Linq to XML

1. Linq to XML
1. Components
1. XDocument
2. XName
3. XElement
4. XComment
5. XCData
6. XNode
7. XStreamingElement
8. XText
9. XObject
10. XContainer
2. How to change an XML?
3. XML Annotation
4. XML Events
5. Linq to XML Operators
1. Validation
6. How to obtain an XML Schema (XSD schema file (.xsd))?
7. XPath

• See also, Linq: Operators

• Functional construction: term used to describe the ability to construct an entire XML hierarchy in a single statement
• namespace System.Xml.Linq.
• Linq to XML API is very element centric (dif. from document centric)
  
• document centricity eliminated in favor of element centricity
  
• in W3C DOM: attributes, comments, CData, section, processing instruction, elements must be created from XmlDocument
• System.Xml.XmlConvert
• .ToBoolean()

• The only classes that could have nodes derived from XNode are XDocument and XElement
• Halloween problem:
• there is any problem that occurs by changing data that is being iterated over affect the iteration
• it can occur, for example, when removing elements from an XML tree
• solution: move elements that are removed in a cache list
• XDocument _doc = new XDocument();
  XmlWriter _writer = _doc.CreateWriter();
  XslCompiledTransform _transform = new XslCompiledTransfor();
• IEnumerable<XElement> _elements = new Element[]
  
  { new XElement("Element" , "Element A")
  , new XElement("Element" , "Element B") };
  XElement _root = new XElement("Root" ,
  

  _elements.Select(e => string(e) != "Element A" ? new Element(e.Name, (string)e) : null));
• null - it is used to do not create an XElement - ignore the iteration

Components

XDocument

• XML Documents are not a must anymore (to create attributes, comments, ... )
• You can read, save a doc without needing a Document object
• Namespaces are out
• inside, no longer exists
• IEnumerable<XElements> _members = xDocument.Element("Team StatViewer").Elements("Member");
• XDocument _doc = new XDocument(
  
  new XDeclaration("1.0", "UTF-8", "yes")
  , new XDocumentType("StatViewer", null, "StatViewer.dtd", null)
  , new XProcessingInstruction("Team StatViewer", "to be printed")
  , new XElement ( "Team StatViewer"));
• .Save()
• .SaveOptions.DisableFormatting
• .Load()

XName

• consist of
  
• namespace (XNamespace)
• local name (LocalName)
• there is no need to directly create a name
• has no public ctor
  
• XNamespace _namespace = "http://www.ixiacom.com";
• XElement _member = new XElement(_namespace + "Member");
  

XElement

• XElement _team = new XElement ( "Team StatViewer" 
                               , new XElement ("Name" , "George Lache")
                               , new XElement ("Position" , "Senior Dev"));
• _team.ToString();
• calling .ToString() method of an element outputs the XML string itself and not object type
• XElement _george = new XElement ("Name" , "George Lache");
  //"Name" is a XName
  //"George Lache is a XText
  Console.WriteLine(_george); // <Name>George Lache</Name>
  Console.WriteLine((string)_george); // George Lache - return a XText
  
• XElement _team = new XElement ( "Team StatViewer"
  
  ,  members.Select ( m =>
  

  new XElement("Member",
  

  new XAttribute("Position" , m.Position),
  new XElement("Name", m.Name)));
• .Add(XAttribute), .Add(XComment)
• .Save()
• .Load()
• .LoadOptions
• .Name
• .PreserveWhitespace
• .SetLineInfo //set line & position
• IXmlLineInfo
• ((IXmlLineInfo)name)
• .LineNumber
• .LinePosition
  
• SetBaseUri()
• .Parse()
• used for parsing XML string
• XElement.Parse(_xmlText);
• .AncestorsAndSelf();
• DescendantsAndSelf();

XComment

• creates comments
• XElement _member = ...;
  XComment _comm = new XComment("This is StatViewer team presentation");
  _member.Add(_comm);
  

XCData

• CData content
• XCData _cData = new XCData("<h1>Invalidate member</h1>"
  
  , "<elem>!CData[<h1> .... </h1>] </elem>")
  

XNode

• it is an abstract class - cannot be instantiated
• subclasses: XComment, XContainer, XDocumentType, XProcessingInstruction, XText
• .NextNode()
• traverse forward
• .PreviousNode()
• traverse barward
• .Ancestors()
• recursively traverses up the XML tree
• .NodesAfterSelf()
• traversing forward the current node
• .ElementsAfterSelf()
• .NodesBeforeSelf()
• .ElementsBeforeSelf()
  

XStreamingElement

• defers elecution of the XML tree construction
• when you create an XML tree using XStreamingElement, a reference to the query variable is stored in the XML tree without being iterated.
  
• string _names = {"George Lache", "Anca Suciu", "Marian Gheorghe"};
  XStreamingElement members = new XStreamingElement("Members",
  
  from n in _names
  select new XStreamingElement("Name", n));
  
  Console.WriteLine(_names[0]);

XText

• XText _text = new XText("George Lache");

XObject

• .Document
• obtains the XML Document
• .Parent
• .AddAnnotation()
• .Annotation()
• .Annotations()
• .RemoveAnnotations()
  

XContainer

• Represents a node that can contain other nodes.
  
• This class provides functionality such as finding the next or previous sibling node, or enumerating the direct children of a node.
  
• The two classes that derive from XContainer are XDocument and XElement
  
• .Nodes()
• _doc.Nodes().OfType<XElement>
• .Attributes()
• .Elements()
• accessing an element's child elements
• Element()
• _member.Element("Name");
  
• .Elements()
• .Descendants();
• .Add()
• .AddLast()
• .AddFirst()
  

How to change an XML?

