XmlUrlResolver Class

Definition

Namespace:

System.Xml

Assembly:

System.Xml.ReaderWriter.dll

Source:

XmlUrlResolver.cs

Resolves external XML resources named by a Uniform Resource Identifier (URI).

public ref class XmlUrlResolver : System::Xml::XmlResolver

public class XmlUrlResolver : System.Xml.XmlResolver

type XmlUrlResolver = class
    inherit XmlResolver

Public Class XmlUrlResolver
Inherits XmlResolver

Inheritance

Object

XmlResolver

XmlUrlResolver

Examples

The following example creates an XmlReader that uses an XmlUrlResolver with default credentials.

// Create an XmlUrlResolver with default credentials.
XmlUrlResolver resolver = new XmlUrlResolver();
resolver.Credentials = CredentialCache.DefaultCredentials;

// Create the reader.
XmlReaderSettings settings = new XmlReaderSettings();
settings.XmlResolver = resolver;
XmlReader reader =
     XmlReader.Create("http://serverName/data/books.xml", settings);

' Create an XmlUrlResolver with default credentials.
Dim resolver As New XmlUrlResolver()
resolver.Credentials = CredentialCache.DefaultCredentials

' Create the reader.
Dim settings As New XmlReaderSettings()
settings.XmlResolver = resolver
Dim reader As XmlReader = _
   XmlReader.Create("http://serverName/data/books.xml", settings)

Remarks

XmlUrlResolver is used to resolve external XML resources such as entities, document type definitions (DTDs) or schemas. It is also used to process include and import elements found in Extensible StyleSheet Language (XSL) style sheets or XML Schema definition language (XSD) schemas.

XmlUrlResolver is the default resolver for all classes in the System.Xml namespace. It supports the file:// and http:// protocols and requests from the WebRequest class.

Important

XmlUrlResolver objects can contain sensitive information such as user credentials. You should be careful when you cache XmlUrlResolver objects and should not pass XmlUrlResolver objects to an untrusted component.

Resolving DTDs

If an XML reader (XmlReader) is reading an XML file that contains an external DTD, it calls the XmlUrlResolver.GetEntityAsync method to get a stream representation of the DTD. If the URI of the DTD is a relative URI, the XML reader calls the XmlUrlResolver.ResolveUri method and returns an absolute URI for the given relativeUri and baseURi parameters. If the XmlUrlResolver doesn't know how to resolve the URI, it returns null.

The XmlUrlResolver.GetEntity method uses the information in the Credentials property as appropriate to gain access to the resource. There is no get accessor to this property for security reasons. When overwriting XmlResolver, GetEntity is the method that utilizes the credential information in the Credentials property.

Resolving all other XML resources is very similar to resolving DTDs. XmlResolver negotiates the connection with the external resource and returns a Stream representation of the content. The object that is making the call to XmlResolver interprets the stream.

Extending the XmlUrlResolver class

The default behavior of the XmlUrlResolver class is to resolve an XML data resource from its source, not from cache. In some cases, resolving a data resource from cache can improve the performance of an application by saving a trip to the data resource's server. The performance gains here must be weighed against the need for up-to-date content.

The following example extends XmlUrlResolver and builds a new class, XmlCachingResolver, to retrieve resources from the cache. This is done by overriding the XmlUrlResolver.Credentials property and the XmlUrlResolver.GetEntity method.

class XmlCachingResolver : XmlUrlResolver
{
    bool enableHttpCaching;
    ICredentials credentials;

