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How to: Prepopulate XmlPreloadedResolver with a File Downloaded from URI Location

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Use the XmlPreloadedResolver if you have to prepopulate a cache with content that is going to be referenced more than once. This topic shows how to use XmlPreloadedResolver to prepopulate the cache with a DTD file that is later referenced from an XML file. To get the content of the DTD file from a URI location, you must use an asynchronous Web request.

To configure a Silverlight Visual Studio project to run this example

  1. In Solution Explorer, add assembly references to System.Xml.Linq.dll, System.Xml.Utils.dll, and System.Net.dll.

  2. Modify your page.xaml file so that it includes the following TextBlock element:

    <TextBlock x:Name ="OutputTextBlock" Canvas.Top ="10" TextWrapping="Wrap"/>

  3. Create a DTD file, which you will load from a URI location. The DTD file that is used in this example has the following content:

    <!ENTITY entity 'Replacement text'>

  4. In the page.xaml source file for your application, add the following using statements (Imports in Visual Basic):

    Imports System.IO
Imports System.Xml.Linq
Imports System.Net
Imports System.Xml.Resolvers
Imports System.Xml

    using System.IO;
using System.Xml.Linq;
using System.Net;
using System.Xml.Resolvers;
using System.Xml;

To cache the DTD file using XmlPreloadedResolver

  1. Create the WebClient object, add the handler, and initiate the request. To request a resource as a stream, you must call an OpenReadAsync method overload. Also initialize the httpURI member variable. In the next step, the XmlPreloadedResolver maps a stream with preloaded data to this URI.

    Dim wc As WebClient = New WebClient()
AddHandler wc.OpenReadCompleted, AddressOf wc_OpenReadCompleted
' Initialize httpURI member variable.
httpURI = New Uri(uriString)
wc.OpenReadAsync(httpURI)

    WebClient wc = new WebClient();
wc.OpenReadCompleted += wc_OpenReadCompleted;
// Initialize httpURI member variable.
httpURI = new Uri(uriString);
wc.OpenReadAsync(httpURI);

  2. Implement the wc_OpenReadCompleted callback function. This function does the following:

    1. Checks the Error propertyfor errors.

    2. If there are no errors, gets the data stream.

    3. Adds the stream with preloaded data to the XmlPreloadedResolver store and maps it to httpURI.

    4. Assigns the resolver to the XmlReaderSettings.

    5. Creates an XML string that references the DTD.

    6. Creates an XmlReader using the specified TextReader that contains the XML data and XmlReaderSettings objects.

    7. Creates a new XDocument and passes the XmlReader object to the Load method overload.

    Private Sub wc_OpenReadCompleted(ByVal sender As Object, ByVal e As OpenReadCompletedEventArgs)
    If e.Error IsNot Nothing Then
        OutputTextBlock.Text = e.Error.Message
        Return
    End If
    Using stream As Stream = e.Result
        Dim resolver As XmlPreloadedResolver = New XmlPreloadedResolver()

        ' Add the DTD into the XmlPreloadedResolver.
        ' The XML string below references this DTD Uri using the full path.
        resolver.Add(httpURI, stream)

        ' Assign the resolver to the XmlReaderSettings.
        Dim settings As XmlReaderSettings = New XmlReaderSettings()
        settings.XmlResolver = resolver
        settings.DtdProcessing = DtdProcessing.Parse

        ' To reference the DTD file, include its full URI path. 
        Dim strXml As String = _
            "<!DOCTYPE doc SYSTEM '" & uriString & "'><doc>&entity;</doc>"

        Using reader As XmlReader = XmlReader.Create(New StringReader(strXml), settings)
            Dim doc As XDocument = XDocument.Load(reader)
            ' Output the content of the document object.  
            OutputTextBlock.Text = doc.ToString()
        End Using
    End Using
End Sub

    private void wc_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
{
    if (e.Error != null)
    {
        OutputTextBlock.Text = e.Error.Message;
        return;
    }
    using (Stream stream = e.Result)
    {
        XmlPreloadedResolver resolver = new XmlPreloadedResolver();

        // Add the DTD into the XmlPreloadedResolver.
        // The XML string below references this DTD using the full path.
        resolver.Add(httpURI, stream);

        // Assign the resolver to the XmlReaderSettings.
        XmlReaderSettings settings = new XmlReaderSettings();
        settings.XmlResolver = resolver;
        settings.DtdProcessing = DtdProcessing.Parse;

        // To reference the DTD file, include its full URI path. 
        String strXml = @"<!DOCTYPE doc SYSTEM '" + uriString + "'>" +
             "<doc>&entity;</doc>";

        using (XmlReader reader = XmlReader.Create(new StringReader(strXml), settings))
        {
            XDocument document = XDocument.Load(reader);
            // Output the content of the document object.  
            OutputTextBlock.Text = document.ToString();
        }
    }
}