Reading Xml with XmlReader in C

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    lt ApplicationPool gt       lt Accounts gt           lt Account gt               lt NameOfKin gt  lt  NameOfKin gt               lt StatementsAvailable gt                   lt Statement gt  lt  Statement gt               lt  StatementsAvailable gt           lt  Account gt       lt  Accounts gt   lt  ApplicationPool gt    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

User · Answer

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 lt Account gt  accounts   new List lt Account gt         public List lt Account gt  Accounts  get return accounts      public class Account       public string NameOfKin  get set       private readonly List lt Statement gt  statements   new List lt Statement gt         public List lt Statement gt  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

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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  lt Account gt  element  after parsing the reader will index the  lt  Accounts gt  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  lt StatementsAvailable gt  node  public class Statements       List lt Statement gt   statements   new List lt Statement gt          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

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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                  lt  xml version  1 0   gt               lt  -- This is a sample XML document -- gt               lt Items gt                 lt Item gt test with a child element  lt more  gt  stuff lt  Item gt               lt  Items gt           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          lt bookstore gt           lt book genre  autobiography  publicationdate  1981-03-22  ISBN  1-861003-11-0  gt               lt title gt The Autobiography of Benjamin Franklin lt  title gt               lt author gt                   lt first-name gt Benjamin lt  first-name gt                   lt last-name gt Franklin lt  last-name gt               lt  author gt               lt price gt 8 99 lt  price gt           lt  book gt       lt  bookstore gt        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

User · Answer

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 lt XElement gt  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  amp  amp  reader ReadState    ReadState Interactive                   corrected for bug noted by Wes below            if reader NodeType    XmlNodeType Element  amp  amp  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

User · Answer

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 lt XElement gt  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

User · Answer

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

User · Answer

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

[c#] Reading Xml with XmlReader in C#

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