    //resolve resources from cache (if possible) when enableHttpCaching is set to true
    //resolve resources from source when enableHttpcaching is set to false
    public XmlCachingResolver(bool enableHttpCaching)
    {
        this.enableHttpCaching = enableHttpCaching;
    }

    public override ICredentials Credentials
    {
        set
        {
            credentials = value;
            base.Credentials = value;
        }
    }

    public override object GetEntity(Uri absoluteUri, string role, Type ofObjectToReturn)
    {
        if (absoluteUri == null)
        {
            throw new ArgumentNullException("absoluteUri");
        }
        //resolve resources from cache (if possible)
        if (absoluteUri.Scheme == "http" && enableHttpCaching && (ofObjectToReturn == null || ofObjectToReturn == typeof(Stream)))
        {
            WebRequest webReq = WebRequest.Create(absoluteUri);
            webReq.CachePolicy = new HttpRequestCachePolicy(HttpRequestCacheLevel.Default);
            if (credentials != null)
            {
                webReq.Credentials = credentials;
            }
            WebResponse resp = webReq.GetResponse();
            return resp.GetResponseStream();
        }
        //otherwise use the default behavior of the XmlUrlResolver class (resolve resources from source)
        else
        {
            return base.GetEntity(absoluteUri, role, ofObjectToReturn);
        }
    }
}

Class XmlCachingResolver
    Inherits XmlUrlResolver
    Dim enableHttpCaching As Boolean
    Public Shadows Credentials As ICredentials

    'resolve resources from cache (if possible) when enableHttpCaching is set to true
    'resolve resources from source when enableHttpcaching is set to false
    Public Sub New(ByVal enableHttpCaching As Boolean)
        Me.enableHttpCaching = enableHttpCaching
    End Sub

    Public Shadows Function GetEntity(ByVal absoluteUri As Uri, ByVal role As String, ByVal returnType As Type) As Object
        If absoluteUri = Nothing Then
            Throw New ArgumentNullException("absoluteUri")
        End If

        'resolve resources from cache (if possible)
        If absoluteUri.Scheme = "http" And enableHttpCaching And (returnType Is GetType(Nullable) Or returnType Is GetType(Stream)) Then
            Dim webReq As WebRequest = WebRequest.Create(absoluteUri)
            webReq.CachePolicy = New HttpRequestCachePolicy(HttpRequestCacheLevel.Default)
            If Not (Credentials Is Nothing) Then
                webReq.Credentials = Credentials
            End If
            Dim resp As WebResponse = webReq.GetResponse()
            Return resp.GetResponseStream()
            'otherwise use the default behavior of the XmlUrlResolver class (resolve resources from source)
        Else
            Return MyBase.GetEntity(absoluteUri, role, returnType)
        End If

    End Function
End Class

The caching behavior of the XmlCachingResolver class is implemented in the GetEntity method. This is done by creating new WebRequest and HttpRequestCachePolicy objects. The HttpRequestCachePolicy object is created using the Default member of the HttpRequestCacheLevel enumeration.

The CachePolicy property of the WebRequest object is set with the HttpRequestCachePolicy object.

An instance of the XmlCachingResolver class is created with the Boolean enableHttpCaching. When this value is set to true, the instance resolves a resource from the default cache if possible. When enableHttpCaching is set to false, the instance uses the default behavior and resolves resources from their source.

Note

This example leverages the extensibility of the XML classes in the .NET Framework. Other classes can be extended and customized to suit the needs of a particular application.

Constructors

XmlUrlResolver()

Initializes a new instance of the XmlUrlResolver class.

Properties

CachePolicy	Gets or sets the cache policy for the underlying WebRequest object.
Credentials	Sets credentials used to authenticate web requests.
Proxy	Gets or sets the network proxy for the underlying WebRequest object.

Methods

Equals(Object)	Determines whether the specified object is equal to the current object. (Inherited from Object)
GetEntity(Uri, String, Type)	Maps a URI to an object that contains the actual resource.
GetEntityAsync(Uri, String, Type)	Asynchronously maps a URI to an object that contains the actual resource.
GetHashCode()	Serves as the default hash function. (Inherited from Object)
GetType()	Gets the Type of the current instance. (Inherited from Object)
MemberwiseClone()	Creates a shallow copy of the current Object. (Inherited from Object)
ResolveUri(Uri, String)	Resolves the absolute URI from the base and relative URIs.
SupportsType(Uri, Type)	Enables the resolver to return types other than Stream. (Inherited from XmlResolver)
ToString()	Returns a string that represents the current object. (Inherited from Object)

See also

• XML Documents and Data