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This chapter is from the book

Generating and Using XSD Schemas

In Chapter 2, "Creating and Manipulating DataSets," you learned how to create an XSD schema in the Visual Studio .NET user interface by dragging and dropping XML elements from the Toolbox. This method is useful when you need to create a schema from scratch. But there will be times when you want to create a schema to match an existing object. In this section, you'll learn about the methods that are available to programmatically generate XSD schemas.

Generating an XSD Schema

One obvious source for an XML schema is an XML file. An XML file can contain explicit schema information (in the form of an embedded schema), or it can contain implicit schema information in its structure. Here's a sample file, Products.xml, that contains embedded schema information:

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:od="urn:schemas-microsoft-com:officedata">
<xsd:schema>
<xsd:element name="dataroot">
<xsd:complexType>
<xsd:choice maxOccurs="unbounded">
<xsd:element ref="Products"/>
</xsd:choice>
</xsd:complexType>
</xsd:element>
<xsd:element name="Products">
<xsd:annotation>
<xsd:appinfo/>
</xsd:annotation>
<xsd:complexType>
<xsd:sequence>
<xsd:element name="ProductID" od:jetType="autonumber"
od:sqlSType="int" od:autoUnique="yes"
od:nonNullable="yes">
<xsd:simpleType>
<xsd:restriction base="xsd:integer"/>
</xsd:simpleType>
</xsd:element>
<xsd:element name="ProductName" minOccurs="0"
od:jetType="text" od:sqlSType="nvarchar">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="40"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>
<dataroot
xmlns:xsi=
"http://www.w3.org/2000/10/XMLSchema-instance">
<Products>
<ProductID>1</ProductID>
<ProductName>Chai</ProductName>
</Products>
</dataroot>
</root>

If the file contains explicit schema information, you can use the DataSet object to read that information and create the corresponding schema as a separate file, as shown here:

' Load the XML file with inline schema info
Dim xtr As XmlTextReader = _
 New XmlTextReader("Products.xml")
' Read the schema (only) into a DataSet
Dim ds As DataSet = New DataSet()
ds.ReadXmlSchema(xtr)
' Write the schema out as a separate stream
Dim sw As StringWriter = New StringWriter()
ds.WriteXmlSchema(sw)
txtSchema.Text = sw.ToString()

The DataSet object must have the capability to read an XML schema so that it can construct a matching data structure in memory. The .NET Framework designers thoughtfully exposed this capability to you through the ReadXmlSchema and WriteXmlSchema methods of the DataSet object. But what if the file does not contain explicit schema information? It turns out that you can still use the DataSet object because this object also has the capability to infer an XML schema based on the data in an XML file. For example, here's how to use the DataSet object to infer a schema for the Books.xml file:

' Load an XML file with no schema information
Dim xtr As XmlTextReader = _
 New XmlTextReader("Books.xml")
' Read the schema (only) into a DataSet
Dim ds As DataSet = New DataSet()
Dim ns As String()
ds.InferXmlSchema(xtr, ns)
' Write the schema out as a separate stream
Dim sw As StringWriter = New StringWriter()
ds.WriteXmlSchema(sw)
txtSchema.Text = sw.ToString()

You have at least four ways to obtain XSD files for your applications:

  • You can use a file generated by an external application such as Microsoft SQL Server or Microsoft Access.

  • You can create your own schema files from scratch using the techniques you learned in Chapter 2.

  • You can extract inline schema information from an XML file using the DataSet.ReadXmlSchema method.

  • You can infer schema information from an XML file using the DataSet.InferXmlSchema method.

Using an XSD Schema

The prime use of a schema file is to validate the corresponding XML file. Although any XML file that conforms to the syntactical rules for XML is well-formed, this does not automatically make the file valid. A valid XML file is one whose structure conforms to a specification. This specification can be in the form of an XML schema or a Document Type Definition (DTD), for example. Any valid XML file is well-formed, but not every well-formed XML file is valid. The .NET Framework provides good support for validating XML files.

