лектор: Борислава Палева

Galin IlievMCT, MCPD, MCSD.NEThttp://www.galcho.com

LINQ to XML

Agenda

• LINQ Architecture• LINQ to XML ( vs. DOM)

– Create– Traverse– Transform

• LINQ to XSD

LINQ Architecture

Objects

<book> <title/> <author/> <price/></book>

XMLRelational

LINQ enabled data sources

LINQ To Objects

LINQ To XML

LINQ enabled ADO.NET

VB Others…

LINQ To Entities

LINQ To SQL

LINQ To Datasets

.Net Language Integrated Query (LINQ)

C#

LINQ to XML (XLINQ)

• Declarative construction of XML document

• Support for language integrated queries

• Cleaner, simpler, smaller and faster XML API

XML using DOM

XmlDocument doc = new XmlDocument();XmlElement contacts = doc.CreateElement("contacts");foreach (Customer c in customers) if (c.Country == "USA") { XmlElement e = doc.CreateElement("contact"); XmlElement name = doc.CreateElement("name"); name.InnerText = c.CompanyName; e.AppendChild(name); XmlElement phone = doc.CreateElement("phone"); phone.InnerText = c.Phone; e.AppendChild(phone); contacts.AppendChild(e); }doc.AppendChild(contacts);

Programming XML today

<contacts> <contact> <name>Great Lakes Food</name> <phone>(503) 555-7123</phone> </contact> …</contacts>

Imperative model

Document centric

No integrated queries

Memory intensive

LINQ to XML

XElement contacts = new XElement("contacts", from c in customers where c.Country == "USA" select new XElement("contact", new XElement("name", c.CompanyName), new XElement("phone", c.Phone) ));

Programming XML with LINQ Declarative model

Elementcentric

Integrated queries

Smaller and faster

LINQ to XML (XElement) vs. DOM (XmlElement)

• XElement needn’t be part of a “document” object– An XDocument can be created if needed, but XElements can exist without it

• XElement.Name property doesn’t include a namespace prefixes– LINQ to XML resolves namespaces and prefixes at serialization time

• Leaf elements can be cast directly into dataint age = (int?)xAge; //LINQ to XMLint age = (int?)xAge.InnerText; //DOM

• XElement.ToString() outputs the underlying XML

LINQ Query on LINQ to XML

XElement doc = XElement.Load("customers.xml");

var query = from c in doc.Element("Root").Elements("Customers") where c.Element("FullAddress“).Element("Country")

.Value == "Germany"select c;

foreach (XElement result in query)

Console.WriteLine(result);

<?xml version="1.0"?>

<presentation title="LINQ to XML"> <slide>Agenda</slide>

<slide>LINQ Architecture</slide>

<slide>LINQ to XML (XLINQ)</slide>

<slide>XML using DOM</slide>

<slide>LINQ to XML</slide>

<slide>LINQ to XML (XElement) vs. DOM (XmlElement)</slide>

<slide>LINQ Query on LINQ to XML</slide>

<slide>DEMO</slide> <slide>Transforms with DOM</slide>

<slide>Transforms with LINQ to XML</slide>

<slide>Navigation primitives</slide>

<slide>VB 9.0 Literals</slide>

<slide>LINQ to XSD</slide>

<slide>DEMO</slide>

<slide>Q&A</slide>

</presentation>

Transforms with DOM

//Load the stylesheet.XslTransform xslt = new XslTransform();xslt.Load(stylesheet);

//Load the file to transform.XPathDocument doc = new XPathDocument(filename);

//Create an XmlTextWriter which outputs to the console.XmlTextWriter writer = new XmlTextWriter(Console.Out);

//Transform the file and send the output to the console.

xslt.Transform(doc, null, writer, null);writer.Close();

Transforms with LINQ to XML

XElement element = new XElement("salaried_employees", from e in doc.Descendants("employee")

where e.Attribute("salaried").Value == "true" select new XElement("employee", new XElement(e.Element("name")) ) );

