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Generating and Using XSD Schemas

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

  1. Add a new form to the project. Name the new form StepByStep2_10.cs.

  2. Add a Button control (btnGetSchema) and a TextBox control (txtSchema) to the form. Set the MultiLine property of the TextBox to true and set its ScrollBars property to Vertical.

  3. Switch to the code view and add the following using directives:

    using System.Xml;
    using System.Data;
    using System.IO;
  4. Double-click the Button controls and add code to process an XML document when you click the Button control:

    private void btnGetSchema_Click(
      object sender, System.EventArgs e)
    {
      // Load the XML file with inline schema info
      XmlTextReader xtr = new
        XmlTextReader(@"..\..\Products.xml");
      // Read the schema (only) into a DataSet
      DataSet ds = new DataSet();
      ds.ReadXmlSchema(xtr);
      // Write the schema out as a separate stream
      StringWriter sw = new StringWriter();
      ds.WriteXmlSchema(sw);
      txtSchema.Text = sw.ToString();
      // Clean up
      xtr.Close();
    }
  5. 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>

NOTE

Generating an Inline Schema The Products.xml file is generated by exporting a portion of the Products table from the Northwind sample database in Microsoft Access 2002.

  1. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  2. Run the project. Click the button to load the XML file and extract the inline schema information to the text box, as shown in Figure 2.9.

Figure 2.9Figure 2.9 You can extract XSD schema information from an XML file by using the DataSet.ReadXmlSchema() method.

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 can 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

  1. Add a new form to the project. Name the new form StepByStep2_11.cs.

  2. Add a Button control (btnInferSchema) and a TextBox control (txtSchema) to the form. Set the MultiLine property of the TextBox to true and set its ScrollBars property to Vertical.

  3. Switch to the code view and add the following using directives:

    using System.Xml;
    using System.Data;
    using System.IO;
  4. Double-click the Button controls and add code to process an XML document when you click the Button control:

    private void btnInferSchema_Click(
      object sender, System.EventArgs e)
    {
      // Load an XML file with no schema information
      XmlTextReader xtr = new
        XmlTextReader(@"..\..\Books.xml");
      // Read the schema (only) into a DataSet
      DataSet ds = new DataSet();
      String[] ns = {};
      ds.InferXmlSchema(xtr, ns);
      // Write the schema out as a separate stream
      StringWriter sw = new StringWriter();
      ds.WriteXmlSchema(sw);
      txtSchema.Text = sw.ToString();
      // Clean up
      xtr.Close();}
  5. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  6. Run the project. Click the button to load the XML file and infer the schema information to the text box, as shown in Figure 2.10.

Figure 2.10Figure 2.10 You can infer schema information from an XML file by using the DataSet.InferXmlSchema() method.

To summarize, there are at least four ways that you can 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 that you learned in Chapter 1.

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

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

NOTE

XML Schema Definition Tool (xsd.exe) You can use the .NET Framework XML Schema Definition Tool (xsd.exe) to:

Generate an XML schema document from a DLL or EXE.

Generate a C# class file that conforms to the given XML schema file.

Generate an XML schema from an XML Data Reduced (XDR) schema file.

Generate an XML schema from an XML file.

Generate DataSet classes from an XML schema file.

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 Description (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 implements the XmlReader class and provides support for validating XML documents. It can also validate the XML 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 Against an Inline Schema

  1. Add a new form to the project. Name the new form StepByStep2_12.cs.

  2. Add a Button control (btnValidate) and a TextBox control (txtErrors) to the form. Set the MultiLine property of the TextBox to true and set its ScrollBars property to Vertical.

  3. Switch to the code view and add the following using directives:

    using System.Xml;
    using System.Xml.Schema;
  4. Double-click the Button controls and add code to validate an XML document when you click the Button control:

    private void btnValidate_Click(
      object sender, System.EventArgs e)
    {
      // Load a document with an inline schema
      XmlTextReader xtr =
        new XmlTextReader(@"..\..\Products.xml");
      // Prepare to validate it
      XmlValidatingReader xvr =
        new XmlValidatingReader(xtr);
      xvr.ValidationType = ValidationType.Schema;
      // Tell the validator what to do with errors
      xvr.ValidationEventHandler +=
        new ValidationEventHandler(ValidationHandler);
      // Load the document, thus validating
      XmlDocument xd = new XmlDocument();
      xd.Load(xvr);
      // Clean up
      xvr.Close();
    }
    
    public void ValidationHandler(
      object sender, ValidationEventArgs e)
    {
      // Dump any validation errors to the UI
      txtErrors.AppendText(e.Message + "\n");
    }
  5. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  6. Run the project. Click the button to load and simultaneously validate the XML file, as shown in Figure 2.11.

