Data Conformance Checking with XmlReader
XmlReader objects created by the Create method are, by default, more conformant than the XmlTextReader implementation. XmlReader objects created by the Create method support the following features by default:
Normalize new-line characters.
Expand entities.
Add default attributes.
The XmlReaderSettings.CheckCharacters and XmlReaderSettings.ConformanceLevel properties allow you to specify the type of conformance checks you want to enable on the created XmlReader object.
CheckCharacters Setting
The CheckCharacters property on the XmlReaderSettings class instructs the reader to check characters and throw an XmlException if any characters are outside the range of legal XML characters. When character checking is enabled, you are ensured the following:
All characters in the document are within the range of legal XML characters as defined by the W3C XML 1.0 Recommendation.
All XML names are valid. For example, if the name of an element starts with a number, an XmlException is thrown. For more information, see http://www.w3.org/TR/REC-xml\#NT-Name.
By default, character checking is enabled. If the reader is processing text data, it always checks that XML names are valid, even when the CheckCharacters property is set to false.
ConformanceLevel Setting
The ConformanceLevel property on the XmlReaderSettings class configures the XmlReader to check and guarantee that the stream being read complies with a certain set of rules. Depending on the conformance level that is specified, the XML data can be checked to see that it conforms to the rules for a well-formed XML 1.0 document, or document fragment. If the data is not in conformance, an XmlException is thrown. The default setting is ConformanceLevel.Document.
Note
If the reader is configured to support ConformanceLevel.Fragment, but the XML data contains a document type definition (DTD), an XmlException is thrown. The XML 1.0 recommendation requires document-level conformance when a DTD is present.
ConformanceLevel value |
Description |
|---|---|
Document |
The XML data conforms to the rules for a well-formed XML 1.0 document. This level of checking ensures that any processor can consume the stream being read as an XML 1.0 document. The reader checks for the following:
|
Fragment |
The XML data conforms to the rules for a well-formed XML 1.0 document fragment. This setting accepts XML data with multiple root elements, or text nodes at the top-level. This level of checking ensures that any processor can consume the stream being read as an XML 1.0 external parsed entity. Note DTD is not allowed in fragments. |
Auto |
The reader decides which level of conformance checking to apply based on the incoming data. Document conformance checking is applied if the XML data contains DTD information. Fragment conformance checking is applied if the XML data contains one of following:
An XmlException is thrown if there is a conflict, such as when there is a text node and a DTD at the root level. This setting can be used in wrapping scenarios when the Create method is used to add additional features to an existing XmlReader. In this case ConformanceLevel.Auto does not add any new conformance checking. Conformance checking is left to the XmlReader that is being wrapped. |
XmlTextReader, XmlValidatingReader, and XmlNodeReader Objects
The ConformanceLevel setting has the following behavior when working with XmlTextReader, XmlValidatingReader, or XmlNodeReader objects.
If the XmlReader object was not created using the Create method, the assumed conformance level is Document. As a result, if you decide to wrap this XmlReader instance within another XmlReader object, when creating the new XmlReader object, the conformance level specified must be either Document or Auto.
If the underlying XmlTextReader or XmlValidatingReader object was constructed using the XmlParserContext object and XmlNodeType.Element, the Fragment setting can also be applied.
Example
The following code creates a reader that enforces fragment-level conformance.
Dim settings as XmlReaderSettings = new XmlReaderSettings()
settings.ConformanceLevel = ConformanceLevel.Fragment
Dim reader as XmlReader = XmlReader.Create(new StringReader(xmlString), settings)
XmlReaderSettings settings = new XmlReaderSettings();
settings.ConformanceLevel = ConformanceLevel.Fragment;
XmlReader reader = XmlReader.Create(new StringReader(xmlString), settings);
See Also
Concepts
Reading XML with the XmlReader