To validate an XML document, you can use the XmlValidatingReader class. This class provides an additional layer between the XmlReader and the XmlDocument. The extra layer validates the document as it is read in to the XmlDocument object. To use the XmlValidatingReader object to validate an XML document with an inline schema, you should supply a handler for any validation errors, as in this code sample:

Private Sub ValidateIt()
 ' Load a document with an inline schema
 Dim xtr As XmlTextReader = _
  New XmlTextReader("Products.xml")
 ' Prepare to validate it
 Dim xvr As XmlValidatingReader = _
  New XmlValidatingReader(xtr)
 xvr.ValidationType = ValidationType.Schema
 ' Tell the validator what to do with errors
 AddHandler xvr.ValidationEventHandler, _
  AddressOf ValidationHandler
 ' Load the document, thus validating
 Dim xd As XmlDocument = _
  New XmlDocument()
 xd.Load(xvr)
End Sub

Private Sub ValidationHandler( _
 ByVal sender As Object, _
 ByVal e As ValidationEventArgs)
 ' Dump any validation errors to the UI
 MessageBox.Show(e.Message)
End Sub

An inline schema cannot contain an entry for the root element of the document, so even when the document is otherwise valid, you'll get an error from that node. The XmlValidatingReader class is constructed so that it does not stop on validation errors. Rather, it continues processing the file but raises an event for each error. This enables your code to decide how to handle errors while still filling the XmlDocument object.

You can also validate an XML file against an external schema. To do this, you can load the schema and the XML file separately and tell the XmlValidatingReader class to compare one to the other:

Private Sub ValidateIt()
 ' Load a document with an external schema
 Dim xtr As XmlTextReader = _
  New XmlTextReader("Books.xml")
 ' Prepare to validate it
 Dim xvr As XmlValidatingReader = _
  New XmlValidatingReader(xtr)
 xvr.ValidationType = ValidationType.Schema
 ' Tell the validator what to do with errors
 AddHandler xvr.ValidationEventHandler, _
  AddressOf ValidationHandler
 ' Load the schema
 Dim xsc As XmlSchemaCollection = _
  New XmlSchemaCollection()
 xsc.Add("xsdBooks", "Books.xsd")
 ' Tell the validator which schema to use
 xvr.Schemas.Add(xsc)
 ' Load the document, thus validating
 Dim xd As XmlDocument = _
  New XmlDocument()
 xd.Load(xvr)
End Sub

Private Sub ValidationHandler( _
 ByVal sender As Object, _
 ByVal e As ValidationEventArgs)
 ' Dump any validation errors to the UI
 MessageBox.Show(e.Message)
End Sub

Schema files are not the only way to describe the structure of an XML file. An older standard for specifying structure is the Document Type Definition, or DTD. DTDs are part of the Standard Generalized Markup Language (SGML) standard, from which both HTML and XML derive. A DTD file lists the elements that may appear in an XML file, as in this example (Books.dtd):

<!ELEMENT Books (Book)* >
<!ELEMENT Book (Author, Title, Publisher) >
<!ATTLIST Book Pages CDATA #REQUIRED>
<!ELEMENT Author (#PCDATA)>
<!ELEMENT Title (#PCDATA)>
<!ELEMENT Publisher (#PCDATA)>

To use a DTD file as the schema for an XML file, you include a DOCTYPE node in the XML file:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE Books SYSTEM "books.dtd">
<Books>
 <Book Pages="1046">
  <Author>Delaney, Kalen</Author>
  <Title>Inside Microsoft SQL Server 2000</Title>
  <Publisher>Microsoft Press</Publisher>
 </Book>
 <Book Pages="1000">
  <Author>Gunderloy. Michael</Author>
  <Title>ADO and ADO.NET Programming</Title>
  <Publisher>Sybex</Publisher>
 </Book>
 <Book Pages="484">
  <Author>Cooper, James W.</Author>
  <Title>Visual Basic Design Patterns</Title>
  <Publisher>Addison Wesley</Publisher>
 </Book>
</Books>

The XmlValidatingReader class can validate an XML document for conformance with a DTD, as in the following sample code:

Private Sub ValidateIt()
 ' Load a document with an external schema
 Dim xtr As XmlTextReader = _
  New XmlTextReader("Books.xml")
 ' Prepare to validate it
 Dim xvr As XmlValidatingReader = _
  New XmlValidatingReader(xtr)
 xvr.ValidationType = ValidationType.DTD
 ' Tell the validator what to do with errors
 AddHandler xvr.ValidationEventHandler, _
  AddressOf ValidationHandler
 ' Load the document, thus validating
 Dim xd As XmlDocument = _
  New XmlDocument()
 xd.Load(xvr)
End Sub

Private Sub ValidationHandler( _
 ByVal sender As Object, _
 ByVal e As ValidationEventArgs)
 ' Dump any validation errors to the UI
 MessageBox.Show(e.Message)
End Sub

The only difference between validating against a schema file and validating against a DTD is in the constant chosen for the ValidationType property of the XmlValidatingReader.

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