Represents a reader that provides fast, non-cached, forward-only access to XML data.
(in System.Xml.dll)
Visual Basic (Declaration)
Public MustInherit Class XmlReader _
Implements IDisposable
Dim instance As XmlReader
public abstract class XmlReader : IDisposable
XmlReader provides forward-only, read-only access to a stream of XML data. The XmlReader class conforms to the W3C Extensible Markup Language (XML) 1.0 and the Namespaces in XML recommendations.
The current node refers to the node on which the reader is positioned. The reader is advanced using any of the read methods and properties reflect the value of the current node.
.gif) Important Note: |
|---|
You must use Create method to create XmlReader instances. |
XmlReader throws an XmlException on XML parse errors. After an exception is thrown the state of the reader is not predictable. For example, the reported node type may be different than the actual node type of the current node. Use the ReadState property to check whether the reader is in error state.
Security Considerations
The following items are things to consider when working with the XmlReader class.
Exceptions thrown the XmlReader can disclose path information that you do not want bubbled up to the application. Your applications must catch exceptions and process them appropriately.
Do not enable DTD processing if you are concerned about denial of service issues or if you are dealing with untrusted sources. DTD processing is disabled by default for XmlReader objects created by the Create method.
You can design your application so that the XML processing is memory and time constrained. For example, configure time-out limits in your ASP.NET application.
XML data can contain a large number of attributes, namespace declarations, nested elements and so on that require a substantial amount of time to process. To limit the size of the input that is sent to the XmlReader, create a custom IStream implementation and supply it the XmlReader.
The ReadValueChunk method can be used to handle large streams of data. This method reads a small number of characters at a time instead of allocating a single string for the whole value.
When reading an XML document with a large number of unique local names, namespaces, or prefixes, a problem can occur. If you are using class that derives from XmlReader, and you call either the LocalName, Prefix, or NamespaceURI property for each item, the returned string is added to a NameTable. The collection held by the NameTable never decreases in size, creating a virtual "memory leak" of the string handles. One mitigation for this is to derive from the NameTable class and enforce a maximum size quota. (There is no way to prevent the use of a NameTable, or to switch the NameTable when it is full). Another mitigation is to avoid using the properties mentioned and instead use the MoveToAttribute method with the IsStartElement method where possible; those methods do not return strings and thus avoid the problem of overfilling the NameTable collection.
The following example navigates through the stream to determine the current node type, and then uses XmlWriter to output the XmlReader content.
Dim output As StringBuilder = New StringBuilder()
Dim xmlString As String = "<?xml version='1.0'?>" & _
"<!-- This is a sample XML document -->" & _
"<Items>" & _
"<Item>test with a child element <more/> stuff</Item>" & _
"</Items>"
' Create an XmlReader
Using reader As XmlReader = XmlReader.Create(New StringReader(xmlString))
Dim ws As XmlWriterSettings = New XmlWriterSettings()
ws.Indent = True
Using writer As XmlWriter = XmlWriter.Create(output, ws)
' Parse the file and display each of the nodes.
While reader.Read()
Select Case reader.NodeType
Case XmlNodeType.Element
writer.WriteStartElement(reader.Name)
Case XmlNodeType.Text
writer.WriteString(reader.Value)
Case XmlNodeType.XmlDeclaration
Case XmlNodeType.ProcessingInstruction
writer.WriteProcessingInstruction(reader.Name, reader.Value)
Case XmlNodeType.Comment
writer.WriteComment(reader.Value)
Case XmlNodeType.EndElement
writer.WriteFullEndElement()
End Select
End While
End Using
End Using
OutputTextBlock.Text = output.ToString()
StringBuilder output = new StringBuilder();
String xmlString =
@"<?xml version='1.0'?>
<!-- This is a sample XML document -->
<Items>
<Item>test with a child element <more/> stuff</Item>
</Items>";
// 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();
System..::.Object
System.Xml..::.XmlReader
System.Xml..::.XmlDictionaryReader
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.
Reference
Other Resources