Access and Manipulate XML Data: Generate and use an XSD schema.
In Chapter 1, "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. 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 in Step By Step 2.10.
STEP BY STEP 2.10 Extracting an XML Schema -
Add a new form to the project. Name the new form StepByStep2-10.vb. -
Add a Button control named btnGetSchema and a TextBox control named txtSchema to the form. Set the Multiline property of the TextBox to True and set its ScrollBars property to Vertical . -
Double-click the Button control to open the form's module. Add these lines of code at the top of the module: Imports System.Data Imports System.IO Imports System.Xml.Xpath Imports System.Xml -
Add code to process an XML document when you click the Button control: Dim xpd As XPathDocument Private Sub btnGetSchema_Click(_ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles btnGetSchema.Click ' 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() End Sub -
Add a new XML file to the project. Name the new file Products.xml. Add this XML to the new file: <?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:element name="SupplierID" minOccurs="0" od:jetType="longinteger" od:sqlSType="int"> <xsd:simpleType> <xsd:restriction base="xsd:integer"/> </xsd:simpleType> </xsd:element> <xsd:element name="CategoryID" minOccurs="0" od:jetType="longinteger" od:sqlSType="int"> <xsd:simpleType> <xsd:restriction base="xsd:integer"/> </xsd:simpleType> </xsd:element> <xsd:element name="QuantityPerUnit" minOccurs="0" od:jetType="text" od:sqlSType="nvarchar"> <xsd:simpleType> <xsd:restriction base="xsd:string"> <xsd:maxLength value="20"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="UnitPrice" minOccurs="0" od:jetType="currency" od:sqlSType="money" type="xsd:double"/> <xsd:element name="UnitsInStock" minOccurs="0" od:jetType="integer" od:sqlSType="smallint" type="xsd:short"/> <xsd:element name="UnitsOnOrder" minOccurs="0" od:jetType="integer" od:sqlSType="smallint" type="xsd:short"/> <xsd:element name="ReorderLevel" minOccurs="0" od:jetType="integer" od:sqlSType="smallint" type="xsd:short"/> <xsd:element name="Discontinued" od:jetType="yesno" od:sqlSType="bit" od:nonNullable="yes" type="xsd:byte"/> </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> <SupplierID>1</SupplierID> <CategoryID>1</CategoryID> <QuantityPerUnit>10 boxes x 20 bags</QuantityPerUnit> <UnitPrice>18</UnitPrice> <UnitsInStock>39</UnitsInStock> <UnitsOnOrder>0</UnitsOnOrder> <ReorderLevel>10</ReorderLevel> <Discontinued>0</Discontinued> </Products> <Products> <ProductID>2</ProductID> <ProductName>Chang</ProductName> <SupplierID>1</SupplierID> <CategoryID>1</CategoryID> <QuantityPerUnit>24 - 12 oz bottles</QuantityPerUnit> <UnitPrice>19</UnitPrice> <UnitsInStock>17</UnitsInStock> <UnitsOnOrder>40</UnitsOnOrder> <ReorderLevel>25</ReorderLevel> <Discontinued>0</Discontinued> </Products> <Products> <ProductID>3</ProductID> <ProductName>Aniseed Syrup</ProductName> <SupplierID>1</SupplierID> <CategoryID>2</CategoryID> <QuantityPerUnit>12 - 550 ml bottles</QuantityPerUnit> <UnitPrice>10</UnitPrice> <UnitsInStock>13</UnitsInStock> <UnitsOnOrder>70</UnitsOnOrder> <ReorderLevel>25</ReorderLevel> <Discontinued>0</Discontinued> </Products> <Products> <ProductID>4</ProductID> <ProductName><![CDATA[Chef Anton's Cajun Seasoning]]> </ProductName> <SupplierID>2</SupplierID> <CategoryID>2</CategoryID> <QuantityPerUnit>48 - 6 oz jars</QuantityPerUnit> <UnitPrice>22</UnitPrice> <UnitsInStock>53</UnitsInStock> <UnitsOnOrder>0</UnitsOnOrder> <ReorderLevel>0</ReorderLevel> <Discontinued>0</Discontinued> </Products> <Products> <ProductID>5</ProductID> <ProductName><![CDATA[Chef Anton's Gumbo Mix]]> </ProductName> <SupplierID>2</SupplierID> <CategoryID>2</CategoryID> <QuantityPerUnit>36 boxes</QuantityPerUnit> <UnitPrice>21.35</UnitPrice> <UnitsInStock>0</UnitsInStock> <UnitsOnOrder>0</UnitsOnOrder> <ReorderLevel>0</ReorderLevel> <Discontinued>1</Discontinued> </Products> <Products> <ProductID>6</ProductID> <ProductName><![CDATA[Grandma's Boysenberry Spread]]> </ProductName> <SupplierID>3</SupplierID> <CategoryID>2</CategoryID> <QuantityPerUnit>12 - 8 oz jars</QuantityPerUnit> <UnitPrice>25</UnitPrice> <UnitsInStock>120</UnitsInStock> <UnitsOnOrder>0</UnitsOnOrder> <ReorderLevel>25</ReorderLevel> <Discontinued>0</Discontinued> </Products> </dataroot> </root> -
Set the form as the startup form for the project. -
Run the project. Click the button to load the XML file and extract the inline schema information to the TextBox, as shown in Figure 2.9. Figure 2.9. XSD Schema information extracted from an XML file.  |
NOTE
Generating Inline Schema This particular XML file was originally generated by exporting a portion of the Products table from the Northwind sample database in Microsoft Access 2002.