Figure 2.11Figure 2.11 You can validate an XML document by using XmlValidatingReader class.

  1. 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.

  2. 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 2.12Figure 2.12 While the XML document is validated, the XmlValidatingReader object raises a ValidationEventHandler event for each error in the XML document.

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 object 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 lets your code 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 object 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 Against an External Schema

  1. Add a new form to the project. Name the new form StepByStep2_13.cs.

  2. Add a Button control (btnValidate) and a TextBox control (txtErrors) to the form. Set the MultiLine property of the TextBox to true and set its ScrollBars property to Vertical.

  3. Switch to the code view and add the following using directives:

    using System.Xml;
    using System.Xml.Schema;
  4. Double-click the Button controls and add code to validate an XML document when you click the Button control:

    private void btnValidate_Click(
      object sender, System.EventArgs e)
    {
      // Load a document with an external schema
      XmlTextReader xtr =
        new XmlTextReader(@"..\..\Books2.xml");
      // Prepare to validate it
      XmlValidatingReader xvr =
        new XmlValidatingReader(xtr);
      xvr.ValidationType = ValidationType.Schema;
      // Tell the validator what to do with errors
      xvr.ValidationEventHandler +=
        new ValidationEventHandler(ValidationHandler);
      // Load the schema
      XmlSchemaCollection xsc =
        new XmlSchemaCollection();
      xsc.Add("xsdBooks", @"..\..\Books2.xsd");
      // Tell the validator which schema to use
      xvr.Schemas.Add(xsc);
      // Load the document, thus validating
      XmlDocument xd = new XmlDocument();
      xd.Load(xvr);
      // Clean up
      xvr.Close();
    }
    public void ValidationHandler(
      object sender, ValidationEventArgs e)
    {
      // Dump any validation errors to the UI
      txtErrors.AppendText(e.Message + "\n");
    }
  5. 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="1088">
        <Author>Delaney, Kalen</Author>
        <Title>Inside Microsoft SQL Server 2000</Title>
        <Publisher>Microsoft Press</Publisher>
      </Book>
      <Book Pages="997">
        <Author>Burton, Kevin</Author>
        <Title>.NET Common Language Runtime</Title>
        <Publisher>Sams</Publisher>
      </Book>
      <Book Pages="392">
        <Author>Cooper, James W.</Author>
        <Title>C# Design Patterns</Title>
        <Publisher>Addison Wesley</Publisher>
      </Book>
    </Books>
  6. 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>
  7. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  8. 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.

  9. 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.

  10. 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 2.13Figure 2.13 To validate against an external schema, you need to add the schema to the XmlValidatingReader.Schemas collection.

Validating Against a DTD

Using schema files is 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 Against a DTD

  1. Add a new form to the project. Name the new form StepByStep2_14.cs.

  2. Add a Button control (btnValidate) and a TextBox control (txtErrors) to the form. Set the MultiLine property of the TextBox to true and set its ScrollBars property to Vertical.

  3. Switch to the code view and add the following using directives:

    using System.Xml;
    using System.Xml.Schema;
  4. Double-click the Button controls and add code to validate an XML document when you click the Button control:

    private void btnValidate_Click(
      object sender, System.EventArgs e)
    {
      // Load a document with a DTD
      XmlTextReader xtr =
        new XmlTextReader(@"..\..\Books3.xml");
      // Prepare to validate it
      XmlValidatingReader xvr =
        new XmlValidatingReader(xtr);
      xvr.ValidationType = ValidationType.DTD;
      // Tell the validator what to do with errors
      xvr.ValidationEventHandler +=
        new ValidationEventHandler(ValidationHandler);
      // Load the document, thus validating
      XmlDocument xd = new XmlDocument();
      xd.Load(xvr);
      // Clean up
      xvr.Close();
    }
    
    public void ValidationHandler(
      object sender, ValidationEventArgs e)
    {
      // Dump any validation errors to the UI
      txtErrors.AppendText(e.Message + "\n");
    }
  5. 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="1088">
        <Author>Delaney, Kalen</Author>
        <Title>Inside Microsoft SQL Server 2000</Title>
        <Publisher>Microsoft Press</Publisher>
      </Book>
      <Book Pages="997">
        <Author>Burton, Kevin</Author>
        <Title>.NET Common Language Runtime</Title>
        <Publisher>Sams</Publisher>
      </Book>
      <Book Pages="392">
        <Author>Cooper, James W.</Author>
        <Title>C# Design Patterns</Title>
        <Publisher>Addison Wesley</Publisher>
      </Book>
    </Books>

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.

  1. 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)>
  2. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  3. 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.

  4. 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.

  5. 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 2.14Figure 2.14 You can use the XmlValidatingReader class to validate an XML document against a DTD by setting its ValidationType property to ValidationType.DTD.

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 object.

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.

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