HOW TO: Modify an XML Schema by Using System.Xml.Schema Classes in Visual Basic .NET (317903)

SUMMARY

This step-by-step article shows how to modify an existing XML schema.

The System.Xml.Schema namespace contains the XML classes that provide standards-based support for XML schema definition language (XSD) schemas.

The XmlSchema class contains the definition of a schema. All XSD elements are children of the schema element; this represents the World Wide Web Consortium (W3C) schema element. You can use these classes to generate new XML schemas or modify existing ones.

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Requirements

The following list outlines the recommended hardware, software, network infrastructure, and service packs that are required:

• Microsoft Visual Studio .NET installed on a compatible Microsoft Windows operating system

This article assumes that you are familiar with the following topics:

• Visual Basic .NET language
• XML standards
• XSD schemas

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Create an XML Schema File

1. Paste the following code in a text file, and then save the file as C:\Book.xsd:

   <xs:schema xmlns="urn:bookstore-schema"
            targetNamespace="urn:bookstore-schema"
            xmlns:xs="http://www.w3.org/2001/XMLSchema">
      <xs:element name="book" type="booktype"/>
         <xs:complexType name="booktype">
            <xs:sequence>
               <xs:element name="author" type="xs:string"/>
                  <xs:element name="price" type="xs:decimal" />
                  </xs:sequence>
         </xs:complexType>
   </xs:schema>
   					

2. Modify the author of the element name.
3. Set the minOccurs and maxOccurs properties.
4. Modify the data type of the element price.
5. Add a new element publisher.

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Create a Visual Basic .NET Application to Modify the Schema

To add a new publisher child element to the book element of the schema, follow these steps:

1. Create a new Visual Basic .NET Console application project.
2. Replace the code in Module1.vb with the following:

   Imports System.Xml
   Imports System.Xml.Schema
   Imports System.IO
   
   Module Module1
   
       Sub Main()
           Dim xsd As String = "c:\book.xsd"
           Dim fs As FileStream
           Dim schema As XmlSchema
           Try
               fs = New FileStream(xsd, FileMode.Open)
               schema = XmlSchema.Read(fs, New ValidationEventHandler(AddressOf ShowCompileError))
   
               schema.Compile(AddressOf ShowCompileError)
   
               'Create a SchemaObjectTable with the elements of the schema.
               Dim schematable As XmlSchemaObjectTable
               schematable = schema.Elements
   
               Dim qname As New XmlQualifiedName("book", "urn:bookstore-schema")
               Dim bookelement As New XmlSchemaElement()
               Dim complexelement As New XmlSchemaComplexType()
               If schematable.Contains(qname) Then
                   bookelement = CType(schematable.Item(qname), XmlSchemaElement)
   
                   Dim simpletype As XmlSchemaSimpleType
   
                   If (bookelement.SchemaTypeName.ToString = "urn:bookstore-schema:booktype") Then
                       complexelement = schema.SchemaTypes.Item(bookelement.SchemaTypeName)
                       Dim seqelement As New XmlSchemaSequence()
                       seqelement = complexelement.Particle
                       'Modify author element to show maxOccurs and minOccurs.
                       Dim eleauthor As New XmlSchemaElement()
                       eleauthor = seqelement.Items(0)
                       'Modify the author element name.
                       eleauthor.Name = "authorname"
                       'Add min and max.
                       eleauthor.MaxOccurs = 1
                       eleauthor.MinOccurs = 1
   
                       Dim eleprice As New XmlSchemaElement()
                       eleprice = seqelement.Items(1)
                       'Modify the type of price element.
                       eleprice.SchemaTypeName = New XmlQualifiedName("decimal", "http://www.w3.org/2001/XMLSchema")
   
                       'Create a new element and add to the schema
                       Dim Newelement As New XmlSchemaElement()
                       Newelement.Name = "Publisher"
   
                       Newelement.SchemaTypeName = New XmlQualifiedName("string", "http://www.w3.org/2001/XMLSchema")
   
                       seqelement.Items.Add(Newelement)
                   End If
   
               End If
   
   
               'Display the schema in the Output window.
               schema.Write(Console.Out)
               Console.Read()
               'Save to the hard disk.
               Dim Newschema As New StreamWriter("c:\Newbooks.xsd", False)
               schema.Write(Newschema)
   
           Catch exp As XmlSchemaException
               Console.WriteLine(exp.Message)
           Catch exp As XmlException
               Console.WriteLine(exp.Message)
           Catch exp As Exception
               Console.WriteLine(exp.Message)
           Finally
               Console.Read()
           End Try
   
   
       End Sub
       Public Sub ShowCompileError(ByVal sender As Object, ByVal e As ValidationEventArgs)
           Console.WriteLine("Validation Error: {0}", e.Message)
       End Sub
   End Module
   						

   XmlSchemaObjectTable is a collection class that provides read-only helpers for XmlSchemaObject objects. This class is used to provide the collections for contained elements that are in the schema as collections that are accessed from the XmlSchema class; for example, Attributes, AttributeGroups, and Elements.
   
   Because you are trying to add a child element publisher to a book element, add the new element to the XmlSchemaSequence. The XmlSchemaSequence class represents the World Wide Web Consortium SEQUENCE (compositor) element.
3. Press F5 to build and run the application.
   
   The new schema that includes the publisher element appears in the Console output area. Also, the XSD file named Newbook.xsd is created in the C:\ folder.

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