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 ability to infer an XML schema based on the data in an XML file. Step By Step 2.11 demonstrates this technique.
STEP BY STEP 2.11 Inferring an XML Schema -
Add a new form to the project. Name the new form StepByStep2-11.vb. -
Add a Button control named btnInferSchema and a TextBox control named txtSchema to the form. Set the Multiline property of the TextBox to True and set its ScrollBars property to Vertical . -
Double-click the Button control to open the form's module. Add these lines of code at the top of the module: Imports System.Data Imports System.IO Imports System.Xml -
Add code to process an XML document when you click the Button control: Private Sub btnInferSchema_Click(_ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles btnInferSchema.Click ' 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() End Sub -
Set the form as the startup form for the project. -
Run the project. Click the button to load the XML file and infer the schema information to the TextBox, as shown in Figure 2.10. Figure 2.10. XSD Schema information inferred from an XML file.  |
To summarize, there are at least four ways that you can obtain XSD files for your applications:
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You can use a file generated by an external application such as Microsoft SQL Server or Microsoft Access.
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You can create your own schema files from scratch, using the techniques that you learned in Chapter 1.
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You can extract inline schema information from an XML file, using the DataSet.ReadXmlSchema method.
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You can infer schema information from an XML file, using the DataSet.InferXmlSchema method.
Using an XSD Schema
Access and Manipulate XML Data: Validate an XML Document.
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.
In this section, you'll see the programmatic support that the .NET Framework provides for validating XML files.
Validating Against XSD
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. Step By Step 2.12 shows how you can use the XmlValidatingReader object to validate an XML document with an inline schema.
STEP BY STEP 2.12 Validating an XML Document with an Inline Schema -
Add a new form to the project. Name the new form StepByStep2-12.vb. -
Add a Button control named btnValidate and a TextBox control named txtErrors to the form. Set the Multiline property of the TextBox to True and set its ScrollBars property to Vertical . -
Double-click the Button control to open the form's module. Add these lines of code at the top of the module: Imports System.Xml Imports System.Xml.Schema -
Add code to process an XML document when you click the Button control: Private Sub btnValidate_Click(_ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles btnValidate.Click ' 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 txtErrors.AppendText(e.Message & vbCrLf) End Sub -
Set the form as the startup form for the project. -
Run the project. Click the button to load and simultaneously validate the XML file, as shown in Figure 2.11. Figure 2.11. Validating with inline schema information.  -
Stop the project. Open the Products.xml file and make a change. For example, change the name of a child element from SupplierID to SupplierIdentifier . -
Run the project. Click the button to load and simultaneously validate the XML file. You'll see additional validation errors, as shown in Figure 2.12. Figure 2.12. Errors caused by invalid XML.  |
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. As you can see, the XmlValidatingReader 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.
When you change the name of an element in the XML, the XML remains well-formed. But because that name doesn't match the name in the schema file, that portion of the XML document becomes invalid. The XmlValidatingReader responds by raising additional events.
You can also validate an XML file against an external schema. Step By Step 2.13 shows this technique in action.
