Visual Basic.NET How to Program (PDF)

Chapter 18 Extensible Markup Language (XML) 867
contain child elements. Setting the model attribute to "closed" specifies that a conforming
XML document can contain only elements defined in this Schema. Line 10 defines
element book; this element’s content is “elements only” (i.e., eltOnly). This means
that the element cannot contain mixed content (i.e., text and other elements). Within the ElementType
element named book, the element element indicates that title is a child
element of book. Attributes minOccurs and maxOccurs are set to "1", indicating that
a book element must contain exactly one title element. The asterisk (*) in line 15 indicates
that the Schema permits any number of book elements in element books.
Class XmlValidatingReader validates an XML document against a Schema. The
program in Fig. 18.18 validates an XML document that the user provides (such as,
Fig. 18.16 or Fig. 18.19) against a Microsoft Schema document (Fig. 18.17).
Line 17 creates an XmlSchemaCollection reference named schemas. Line 33
calls its Add method to add an XmlSchema object to the Schema collection. Method Add
receives as arguments a name that identifies the Schema (e.g., "book") and the name of
the Schema file (e.g., "book.xdr").
The XML document to be validated against the Schema(s) contained in the
XmlSchemaCollection must be passed to the XmlValidatingReader constructor
(line 48–49). Lines 44–45 create an XmlReader for the file that the user selected
from filesComboBox. The XmlReader passed to this constructor is created using the
file name selected from cboFiles (lines 44–45).
Line 52 Adds the Schema collection referenced by Schemas to the Schemas property.
This property sets the Schema(s) used to validate the document. The Validation-
Type property (line 55) is set to the ValidationType enumeration constant for
Microsoft Schema. Lines 58–59 register method ValidationError with ValidationEventHandler.
Method ValidationError (lines 79–84) is called if the document
is invalid or an error occurs, such as if the document cannot be found. Failure to
register a method with ValidationEventHandler causes an exception to be thrown
when the document is missing or invalid.
1 ' Fig. 18:18: ValidationTest.vb
2 ' Validating XML documents against Schemas.
3
4 Imports System.Windows.Forms
5 Imports System.Xml
6 Imports System.Xml.Schema ' contains Schema classes
7
8 ' determines XML document Schema validity
9 Public Class FrmValidationTest
10 Inherits Form
11
12 ' Controls for validating XML document
13 Friend WithEvents cboFiles As ComboBox
14 Friend WithEvents cmdValidate As Button
15 Friend WithEvents lblConsole As Label
16
17 Private schemas As XmlSchemaCollection ' Schemas
18 Private valid As Boolean ' validation result
Fig. 18.18 Schema-validation example (part 1 of 3).