You can validate an XML document against a schema, by using built in XmlReader’s validation features. Note: You can find more details about XML schemas from the article How to use XML Schemas in XML document. You can follow the next steps when performing validation:
1. You need to import the System.Xml.Schema namespace, which contains types such as XmlSchema and XmlSchemaCollection:
Imports System.Xml.Schema
2. You need to create the validation reader, by using XmlReaderSettings object that specifically indicates that you want to perform validation. You can do this by setting the ValidationType property and loading your XSD schema file into the Schemas collection, as shown here:
‘ Configure the reader to use validation.
Dim Settings As XmlReaderSettings = New XmlReaderSettings()
Settings.ValidationType = ValidationType.Schema
‘ Create the path for the schema file.
Dim schemaFile As String = Path.Combine(Request.PhysicalApplicationPath, “App_Data\BooksList.xsd”);
‘ Indicate that elements in the namespace
‘ https://www.Books.com/BooksList should be validated using the schema file.
Settings.Schemas.Add(“urn:books”,SchemaFile);
3. You need to create the validation reader, by using the static XmlReader.Create() method. This method has several overloads, but you can use a FileStream (with the XML document) and the XmlReaderSettings object that has your validation settings:
‘ Open the XML file.
Dim Fs As FileStream = New FileStream(File, FileMode.Open)
‘ Create the validating reader.
Dim R As XmlReader = XmlReader.Create(Fs, Settings)
The XmlReader in this example works in the same way as is explained in the articles How to use XmlTextReader to read XML file as text in VB.NET or How to use XmlTextReader to read XML file as objects in VB.NET, but it adds the ability to verify that the XML document follows the schema rules. This reader throws an exception (or raises an event) to indicate errors as you move through the XML file.
The following example shows how you can create a validating reader that uses the BooksList.xsd file to verify that the XML in BooksList.xml is valid:
Imports System.Collections.Generic
Imports System.Linq
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.IO
Imports System.Xml
Imports System.Xml.Schema
Public Class XMLValidator
Inherits System.Web.UI.Page
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not Me.IsPostBack Then
XMLValidator()
End If
End Sub
Protected Sub XMLValidator()
‘ Configure the reader to use validation.
Dim Settings As XmlReaderSettings = New XmlReaderSettings()
Settings.ValidationType = ValidationType.Schema
‘ Create the path for the schema file.
Dim SchemaFile As String = Path.Combine(Request.PhysicalApplicationPath, “App_Data\BooksList.xsd”)
‘Indicate that elements in the namespace
‘ https://www.Books.com/BooksList should be validated using the schema file.
Settings.Schemas.Add(“urn:books”, SchemaFile)
Settings.ValidationFlags = Settings.ValidationFlags Or XmlSchemaValidationFlags.ProcessInlineSchema
Settings.ValidationFlags = Settings.ValidationFlags Or XmlSchemaValidationFlags.ReportValidationWarnings
‘ Open a stream to the file.
Dim File As String = Path.Combine(Request.PhysicalApplicationPath, “App_Data\BooksList.xml”)
Dim Fs As New FileStream(File, FileMode.Open)
Dim R As XmlReader = XmlReader.Create(Fs, Settings)
‘ Create a generic collection of books.
Dim Books As New List(Of Book)()
‘ Loop through the books.
LblStatus.Text = “”
While R.Read()
If R.NodeType = XmlNodeType.Element AndAlso R.Name = “Book” Then
Dim newBook As New Book()
newBook.Isbn13 = R.GetAttribute(“ISBN-13”)
newBook.Title = R.GetAttribute(“Title”)
‘ Get the rest of the subtags for this book
While R.NodeType <> XmlNodeType.EndElement
R.Read()
‘ Look for Author subtags.
If R.Name = “Author” Then
While R.NodeType <> XmlNodeType.EndElement
R.Read()
If R.NodeType = XmlNodeType.Text Then
newBook.Author = R.Value
End If
End While
End If
R.Read()
If R.Name = “Price” Then
While R.NodeType <> XmlNodeType.EndElement
R.Read()
If R.NodeType = XmlNodeType.Text Then
newBook.Price = Decimal.Parse(R.Value)
End If
End While
End If
R.Read()
If R.Name = “Available” Then
While R.NodeType <> XmlNodeType.EndElement
R.Read()
If R.NodeType = XmlNodeType.Text Then
newBook.Available = Boolean.Parse(R.Value)
End If
End While
End If
End While
Books.Add(newBook)
End If
End While
‘ end of while
Fs.Close()
If (Not LblStatus.Text.Contains(“Error”)) Then
GridResult.DataSource = Books
GridResult.DataBind()
End If
End Sub
End Class
Using the current file, this code will succeed, and you’ll be able to access each node in the document. However, consider what happens if you make the minor modification shown here:
<Author>J.K. Rowling</Author>
<Price>1x.99</Price>
Now when you try to validate the document, an XmlSchemaException (from the System.Xml.Schema namespace) will be thrown, alerting you to the invalid data type, as shown in:
Validating of an XML document with XML Schemas in VB.NET
Instead of catching errors, you can react to the XmlReaderSettings.ValidationEventHandler event. If you react to this event, you’ll be provided with information about the error, but no exception will be thrown. To connect an event handler to this event, you can attach an event handler before you create the XmlReader:
‘ Connect to the method named ValidateHandler.
AddHandler Settings.ValidationEventHandler, AddressOf ValidateHandler
The event handler receives a ValidationEventArgs object as a parameter, which contains the exception, a message, and a number representing the severity:
Public Sub ValidateHandler(sender As Object, args As ValidationEventArgs)
If (args.Severity = XmlSeverityType.Error) Then
LblStatus.Text += “Error: ” + ” Validation error: ” + args.Message
End If
End Sub
The next picture shows the result of failed validation attempt:
Using of event handler in XML document validation against XML schema in VB.NET