STEP BY STEP 2.13 Validating an XML Document with an External Schema -
Add a new form to the project. Name the new form StepByStep2-13.vb. -
Add a Button control named btnValidate and a TextBox control named txtErrors to the form. Set the Multiline property of the TextBox to True and set its ScrollBars property to Vertical . -
Double-click the Button control to open the form's module. Add these lines of code at the top of the module: Imports System.Xml Imports System.Xml.Schema -
Add code to process an XML document when you click the Button control: Private Sub btnValidate_Click(_ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles btnValidate.Click ' Load a document with an external schema Dim xtr As XmlTextReader = _ New XmlTextReader("..\Books2.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", "..\Books2.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 txtErrors.AppendText(e.Message & vbCrLf) End Sub -
Add a new XML file to the project. Name the new file Books2.xml. Enter the following text for Books2.xml: <?xml version="1.0" encoding="UTF-8" ?> <Books xmlns="xsdBooks"> <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> -
Add a new schema file to the project. Name the new schema file Books2.xsd. Enter the following text for Books2.xsd: <?xml version="1.0" encoding="utf-8" ?> <xs:schema id="Books" xmlns="xsdBooks" elementFormDefault="qualified" targetNamespace="xsdBooks" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="Books"> <xs:complexType> <xs:choice maxOccurs="unbounded"> <xs:element name="Book"> <xs:complexType> <xs:sequence> <xs:element name="Author" type="xs:string" /> <xs:element name="Title" type="xs:string" /> <xs:element name="Publisher" type="xs:string" /> </xs:sequence> <xs:attribute name="Pages" type="xs:string" /> </xs:complexType> </xs:element> </xs:choice> </xs:complexType> </xs:element> </xs:schema> -
Set the form as the startup form for the project. -
Run the project. Click the button to load and simultaneously validate the XML file. Because the file exactly matches the schema, you won't see any errors. -
Stop the project. Open the Books2.xml file and make a change. For example, change the name of a child element from Author to Writer . -
Run the project. Click the button to load and simultaneously validate the XML file. You'll see validation errors, as shown in Figure 2.13. Figure 2.13. Validating against an external schema file.  |
Validating Against a DTD
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 Standardized Generalized Markup Language (SGML) standard, from which both HTML and XML derive. The XmlValidatingReader class can also validate an XML document for conformance with a DTD, as you'll see in Step By Step 2.14.
NOTE
DTD Tutorial A good source for more information on DTDs is the XMLFiles.com DTD Tutorial, located at http://www.xmlfiles.com/dtd/.
STEP BY STEP 2.14 Validating an XML Document with a DTD -
Add a new form to the project. Name the new form StepByStep2-14.vb. -
Add a Button control named btnValidate and a TextBox control named txtErrors to the form. Set the Multiline property of the TextBox to True and set its ScrollBars property to Vertical . -
Double-click the Button control to open the form's module. Add these lines of code at the top of the module: Imports System.Xml Imports System.Xml.Schema -
Add code to process an XML document when you click the Button control: Private Sub btnValidate_Click(_ ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnValidate.Click ' Load a document with an external schema Dim xtr As XmlTextReader = _ New XmlTextReader("..\Books3.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 txtErrors.AppendText(e.Message & vbCrLf) End Sub -
Add a new XML file to the project. Name the new file Books3.xml. Enter the following text for Books3.xml: <?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> -
Add a new text file to the project. Name the new schema file Books.dtd. Enter the following text for Books.dtd: <!ELEMENT Books (Book)* > <!ELEMENT Book (Author, Title, Publisher) > <!ATTLIST Book Pages CDATA #REQUIRED> <!ELEMENT Author (#PCDATA)> <!ELEMENT Title (#PCDATA)> <!ELEMENT Publisher (#PCDATA)> -
Set the form as the startup form for the project. -
Run the project. Click the button to load and simultaneously validate the XML file. Because the file exactly matches the schema, you won't see any errors. -
Stop the project. Open the Books3.xml file and make a change. For example, change the name of a child element from Author to Writer . -
Run the project. Click the button to load and simultaneously validate the XML file. You'll see validation errors, as shown in Figure 2.14. Figure 2.14. Validating against a DTD.  |
NOTE
XDR Validation The XmlValidatingReader can also validate an XML file for conformance with an XML Data Reduced (XDR) specification. XDR is a standard that Microsoft briefly embraced for describing XML files before they settled on the more standard XSD. You're not likely to find many XDR files in common use.
If you inspect the code, you'll see that 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.
REVIEW BREAK -
You can extract an inline schema from an XML file by using the ReadXmlSchema method of the DataSet class. -
You can infer a schema from the structure of an XML file by using the InferXmlSchema method of the DataSet class. -
You can validate an XML document for conformance with an inline schema, an external schema, a DTD, or an XDR file by using the XmlValidatingReader class. |
