Signs on the Sand

An interesting question has been raised in microsoft.public.dotnet.xml newsgroup: how to compile XPath expression without a XML document at hands? XPathNavigator class does provide such functionality via Compile() method, but XPathNavigator is abstract class hence this functionality is available only to its implementers, such as internal DocumentXPathNavigator and XPathDocumentNavigator classes, which are accessible only via corresponding XmlDocument and XPathDocument.

Therefore obvious solutions are: using dummy XmlDocument or XPathDocument object to get XPathNavigator and make use of its Compile() method or implement dummy XPathNavigator class. Dummy object vs dummy implementation, hehe. Well, dummy implementation at least doesn't allocate memory, so I'm advocating this solution. Below is the implementation and its usage:

public sealed class XPathCompiler {
    private sealed class DummyXpathNavigator : XPathNavigator {
        public override XPathNavigator Clone() {
            return new DummyXpathNavigator();
        }

        public override XPathNodeType NodeType {
            get { return XPathNodeType.Root; }
        }

        public override string LocalName {
            get { return String.Empty; }
        }

        public override string NamespaceURI { 
            get { return String.Empty; }
        }

        public override string Name { 
            get { return String.Empty; }
        }

        public override string Prefix { 
            get { return String.Empty; }
        }

        public override string Value { 
            get { return String.Empty; }
        }

        public override string BaseURI {
            get { return String.Empty; }
        }
                        
        public override String XmlLang { 
            get { return String.Empty; }
        }

        public override bool IsEmptyElement { 
            get { return false; }
        }

        public override XmlNameTable NameTable {
            get { return null; }
        }

        public override bool HasAttributes {
            get { return false; }
        }

        public override string GetAttribute(string localName, 
                                string namespaceURI) {                
            return string.Empty;			
        }

        public override bool MoveToAttribute(string localName, 
                                string namespaceURI) {                
            return false;
        }

        public override bool MoveToFirstAttribute() {
            return false;
        }

        public override bool MoveToNextAttribute() {
            return false;
        }

        public override string GetNamespace(string name) {
            return string.Empty;			
        }

        public override bool MoveToNamespace(string name) {
            return false;
        }

        public override bool MoveToFirstNamespace(XPathNamespaceScope 
            namespaceScope) {
            return false;
        }

        public override bool MoveToNextNamespace(XPathNamespaceScope 
            namespaceScope) {
            return false;
        }

        public override bool HasChildren {
            get { return false; }
        }

        public override bool MoveToNext() {
            return false;
        }

        public override bool MoveToPrevious() {
            return false;
        }

        public override bool MoveToFirst() {
            return false;
        }
 
        public override bool MoveToFirstChild() {
            return false;
        }

        public override bool MoveToParent() {
            return false;
        }

        public override void MoveToRoot() {}

        public override bool MoveTo( XPathNavigator other ) {
            return false;
        }

        public override bool MoveToId(string id) {
            return false;
        }   

        public override bool IsSamePosition(XPathNavigator other) {      
            return false;
        }

        public override XPathNodeIterator SelectDescendants(string name, 
                    string namespaceURI, bool matchSelf) {
            return null;
        }
            
        public override XPathNodeIterator SelectChildren(string name, 
                                    string namespaceURI) {
            return null;
        }
        
        public override XPathNodeIterator 
                SelectChildren(XPathNodeType nodeType) {
            return null;
        }
        
        public override XmlNodeOrder 
                ComparePosition(XPathNavigator navigator) {
            return new XmlNodeOrder();
        }
    }
    
    private static XPathNavigator _nav = 
            new DummyXpathNavigator();
        
    public static XPathExpression Compile(string xpath) {
        return _nav.Compile(xpath);
    }
}

public class XPathCompilerTest {
    static void Main(string[] args) {
        //Document-free compilation
        XPathExpression xe = XPathCompiler.Compile("/foo");
        //Usage of the compiled expression
        XPathDocument doc = 
            new XPathDocument(new StringReader("<foo/>"));
        XPathNavigator nav = doc.CreateNavigator();
        XPathNodeIterator ni = nav.Select(xe);
        while (ni.MoveNext()) {
            Console.WriteLine(ni.Current.Name);
        }
    }
}