Navigation primitives in LINQ to XML

• Similar to the path axes in XPath 1.0– Nodes() : retrieves all the children– Elements(): retrieves all elements children– Elements(“name”): selects children elem. by

name– Attributes()– Parent()– Descendants()– Etc

LINQ to XML Enhancement for VB Only: XML Literals

• VB can create XElements in memory with XElement constructors, same as in C#

Dim contacts As XElement = _ New XElement("contacts", _ New XElement("contact", _ New XElement("name", “Fred Blogs"), _ New XElement("phone", "206-555-0144", _ New XAttribute("type", "home")), _ New XElement("phone", "425-555-0145", _ New XAttribute("type", "work")), _ New XElement("address", _ New XElement("street1", "123 Main St"), _ New XElement("city", "Mercer Island"), _ New XElement("state", "WA"), _ New XElement("postal", "98040"))))

Creating an XElement from an XML Literal

• Wouldn’t you rather do this?

Dim contacts As XElement = _ <contacts> <contact> <name>Fred Blogs</name> <phone type="home">206-555-0144</phone> <phone type="work">425-555-0145</phone> <address> <street1>123 Main St</street1> <city>Mercer Island</city> <state>WA</state> <postal>98040</postal> </address> </contact> </contacts>

• Compiler converts this to same code as previous slide• No need for line continuation characters within the XML

VB XML Literals Can Contain Expressions Evaluated at Runtime

• Element Value from VariableDim name As XElement = <name><%=someVariable%></name>

• Element Value from Function Call

Dim name As XElement = <name><%=SomeFunction()%></name>

• Attribute Value from Variable

Dim phone as XElement =<phone type=(someVariable)>234-5678</phone>

• Element Name from VariableDim contact as XElement =<contact>

<(someVariable)>Foo</></contact>

Embedding LINQ to XML Query Inside XML Literal

Dim root As XElement = _ <Products> <%= From p In dc.Products _ Select <Product ProductId=<%= p.ProductID %>> <Name><%= p.Name %></Name> <ProductNumber><%= p.ProductNumber %></ProductNumber> </Product> %> </Products>

LINQ to XML Enhancement for VB Only: Late Bound XML

• VB can apply three XPath-style navigational operators to an XElement – Child axis operator: “.”– Descendants axis operator: “…”– Attribute axis operator: “@”

• This makes the syntax simpler than the index-based XElement methods– However, VB applies these operators at runtime using late binding, so their use is not type-safe

Late Bound XML ExampleFor Each Dim phone In contact.phone

Console.WriteLine(CStr(phone.@type))NextConsole.WriteLine(CStr(contacts...<city>(0)))

• … which is equivalent to

For Each Dim phone In Contact.Element("phone")Console.WriteLine(CStr(phone.Attribute("type"))

NextConsole.WriteLine(CStr(contact.Descendants("city")(0))

Schema-Aware LINQ to XML?• Current LINQ to XML release are unaware of XML structure at compile time

– Nodes must be located at run time using indexes, and values must be cast to correct data typestring zip = (string)name.Element(“zip”)

• Microsoft is working toward to construct “strongly typed” XML objectsstring zip = name.zip

• LINQ to XSD – alpha v0.2

<?xml version="1.0"?>

<presentation title="LINQ to XML"> <slide>Agenda</slide> <slide>LINQ Architecture</slide> <slide>LINQ to XML (XLINQ)</slide> <slide>XML using DOM</slide> <slide>LINQ to XML</slide> <slide>LINQ to XML (XElement) vs. DOM (XmlElement)</slide> <slide>LINQ Query on LINQ to XML</slide> <slide>DEMO</slide> <slide>Transforms with DOM</slide> <slide>Transforms with LINQ to XML</slide> <slide>Navigation primitives</slide> <slide>VB 9.0 Literals</slide> <slide>LINQ to XSD</slide>

<slide>DEMO</slide> <slide>Q&A</slide>

</presentation>

Resources

• .NET Language-Integrated Query for XML Data• Microsoft XML Team's WebLog• LINQ Tutorial (In Bulgarian)• LINQ Project Forum

Q&A