answer 2

Three years later, perhaps with the renewed emphasis on WebApi and xml data, I came across this question. Since codewise I am inclined to follow Skeet out of an airplane without a parachute, and seeing his initial code doubly corraborated by the MS Xml team article as well as an example in BOL Streaming Transform of Large Xml Docs , I very quickly overlooked the other comments, most specifically from 'pbz', who pointed out that if you have the same elements by name in succession, every other one is skipped because of the double read. And in fact, the BOL and MS blog articles both were parsing source documents with target elements nested deeper than second level, masking this side-effect.

The other answers address this problem. I just wanted to offer a slightly simpler revision that seems to work well so far, and takes into account that the xml might come from different sources, not just a uri, and so the extension works on the user managed XmlReader. The one assumption is that the reader is in its initial state, since otherwise the first 'Read()' might advance past a desired node:

public static IEnumerable<XElement> ElementsNamed(this XmlReader reader, string elementName)
{
    reader.MoveToContent(); // will not advance reader if already on a content node; if successful, ReadState is Interactive
    reader.Read();          // this is needed, even with MoveToContent and ReadState.Interactive
    while(!reader.EOF && reader.ReadState == ReadState.Interactive)
    {
        // corrected for bug noted by Wes below...
        if(reader.NodeType == XmlNodeType.Element && reader.Name.Equals(elementName))
        {
             // this advances the reader...so it's either XNode.ReadFrom() or reader.Read(), but not both
             var matchedElement = XNode.ReadFrom(reader) as XElement;
             if(matchedElement != null)
                 yield return matchedElement;
        }
        else
            reader.Read();
    }
}

Answer 3

We do this kind of XML parsing all the time. The key is defining where the parsing method will leave the reader on exit. If you always leave the reader on the next element following the element that was first read then you can safely and predictably read in the XML stream. So if the reader is currently indexing the <Account> element, after parsing the reader will index the </Accounts> closing tag.

The parsing code looks something like this:

public class Account
{
    string _accountId;
    string _nameOfKin;
    Statements _statmentsAvailable;

    public void ReadFromXml( XmlReader reader )
    {
        reader.MoveToContent();

        // Read node attributes
        _accountId = reader.GetAttribute( "accountId" );
        ...

        if( reader.IsEmptyElement ) { reader.Read(); return; }

        reader.Read();
        while( ! reader.EOF )
        {
            if( reader.IsStartElement() )
            {
                switch( reader.Name )
                {
                    // Read element for a property of this class
                    case "NameOfKin":
                        _nameOfKin = reader.ReadElementContentAsString();
                        break;

                    // Starting sub-list
                case "StatementsAvailable":
                    _statementsAvailable = new Statements();
                    _statementsAvailable.Read( reader );
                    break;

                    default:
                        reader.Skip();
                }
            }
            else
            {
                reader.Read();
                break;
            }
        }       
    }
}

The Statements class just reads in the <StatementsAvailable> node

public class Statements
{
    List<Statement> _statements = new List<Statement>();

    public void ReadFromXml( XmlReader reader )
    {
        reader.MoveToContent();
        if( reader.IsEmptyElement ) { reader.Read(); return; }

        reader.Read();
        while( ! reader.EOF )
        {
            if( reader.IsStartElement() )
            {
                if( reader.Name == "Statement" )
                {
                    var statement = new Statement();
                    statement.ReadFromXml( reader );
                    _statements.Add( statement );               
                }
                else
                {
                    reader.Skip();
                }
            }
            else
            {
                reader.Read();
                break;
            }
        }
    }
}

The Statement class would look very much the same

public class Statement
{
    string _satementId;

    public void ReadFromXml( XmlReader reader )
    {
        reader.MoveToContent();

        // Read noe attributes
        _statementId = reader.GetAttribute( "statementId" );
        ...

        if( reader.IsEmptyElement ) { reader.Read(); return; }

        reader.Read();
        while( ! reader.EOF )
        {           
            ....same basic loop
        }       
    }
}

Answer 4

For sub-objects, ReadSubtree() gives you an xml-reader limited to the sub-objects, but I really think that you are doing this the hard way. Unless you have very specific requirements for handling unusual / unpredicatable xml, use XmlSerializer (perhaps coupled with sgen.exe if you really want).

