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Klein c08.tex V3 - 12/13/2007 1:57pm Page 177 Chapter 8: Advanced LINQ to XML Programming Topics foreach (XElement el in xmlTree) el.WriteTo(xw); xw.WriteEndElement(); } You should get the same results as the previous example. Summary On the surface, LINQ to XML looks intimidating, but this and the previous three chapters should have dispelled that vicious rumor. Hopefully, as you started working your way through this chapter, you quickly realized that it really wasn’t as bad as you thought. Sure, working with annotations, events, and axes can be tough, but the purpose of this chapter was to show you that working with them also can be delightful and downright fun. You first examined functional construction and the important role it plays in LINQ to XML. Functional construction provides the capability to easily and efficiently construct an XML document within a single statement. Then you saw how to add annotations to elements and attributes in an XML tree using the AddAnnotation method, and how to read annotations once they are applied. You explored LINQ to XML axis methods, which provide the capability to quickly and efficiently query an XML tree to find elements and attributes and return their values. Knowing how to use these is abso- lutely necessary to understanding and writing query expressions. Finally, you saw how to stream XML documents using LINQ to XML and the benefits that streaming provides, such as managing memory and controlling the size of your XML documents. Chapter 9, the last chapter in this section, focuses on LINQ to XML in Visual Basic .NET. 177 Klein c08.tex V3 - 12/13/2007 1:57pm Page 178 Klein C09.tex V3 - 12/13/2007 5:49pm Page 179 LINQ to XML and Visual Basic .NET Visual Basic .NET wasn’t left behind when it comes to working with LINQ to XML. Visual Basic provides profound support for LINQ to XML through XML literals and XML Axis properties. This chapter focuses on the LINQ to XML differences that apply to Visual Basic .NET. You’ll explore the following topics in this chapter: ❑ How to create XML ❑ How to access XML ❑ How to load XML ❑ How to manipulate XML Creating XML You have to agree that LINQ to XML is powerful and flexible. One of the things you have seen is the capability to call LINQ APIs directly, but what you are about to see makes LINQ to XML even better. In Visual Basic, you can declare XML literals and write XML directly in your code. And you also can access XML Axis properties from within your code. However, it is important to understand what XML literals are so that you can have a better appreciation, as well as for creating XML. Klein C09.tex V3 - 12/13/2007 5:49pm Page 180 Part II: LINQ to XML Overview of XML Literals An XML Literal is a piece of XML, such as a complete XML document or XML fragment, that is typed directly into the source code of a Visual Basic .NET module without the use of quotation marks. Using XML literals lets you write XML directly within your Visual Basic code providing the same structure and layout as the resulting XML. This means that you can create XML documents and fragments easily and efficiently right in your code. XML literal syntax is a representation of the LINQ to XML objects, with Visual Basic compiling the literals into LINQ to XML objects. This functionality is provided via the LINQ to XML object model, which lets you create and work with XML easily. To get a feel for XML Literals, create a new Windows Forms Visual Basic project in Visual Studio. When the project is created, add a reference to the System.Xml.Linq namespace. Open the form in design view and add a few buttons and a text box. Set the Multiline property of the text box to True and size the text box so that you can easily view an XML document or fragment. Next, double-click on the form to view the code behind it, and add the following statement in the declarations section: Imports System.Xml.Linq Next, behind button1, add the following highlighted code. Pay attention to what happens as you type the code for the XML element. Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim emp As XElement = _ < Employee > < EmployeeID > 1 < /EmployeeID > < FirstName > Scott < /FirstName > < LastName > Klein < /LastName > < Title > Geek < /Title > < /Employee > Me.TextBox1.Text = emp.ToString End Sub Slick, isn’t it? The XML you typed into the Visual Basic code looks exactly like the XML fragment that appears in the text box when you run the application and click button1: < Employee > < EmployeeID > 1 < /EmployeeID > < FirstName > Scott < /FirstName > < LastName > Klein < /LastName > < Title > Geek < /Title > < /Employee > How cool is that? 180 Klein C09.tex V3 - 12/13/2007 5:49pm Page 181 Chapter 9: LINQ to XML and Visual Basic .NET You can even create XML documents using XML literals. To do so, simply add the highlighted code to your XML from the previous example: Dim emp As XElement = _ < ?xml version="1.0"? > < Employee > < EmployeeID > 1 < /EmployeeID > < FirstName > Scott < /FirstName > < LastName > Klein < /LastName > < Title > Geek < /Title > < /Employee > When you run this code, you will receive an error stating "Value of type ’System.Xml.Linq.XDocument’ cannot be converted to ’System.Xml.Linq.XElement’." That’s because the compiler is smart enough to know that this is