• XElement
• .SetElementValue()
• .RemoveAll()
• deletes the content of an element, but not he element itself
• .Value
• also, for XText, XComment
• _member.Element("Name").Single().Name = "George";
• .ReplaceAll()
• replaces an element's entire subtree of XML
• SetElementValue()
• it is a method on a parent element to affect its content (child elements)
• if you pass null the child elements will be removed
• it will affect only the first child element that matches the specified name
• Sample:
  
  _member.SetElementValue("Name", "George V. Lache");
• it can be used for adding/updating/removing an element
• .Attribute()
• .Attributes()
• .Add()
• .AddFirst()
• .AddBeforeThis()
• .AddAfterThis()
• .AddAttributeValue()
  
• XContainer: .Add(), .AddLast(), .AddFirst()
• XNode:
  
• .AddBeforeSelf()
• inserts a node into a node's list of child nodes in a specific location
• .AddAfterSelf(),
  
• .Remove()
• Remove any node as well as its child nodes and its attributes from an XML tree
• _members.Descendants().Where(m => m.Name.Contains("George")).Remove();
• XDocumentType
• .Name
• update document type
• .PublicId // "http:// ... .DTD"
• .SystemId
  
• XProcessingInstruction
• Sample: new XProcessingInstruction("Team StatViewer", "to be printed")
• .Target //"Team StatViewer"
• .Data //"to be printed"
• XAttribude
• does not inherit from XNode
• .FirstAttribute()
• .LastAttribute()
• .NextAttribute()
• .PreviousAttibute()
• .Remove()
• IEnumerable<T>.Remove()
  
• _member.Attributes().Remove()
• to update:
  
• .Value
• .SetAttributeValue()
• also, used for updating and deleting

XML Annotation

• provides the ability to associate a user data object with any class inheriting from XObject class via annotations
• assigns whatever data type to an element, document, ...
• XObject
• .AddAnnotation()
• .Annotation()
• .Annotations()
• .RemoveAnnotations()
• Sample:
  MemberClass _memberClass = new MemberClass();
  _memberElement.AddAnnotation(_memberClass);
  _memberElement.GetAnnotation<MemberClass>();
  _memberElement.RemoveAnnotation<MemberClass>();
  
  

XML Events

• the events will be raised on the object if that object or any descendant object is changing
• XObject
• .Changing
• myXObj.Changing = new EventHandler<XObjectChangeEventArgs>(MyHandlingMethod);
• Changed
• XObjectChangeEventArgs
• .ObjectChange
• of type XObjectChange
• .Add
• .Name
• .Remove
• .Value
• !!! when you change the string value of an element, an XText object is removed and then added back

Linq to XML Operators

• Xml Operators
• are extension methods
• are defined in System.Xml.Linq.Extension class
• extension methods are somehow equivalent to operators
• Element
  
• IEnumerable<XElement> Element<T>
            (this IEnumerable<T> source)
  where T : XElement
• Ancestors
• IEnumerable<XElement> Ancestors<T>
             (this IEnumerable<T> source)
  
  [,Xname name)]
  
             where T : XNode
• _xDoc.Element("Members").Descendand("Name").Ancestors()
• AncestorsAndSelf
  
• it can be called on XElement
• as opposed to Ancestor which is called on Node
• Attributes
• retrives a sequence containint the attributes of each source element
• DescendantNodes
• return a sequence containing the descendant nodes if each element or document
• public static IEnumerable<Xnode> DescendingNodes<T>
                               (this IEnumerable<T> source)
                     where T : XContainer
• DescendantNodesAndSelf
• Descendants
• can be called on a sequence of elements or documents and return a sequence of elements
  
• DescendantsAndSelf
• Elements
• returns a sequence of elements containing each source element's or document's child element
• returns only the immediate child elements of each element in the source sequence
• InDocumentOrder
• returns a sequence containing each source node's child nodes in document order
• Nodes
• returns only the immediate child elements
• public static IEnumerable<Xnode> Nodes<T>
                               (this IEnumerable<T> source)
                     where T : XContainer
• Remove
  

Validation

• in system.Xml.Schema.Extension
• .Validate
• (this XDocument [or XElement or XAttribure] source,
  XmlSchemeSet schemas,
  ValidationEventHandler validationEH)
  [,bool addShemaInfo)]
• .GetSchemaInfo
• XmlSchemaValidationException

How to obtain an XML Schema (XSD schema file (.xsd))?

XmlSchemaInference inference = new XmlSchemaInference();
XmlSchemaSet schemaSet = inference.InferSchema ( new XmlTextReader ("StatViewer.xml") );
XmlWriter writer = XmlWriter.Create("StatViewer.xsd");
foreach(XmlSchema schema in schemaSet.Schemas)
{
    schema.Write(writer);
}

xDocument.Validate(schemaSet,
    (o , validExceptArgs ) =>
        {
            Console.WriteLine(o.GetType().Name +validExceptArgs.Message);
        });

XPath

• XPath allows you to create a query by assembling a number of expressions and predicates, each one acting on a node-set 
  
• XPathNavigator
  
• System.Xml.XPath.Extension class
• .CreateNavigator
  
  (this XNode node
  [, XmlNameTable nameTable])
• XPathEvaluate()
• XPathSelectElement()
• XPathSelectElements()
• Sample:
• xDocument.XPathSelectElement("//StatViewer/Members/Member[Name = 'George Lache']")