Reading XML
As mentioned previously, XmlReader is an abstract base class for XML reader classes. This class provides fast, non-cached, forward-only cursors to read XML documents.
The XmlTextReader, XmlNodeReader, and XmlValidatingReader classes are defined from the XmlReader class. Figure 6-2 shows XmlReader and its derived classes.
Figure 6-2: XmlReader and its derived classes
You use the XmlTextReader, XmlNodeReader, and XmlValidatingReader classes to read XML documents. These classes define overloaded constructors to read XML files, strings, streams, TextReader objects, XmlNameTable, and combinations of these. After creating an instance, you simply call the Read method of the class to read the document. The Read method starts reading the document from the root node and continues until Read returns False, which indicates there's no node left to read in the document. Listing 6-1 reads an XML file and displays some information about the file. In this example, we use the books.xml file. You can use any XML by replacing the string name.
Listing 6-1: Reading an XML File
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Dim reader As XmlTextReader = New XmlTextReader("C:\\books.xml") Console.WriteLine("General Information") Console.WriteLine("===================") Console.WriteLine(reader.Name) Console.WriteLine(reader.BaseURI) Console.WriteLine(reader.LocalName) |
Getting Node Information
The Name property returns the name of the node with the namespace prefix, and the LocalName property returns the name of the node without the prefix.
The Item property works as an indexer and returns the value of the attribute at the specified index. The Value property returns the value of current node. You can even get the level of the node by using the Depth property, as shown in Listing 6-2.
Listing 6-2: Getting XML Node Information
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Dim reader As XmlTextReader = New XmlTextReader("C:\\books.xml") While reader.Read() If reader.HasValue Then Console.WriteLine("Name: "+reader.Name) Console.WriteLine("Node Depth: " +reader.Depth.ToString()) Console.WriteLine("Value: "+reader.Value) End If End While |
The NodeType property returns the type of the current node in the form of an XmlNodeType enumeration:
XmlNodeType type = reader.NodeType;
which defines the type of a node. The XmlNodeType enumeration members are Attribute, CDATA, Comment, Document, Element, WhiteSpace, and so on. These represent XML document node types.
In Listing 6-3, you read a document's nodes one by one and count them. Once the reading and counting are done, you see how many comments, processing instructions, CDATAs, elements, whitespaces, and so on that a document has and display them on the console. The XmlReader.NodeType property returns the type of node in the form of an XmlNodeType enumeration. The XmlNodeType enumeration contains a member corresponding to each node type. You can compare the return value with the XmlNodeType members to find out the node's type.
Listing 6-3: Getting Node Information
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Sub Main() Dim DecCounter As Integer = 0 Dim PICounter As Integer = 0 Dim DocCounter As Integer = 0 Dim CommentCounter As Integer = 0 Dim ElementCounter As Integer = 0 Dim AttributeCounter As Integer = 0 Dim TextCounter As Integer = 0 Dim WhitespaceCounter As Integer = 0 Dim reader As XmlTextReader = New XmlTextReader("C:\\books.xml") While reader.Read() Dim nodetype As XmlNodeType = reader.NodeType Select Case nodetype Case XmlNodeType.XmlDeclaration DecCounter = DecCounter + 1 Exit Select Case XmlNodeType.ProcessingInstruction PICounter = PICounter + 1 Exit Select Case XmlNodeType.DocumentType DocCounter = DocCounter + 1 Exit Select Case XmlNodeType.Comment CommentCounter = CommentCounter + 1 Exit Select Case XmlNodeType.Element ElementCounter = ElementCounter + 1 If reader.HasAttributes Then AttributeCounter += reader.AttributeCount End If Exit Select Case XmlNodeType.Text TextCounter = TextCounter + 1 Exit Select Case XmlNodeType.Whitespace WhitespaceCounter = WhitespaceCounter + 1 Exit Select End Select End While ' Print the info Console.WriteLine("White Spaces:" + WhitespaceCounter.ToString()) Console.WriteLine("Process Instructions:" + PICounter.ToString()) Console.WriteLine("Declaration:" + DecCounter.ToString()) Console.WriteLine("White Spaces:" + DocCounter.ToString()) Console.WriteLine("Comments:" + CommentCounter.ToString()) Console.WriteLine("Attributes:" + AttributeCounter.ToString()) End Sub |
The Case statement can have the values XmlNodeType.XmlDeclaration, XmlNodeType.ProcessingInstruction, XmlNodeType.DocumentType, XmlNodeType.Comment, XmlNodeType.Element, XmlNodeType.Text, XmlNodeType.Whitespace , and so on.
The XmlNodeType enumeration specifies the type of node. Table 6-1 describes its members.
| MEMBER NAME | DESCRIPTION |
|---|---|
| Attribute | Attribute node |
| CDATA | CDATA section |
| Comment | Comment node |
| Document | Document object |
| DocumentFragment | Document fragment |
| DocumentType | The DTD, indicated by the <!DOCTYPE> tag |
| Element | Element node |
| EndElement | End of element |
| EndEntity | End of an entity |
| Entity | Entity declaration |
| EntityReference | Reference to an entity |
| None | Returned if XmlReader is not called yet |
| Notation | A notation in the document type |
| ProcessingInstruction | Represents a processing instruction (PI) node |
| SignificantWhitespace | Represents whitespace between markup in a mixed content model |
| Text | Represents the text content of an element |
| Whitespace | Represents whitespace between markup |
| XmlDeclaration | Represents an XML declaration node |
Moving to a Content Node
You can use the MoveToMethod to move from the current node to the next content node of an XML document. A content's node is an item of the following type: Text, CDATA, Element, EntityReference, or Entity. So, if you call the MoveToContent method, it skips other types of nodes besides the content type nodes. For example, if the next node of the current node is PI, DxlDeclaration, or DocumentType, it'll skip these nodes until it finds a content type node. Listing 6-4 reads books.xml and moves through its nodes using the MoveToContent method.