an XML document literal, and you are trying to stuff an XML document into an XML element, which won’t work. To fix this, simply change the XElement to an XDocument as highlighted here: Dim empDoc As XDocument = _ < ?xml version="1.0"? > < Employee > < EmployeeID > 1 < /EmployeeID > < FirstName > Scott < /FirstName > < LastName > Klein < /LastName > < Title > Geek < /Title > < /Employee > How does the Visual Basic compiler create objects from these XML literals? When the compiler encoun- ters an XML literal, it translates the literal into calls for the equivalent LINQ to XML constructors. Those LINQ to XML constructors are used to construct the LINQ to XML object. Depending on the XML literal, the XML literal will be translated into calls to one or several constructors. At the least, it will be converted to a call to the XElement constructor, and if your XML lit- eral contains attributes, then there will be calls to the XAttribute constructor as well. If your XML literal contains version instructions, it will be translated into a call to the XProcessingInstruction constructor. Each class, such as the XElement and XAttribute , has an overloaded New constructor. These constructors are called for each type found, meaning that the XML literal will be translated into a call and passed to the corresponding constructor. For example, if you have an element that contains two attributes, the XElement constructor will be called once and the XAttribute constructor will be called twice. Now that you understand how XML literals work, let’s look at some examples of creating XML. The following example is taken from the first example in this chapter and illustrates the simple way to create XML. It uses the XElement class to create a simple XML fragment containing employee information. You should be quite familiar with the XElement class by now. It’s used to represent an XML element. The only difference here is the capability to use it to create XML literals in Visual Basic. For example, the following code creates an XML fragment in Visual Basic .NET. Notice the exclusion of any quotes. The XML is typed directly into the source code of the project. 181 Klein C09.tex V3 - 12/13/2007 5:49pm Page 182 Part II: LINQ to XML Dim emp As XElement = _ < Employee > < EmployeeID > 1 < /EmployeeID > < FirstName > Scott < /FirstName > < LastName > Klein < /LastName > < Title > Geek < /Title > < /Employee > The following example creates an XML document with a comment and version information. Again, this is quite similar to the way the XDocument class was used in previous chapters, but it’s used in this example in an XML literal. Dim empDoc As XDocument = _ < ?xml version="1.0" encoding="UTF-8"? > < ! Test > < Employee > < EmployeeID > 1 < /EmployeeID > < FirstName > Scott < /FirstName > < LastName > Klein < /LastName > < Title > Geek < /Title > < /Employee > Expressions There is so much more that XML literals can do, such as using embedded expressions. Visual Basic supports the concept of embedded expressions. These expressions have the following syntax: < %=expression% > With Visual Basic you can place embedded expressions directly within your XML literals. These expressions are then evaluated at runtime. For example, the following code defines two variables, one that contains the employee ID, and the other that contains the employee name. These two variables are then used as embedded expressions in the creation of the XML literal to specify the value of the empID attribute value and the FirstName element value. Dim empID As Integer Dim empName As String empID = 1 empName = "Scott" Dim emp As XElement = _ < Employee empID= < %= empID % >> < FirstName > < %= empName % > < /FirstName > < /Employee > 182 Klein C09.tex V3 - 12/13/2007 5:49pm Page 183 Chapter 9: LINQ to XML and Visual Basic .NET When this code is run, you’ll get the following output: < Employee empID = "1" > < FirstName > Scott < /FirstName > < /Employee > In this example, embedded expressions were used as an attribute value on the XML element and as XML element content value. Likewise, you can use embedded expressions as names in the name/value pair, as shown here: Dim empID As String Dim empName As String empID = "ID" empName = "FirstName" Dim emp As XElement = _ < Employee < %= empID % > ="1" > << %= empName % > / > < /Employee > When this code runs, you get the following output: < Employee ID = "1" > < FirstName / > < /Employee > This can be taken one step further by including values for the name/value pair, like this: Dim empID As String Dim empName As String empID = "ID" empName = "FirstName" Dim emp As XElement = _ < Employee < %= empID % > ="1" > << %= empName % >> Scott < / > < /Employee > This code produces the following: < Employee ID = "1" > < FirstName > Scott < /FirstName > < /Employee > There are a couple of noteworthy items of which you should be aware. First, be careful how you use Option Strict in this scenario. Option Strict will cause the compiler to check each type to ensure that it has widened to the required type. This applies to everything except the root element of an XML document. If you leave Option Strict off, expressions of type Object can be embedded, in which case their type is verified at runtime. 