XmlReader is... tricky. Contrast to:

using System;
using System.Collections.Generic;
using System.Xml.Serialization;
public class ApplicationPool {
    private readonly List<Account> accounts = new List<Account>();
    public List<Account> Accounts {get{return accounts;}}
}
public class Account {
    public string NameOfKin {get;set;}
    private readonly List<Statement> statements = new List<Statement>();
    public List<Statement> StatementsAvailable {get{return statements;}}
}
public class Statement {}
static class Program {
    static void Main() {
        XmlSerializer ser = new XmlSerializer(typeof(ApplicationPool));
        ser.Serialize(Console.Out, new ApplicationPool {
            Accounts = { new Account { NameOfKin = "Fred",
                StatementsAvailable = { new Statement {}, new Statement {}}}}
        });
    }
}

Answer 5

The following example navigates through the stream to determine the current node type, and then uses XmlWriter to output the XmlReader content.

    StringBuilder output = new StringBuilder();

    String xmlString =
            @"<?xml version='1.0'?>
            <!-- This is a sample XML document -->
            <Items>
              <Item>test with a child element <more/> stuff</Item>
            </Items>";
    // Create an XmlReader
    using (XmlReader reader = XmlReader.Create(new StringReader(xmlString)))
    {
        XmlWriterSettings ws = new XmlWriterSettings();
        ws.Indent = true;
        using (XmlWriter writer = XmlWriter.Create(output, ws))
        {

            // Parse the file and display each of the nodes.
            while (reader.Read())
            {
                switch (reader.NodeType)
                {
                    case XmlNodeType.Element:
                        writer.WriteStartElement(reader.Name);
                        break;
                    case XmlNodeType.Text:
                        writer.WriteString(reader.Value);
                        break;
                    case XmlNodeType.XmlDeclaration:
                    case XmlNodeType.ProcessingInstruction:
                        writer.WriteProcessingInstruction(reader.Name, reader.Value);
                        break;
                    case XmlNodeType.Comment:
                        writer.WriteComment(reader.Value);
                        break;
                    case XmlNodeType.EndElement:
                        writer.WriteFullEndElement();
                        break;
                }
            }

        }
    }
    OutputTextBlock.Text = output.ToString();

The following example uses the XmlReader methods to read the content of elements and attributes.

StringBuilder output = new StringBuilder();

String xmlString =
    @"<bookstore>
        <book genre='autobiography' publicationdate='1981-03-22' ISBN='1-861003-11-0'>
            <title>The Autobiography of Benjamin Franklin</title>
            <author>
                <first-name>Benjamin</first-name>
                <last-name>Franklin</last-name>
            </author>
            <price>8.99</price>
        </book>
    </bookstore>";

// Create an XmlReader
using (XmlReader reader = XmlReader.Create(new StringReader(xmlString)))
{
    reader.ReadToFollowing("book");
    reader.MoveToFirstAttribute();
    string genre = reader.Value;
    output.AppendLine("The genre value: " + genre);

    reader.ReadToFollowing("title");
    output.AppendLine("Content of the title element: " + reader.ReadElementContentAsString());
}

OutputTextBlock.Text = output.ToString();

Answer 6

    XmlDataDocument xmldoc = new XmlDataDocument();
    XmlNodeList xmlnode ;
    int i = 0;
    string str = null;
    FileStream fs = new FileStream("product.xml", FileMode.Open, FileAccess.Read);
    xmldoc.Load(fs);
    xmlnode = xmldoc.GetElementsByTagName("Product");

You can loop through xmlnode and get the data...... C# XML Reader

Answer 7

I am not experiented .But i think XmlReader is unnecessary. It is very hard to use.
XElement is very easy to use.
If you need performance ( faster ) you must change file format and use StreamReader and StreamWriter classes.