Listing 6-4: Using the MoveToContent Method
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Dim reader As XmlTextReader = New XmlTextReader("C:\\books.xml") While reader.Read() Console.WriteLine(reader.Name) reader.MoveToContent() Console.WriteLine(reader.Name) End While |
Using the GetAttributes of a Node
The GetAttribute method is an overloaded method. You can use this method to return attributes with the specified name, index, local name, or namespace Uniform Resource Indicator (URI). You use the HasAttributes property to check if a node has attributes, and AttributeCount returns the number of attributes on the node. The local name is the name of the current node without prefixes. For example, if <bk:book> represents a name of a node, where bk is a namespace and : refers to the namespace, the local name for the <bk:book> element is book. MoveToFirstAttribute moves to the first attribute. The MoveToElement method moves to the element that contains the current attribute node (see Listing 6-5).
Listing 6-5: Using the GetAttribute of a Node
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Imports System.Xml Module Module1 Sub Main() Dim reader As XmlTextReader = _ New XmlTextReader("C:\\books.xml") reader.MoveToContent() reader.MoveToFirstAttribute() Console.WriteLine("First Attribute Value" + reader.Value) Console.WriteLine("First Attribute Name" + reader.Name) While reader.Read() If reader.HasAttributes Then Console.WriteLine(reader.Name + " Attribute") Dim i As Integer Dim counter As Integer = reader.AttributeCount - 1 For i = 0 To counter reader.MoveToAttribute(i) Console.WriteLine("Name: " + reader.Name) Next i reader.MoveToElement() End If End While End Sub End Module |
You can move to attributes by using MoveToAttribute, MoveToFirstAttribute, and MoveToNextAttribute. MoveToFirstAttribute and MoveToNextAttribute move to the first and next attributes, respectively. After calling MoveToAttribute, the Name, Namespace, and Prefix properties will reflect the properties of the specified attribute.
Searching for a Node
The Skip method skips the current node. It's useful when you're looking for a particular node and want to skip other nodes. In Listing 6-6, you read the books.xml document and compare its XmlReader.name (through XmlTextReader) to look for a node with the name bookstore and display the name, level, and value of that node using XmlReader's Name, Depth, and Value properties.
Listing 6-6: Using the Skip Method
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New XmlTextReader("C:\\books.xml") While reader.Read() ' Look for a node with name bookstore If reader.Name <> "bookstore" Then reader.Skip() Else Console.WriteLine("Name: " + reader.Name) Console.WriteLine("Level of the node: " + reader.Depth.ToString()) Console.WriteLine("Value: " + reader.Value) End If End While reader.Close() |
Closing the Document
Finally, you use Close to close the opened XML document.
Tables 6-2 and 6-3 describe the XmlReader class properties and methods. We already discussed some of them in the previous discussion.
| PUBLIC INSTANCE PROPERTY | DESCRIPTION |
|---|---|
| AttributeCount | Returns the number of attributes on the current node |
| BaseURI | Returns the base URI of the current node |
| Depth | Returns the level of the current node |
| EOF | Indicates whether its pointer is at the end of the stream |
| HasAttributes | Indicates if a node has attributes |
| HasValue | Indicates if a node has a value |
| IsDefault | Indicates whether the current node is an attribute generated from the default value defined in the DTD or schema |
| IsEmptyTag | Returns if the current node is empty or not |
| Item | Returns the value of the attribute |
| LocalName | Name of the current node without the namespace prefix |
| Name | Name of the current node with the namespace prefix |
| NamespaceURI | Namespace Uniform Resource Name (URN) of the current namespace scope |
| NameTable | Returns the XmlNameTable associated with this implementation |
| NodeType | Returns the type of node |
| Prefix | Returns the namespace associated with a node |
| ReadState | Read state |
| Value | Returns the text value of a node |
| XmlLang | Returns the current xml:lang scope |
| XmlSpace | Returns the current xml:space scope |
| PUBLIC INSTANCE METHOD | DESCRIPTION |
|---|---|
| Close | Closes the stream and changes ReadState to Closed |
| GetAttribute | Returns the value of an attribute |
| IsStartElement | Checks if a node has start tag |
| LookupNamespace | Resolves a namespace prefix in the current element's scope |
| MoveToAttribute, MoveToContent, MoveToElement | Moves to specified attribute, content, and element |
| MoveToFirstAttribute, MoveToNextAttribute | Moves to the first and next attributes |
| Read | Reads a node |
| ReadAttributeValue | Parses the attribute value into one or more Text and/or EntityReference node types |
| ReadXXX (ReadChar, ReadBoolean, ReadDate, ReadInt32, and so on) | Reads the contents of an element into the specified type including char, integer, double, string, date, and so on |
| ReadInnerXml | Reads all the content as a string |
| Skip | Skips the current element |
EAN: 2147483647
Pages: 214