183 Klein C09.tex V3 - 12/13/2007 5:49pm Page 184 Part II: LINQ to XML Second, you will probably run into situations where content is optional. In those cases, embedded expressions that contain Nothing are ignored. Thus, there is no need to check that the values of elements or attributes are not Nothing as you use XML literals. In other words, required values such as element and attribute names cannot be Nothing , but empty embedded expressions are ignored. Sweet. Embedding Queries The fact that you can embed expressions within your XML literal should tell you that you can also embed queries within your XML literal. When queries are embedded within the XML literal, all ele- ments returned by the query are added to the XML. This lets you create and add dynamic content to your XML literal. In the following example, a standard LINQ query is embedded in the XML literal, which will create an XML tree based on the value returned from the query. Dim emp As XElement = _ < Employee > < %= From con in Contact Select < name >< %= con.FirstName % >< /name > % > < /Employee > That gives you an idea of what is possible when LINQ queries are embedded in XML literals. However, take it a step further and add an attribute to the XML. Modify the XML as highlighted: Dim emp As XElement = _ < Employee > < %= From con in Contact Select < name id= < %= con.ContactID % >>< %= con.FirstName % >< /name > % > < /Employee > Now you have a good understanding of how easy it is to dynamically create and add content to your XML. Understanding Whitespace in Visual Basic XML Literals Only significant whitespace is included in an XML literal by the Visual Basic compiler when a LINQ to XML object is created. Any insignificant whitespace is ignored. To illustrate how whitespace is used, add the following code to the click event of one of the buttons on your form: Dim empID As String Dim empName As String empID = "ID" empName = "FirstName" Dim emp As XElement = _ 184 Klein C09.tex V3 - 12/13/2007 5:49pm Page 185 Chapter 9: LINQ to XML and Visual Basic .NET < Employee < %= empID % > ="1" > << %= empName % >> Scott < / > < /Employee > Me.Textbox1.Text = emp.ToString Run the application and click the button. The text box on the form will be populated with the following XML: < Employee ID = "1" > < FirstName > Scott < /FirstName > < /Employee > Notice all of the whitespace to the left of ‘‘Scott’’ and to the left of the < FirstName > closing tag. The output is like this because the inner element, < FirstName >, contains significant whitespace and the output text, while the outer element, < Employee >, contains insignificant whitespace. So, you are probably asking yourself, what is the difference between ‘‘significant’’ and ‘‘insignificant’’ whitespace? Whitespace in XML literals is considered significant when: ❑ It is in an attribute value. ❑ It is part of an element’s text content. ❑ It is in an embedded expression for an element’s text content. You can add the xml:space attribute in the XML element literal, but it won’t change how the compiler handles the whitespace. Accessing XML Creating XML is one thing, but accessing it is another, and this section shows you how to do that in Visual Basic. Navigating XML structures in LINQ to XML is quite easy in Visual Basic. To illustrate, add another button to your form and place the following code in its click event: Dim employee As XElement = _ < Employees > < Employee EmpID="1" > < Title > Geek < /Title > < FirstName > Scott < /FirstName > < MiddleName > L < /MiddleName > < LastName > Klein < /LastName > < EmailAddress > ScottKlein@SqlXml.com < /EmailAddress > < Address > < Street > 111 Main St. < /Street > < City > Wellington < /City > < State > FL < /State > < Zip > 33414 < /Zip > 185 Klein C09.tex V3 - 12/13/2007 5:49pm Page 186 Part II: LINQ to XML < /Address > < /Employee > < /Employees > Me.TextBox1.Text = employee < City > .Value Run the project and click the button. The text box should display the following value: Wellington Accessing element and attribute values is done through XML axis properties, which provide the capabil- ity to navigate XML structures. These properties use a special syntax that helps access any element and attribute by specifying XML names within your XML document. Following are the available properties for accessing element and attribute values: ❑ Descendant axis—Returns all the child elements of the specified element regardless how deep in the hierarchy the child elements are found. The preceding example illustrates this point. The < City > element is several nodes deep in the XML fragment but through the use of the descendant axis property you can easily access the value of that node, as shown above. Because it doesn’t matter how deep in the hierarchy the element is found, you can use the same syntax to return the value of the < Title > as well as any other element. ❑ Child axis—Returns all the elements that are child elements of the specified element. This prop- erty is used to return all the child elements of a specified element. For example: employee. < FirstName > ❑ Attribute axis—Returns all the attributes of the specified element. From the preceding XML fragment, this property can be used to return the EmpID attribute of < Employee > node: Employee. < Employee > .@EmpID ❑ Value—Returns a string containing the object (value) for the first object found in the sequence. It returns Nothing ifthesequenceisempty.Asyouhaveseen,thispropertyreturnsthevalueof thespecifiednode(s),suchasthevalueofthe < City > element: employee < City > .Value ❑ Extension indexer—Returns the first element from the sequence. To illustrate this property, modify the XML fragment at the beginning of this section by adding a second employee, as shown in the following highlighted code: Dim employee As XElement = _ < Employees > < Employee EmpID="1" > < Title > Geek < /Title > < FirstName > Scott < /FirstName > < MiddleName > L < /MiddleName > < LastName > Klein < /LastName > < EmailAddress > ScottKlein@SqlXml.com < /EmailAddress > 186 [...]... information In the next chapter, the focus turns to working with LINQ to SQL 195 Page 195 Klein C09.tex V3 - 12/13/2007 5:49pm Page 1 96 Klein Part III LINQ to SQL Chapter 10: LINQ to SQL Overview Chapter 11: LINQ to SQL Queries Chapter 12: Advanced Query Concepts Chapter 13: More about Entity Classes Chapter 14: LINQ to DataSet Chapter 15: Advanced LINQ to SQL Topics p03.tex V3 - 12/13/2007 2:00pm Page 197... V3 - 12/13/2007 2:01pm LINQ to SQL Over view LINQ to SQL is a component of LINQ and part of ADO.NET that provides a run-time infrastructure for mapping relational data as objects This chapter provides an overview of LINQ to SQL, and the rest of the chapters in this section of the book then dig deeper into the individual aspects of LINQ to SQL, including LINQ to SQL queries and LINQ over DataSets Today’s... basics Page 199 Klein c10.tex V3 - 12/13/2007 Part III: LINQ to SQL Understanding LINQ to SQL As mentioned, LINQ to SQL is a part of ADO.NET and a component of LINQ As such, you get the benefit of the unified programming model, standard query operators, and standard query facilities provided by LINQ, plus the services provided by the ADO.NET provider model LINQ to SQL works by mapping the data model of a relational... However, to fully understand LINQ to SQL, the LINQ to SQL object model and the concept of attribute-based mapping must be explored LINQ to SQL Object Model The LINQ to SQL object model provides the fundamental elements for working with and managing relational objects It is via this model that a relational model is mapped to and expressed in the developer’s programming language In the LINQ to SQL object model,... namespace, which provides the capability to query a table and even add and delete objects LINQ to SQL works with objects Meaning, in LINQ to SQL a relational database’s object model is directly mapped to an object model expressed in your selected programming language Thus, a translation takes place LINQ to SQL translates the LINQ queries of the object model into statements that SQL can understand and then... same LINQ query operators and query facilities that you have come to love The following code queries the Person.Contact table of the Adventureworks database as defined in the object mapping: var query = (from c in contact where c.FirstName.StartsWith("S") && c.LastName.StartsWith("K") orderby c.LastName select c); As with the other components of LINQ, LINQ to SQL works with both C# and Visual Basic LINQ. .. address data access functionality) While LINQ to SQL may not offer any speed advantages over previous or existing technology, it does offer the capability to build applications more quickly and efficiently This chapter discusses the fundamentals and concepts that programmers need to know to work with LINQ to SQL, including the following: ❑ Overview of LINQ to SQL ❑ LINQ to SQL object model ❑ Attribute-based... 12/13/2007 Part III: LINQ to SQL LINQ to SQL Object Relational Object DataContext Database Entity class Table Class member Column Association Foreign-key relationship The following section explores LINQ to SQL object model mapping Attribute-Based Mapping It is all about attributes when mapping SQL Server objects to the object model This attribute-based approach is utilized heavily by LINQ to SQL to effectively... class with a relationship to the Employee class: [Table(Name = "Person.Contact")] public class Contact { [Column(DBType = "nvarchar(8) not null")] public string Title; 2 06 2:01pm Page 2 06 Klein c10.tex V3 - 12/13/2007 2:01pm Chapter 10: LINQ to SQL Overview [Column(DBType = "nvarchar(50) not null")] public string FirstName; [Column(DBType = "nvarchar(50) not null")] public string MiddleName; [Column(DBType... c10.tex V3 - 12/13/2007 2:01pm Chapter 10: LINQ to SQL Overview You use DataContext much the same way that you use an ADO.NET connection, in that it is initialized with a connection or connection string Once the DataContext is created, a table variable is constructed using the Contact class created above This is done by using the Table class of the System.Data .Linq namespace, which provides the capability . 12/13/2007 5:49pm Page 179 LINQ to XML and Visual Basic .NET Visual Basic .NET wasn’t left behind when it comes to working with LINQ to XML. Visual Basic provides profound support for LINQ to XML through. XML You have to agree that LINQ to XML is powerful and flexible. One of the things you have seen is the capability to call LINQ APIs directly, but what you are about to see makes LINQ to XML even better syntax is a representation of the LINQ to XML objects, with Visual Basic compiling the literals into LINQ to XML objects. This functionality is provided via the LINQ to XML object model, which