How to use XPath on TXMLDocument which has namespa

Question

I have an XML packet received from a third-party web server:

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <SomeResponse xmlns="http://someurl">
      <SomeResult>
        .....
      </SomeResult>
    </SomeResponse>
  </soap:Body>
</soap:Envelope>

To be cross-platform capable, this XML is loaded into Delphi's IXMLDocument:

XmlDoc.LoadFromXML(XmlString);

I'm using a solution to find an XML node using XPath. The solution works in other cases, however when the XML document contains namespace prefixes, it fails.

I'm trying to access path:

/soap:Envelope/soap:Body/SomeResponse/SomeResult

From the linked answer:

function selectNode(xnRoot: IXmlNode; const nodePath: WideString): IXmlNode;
var
  intfSelect : IDomNodeSelect;
  dnResult : IDomNode;
  intfDocAccess : IXmlDocumentAccess;
  doc: TXmlDocument;
begin
  Result := nil;
  if not Assigned(xnRoot) or not Supports(xnRoot.DOMNode, IDomNodeSelect, intfSelect) then
    Exit;
  dnResult := intfSelect.selectNode(nodePath);
  if Assigned(dnResult) then
  begin
    if Supports(xnRoot.OwnerDocument, IXmlDocumentAccess, intfDocAccess) then
      doc := intfDocAccess.DocumentObject
    else
      doc := nil;
    Result := TXmlNode.Create(dnResult, nil, doc);
  end;
end;

It fails at dnResult := intfSelect.selectNode(nodePath); with EOleException: Reference to undeclared namespace prefix: 'soap'

How do I make this work when the node names have a namespace prefix?

Answer 1

As others have pointed out, different vendors handle namespaces differently. Here is an example using MSXML (the windows default) DOMVendor: (which I DO realise is not exactly what the OP was asking, but I felt it was worth documenting)

XML:

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <SomeResponse xmlns="http://someurl">
      <SomeResult>
        Some result here
      </SomeResult>
    </SomeResponse>
  </soap:Body>
</soap:Envelope>

Selection code (for completeness)

// From a post in Embarcadero's Delphi XML forum.
function selectNode(xnRoot: IXmlNode; const nodePath: WideString): IXmlNode;
var
  intfSelect : IDomNodeSelect;
  dnResult : IDomNode;
  intfDocAccess : IXmlDocumentAccess;
  doc: TXmlDocument;
begin
  Result := nil;
  if not Assigned(xnRoot) or not Supports(xnRoot.DOMNode, IDomNodeSelect, intfSelect) then
    Exit;
  dnResult := intfSelect.selectNode(nodePath);
  if Assigned(dnResult) then
  begin
    if Supports(xnRoot.OwnerDocument, IXmlDocumentAccess, intfDocAccess) then
      doc := intfDocAccess.DocumentObject
    else
      doc := nil;
    Result := TXmlNode.Create(dnResult, nil, doc);
  end;
end;

Actual setting of XML search namespaces:

uses Winapi.MSXMLIntf; // NOTE: Use this version of the interface. MSXML2_TLB won't work.
...
procedure TForm1.DoExampleSearch;
var fnd:IXmlNode;
    doc:IXmlDomDocument2;
    msdoc:TMSDOMDocument;
const searchnames = 'xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" '+
                    'xmlns:xsd="http://www.w3.org/2001/XMLSchema" '+
                    'xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" '+
                    'xmlns:some="http://someurl"';

begin
  if Xmldocument1.DOMDocument is TMSDOMDocument then
  begin
    msdoc:=Xmldocument1.DOMDocument as TMSDOMDocument;
    doc:=(msdoc.MSDocument as IXMLDOMDocument2);
    doc.setProperty('SelectionLanguage', 'XPath');
    doc.setProperty('SelectionNamespaces',searchNames);
  end;
  fnd:=selectNode(XmlDocument1.DocumentElement,'/soap:Envelope/soap:Body/some:SomeResponse/some:SomeResult');
  if (fnd=nil) then showmessage('Not found') else showmessage('Found: '+fnd.Text);
end;

Couple of things worth noting: once you add namespaces into the mix at all, Xpath seems to insist on them for everything. Note that I added a 'some' namespace for the search criteria, because the SomResult inherited it from its parent, and I have yet to get XPath to implicitly handle default namespaces.

Answer 2

The OmniXML solution:

I can absolutely confirm the OmniXML XPath does NOT support namespaces per se.

BUT:

since it treats the nodenames as literals, 'soap:Envelope' will work in a query PROVIDED the name in the xml document IS soap:Envelope. So in the OP example, the OmniXML search path '/soap:Envelope/soap:Body/SomeResponse/SomeResult' would work.

Note that you can absolutely NOT rely on inherited or default namespaces, OmniXML matches on the literal nodename.

You could fairly easily implement a loop to either remove or normalize all namespace tags in your document without too much effort.

Answer 3

When I tried this a couple of years ago, I found namespace lookup in XPath was different between xml providers.

If I remember correctly, the Msxml lets you just use the namespace prefixes as they are defined in the xml file.

The ADOM 4 provider requires that you resolve namespace prefixes used in your XPath query to the actual namespaces, independent of the namespace mapping used in the xml file. There is a method pointer for that purpose, OnOx4XPathLookupNamespaceURI. Then you can have a name lookup function like this:

procedure TTestXmlUtil.EventLookupNamespaceURI(
  const AContextNode: IDomNode; const APrefix: WideString;
  var ANamespaceURI: WideString);
begin
  if APrefix = 'soap' then
    ANamespaceURI := 'http://schemas.xmlsoap.org/soap/envelope/'
  else if APrefix = 'some' then
    ANamespaceURI := 'http://someurl'
end;

Using this lookup function, and the selectNode function (which looks like something I may have once posted in a Delphi forum, taken from https://github.com/Midiar/adomxmldom/blob/master/xmldocxpath.pas), I could do the following test (using your xml in a string constant):

procedure TTestXmlUtil.SetUp;
begin
  inherited;
  DefaultDOMVendor := sAdom4XmlVendor;
  docFull := LoadXmlData(csSoapXml);

  OnOx4XPathLookupNamespaceURI := EventLookupNamespaceURI;
end;

procedure TTestXmlUtil.Test_selectNode;
var
  xn: IXmlNode;
begin
  xn := selectNode(docFull.DocumentElement, '/soap:Envelope/soap:Body/some:SomeResponse/some:SomeResult');
  CheckNotNull(xn, 'selectNode returned nil');
end;

I had to modify you XPath query a little for the default namespace.

Answer 4

One solution could be to remove all namespaces before you start processing your XML:

class function TXMLHelper.RemoveNameSpaces(XMLString: String): String;
const
  // An XSLT script for removing the namespaces from any document.
  // From http://wiki.tei-c.org/index.php/Remove-Namespaces.xsl
  cRemoveNSTransform =
    '<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">' +
    '<xsl:output method="xml" encoding="utf-8"/>' +

    '<xsl:template match="/|comment()|processing-instruction()">' +
    '    <xsl:copy>' +
    '      <xsl:apply-templates/>' +
    '    </xsl:copy>' +
    '</xsl:template>' +

    '<xsl:template match="*">' +
    '    <xsl:element name="{local-name()}">' +
    '      <xsl:apply-templates select="@*|node()"/>' +
    '    </xsl:element>' +
    '</xsl:template>' +

    '<xsl:template match="@*">' +
    '    <xsl:attribute name="{local-name()}">' +
    '      <xsl:value-of select="."/>' +
    '    </xsl:attribute>' +
    '</xsl:template>' +

    '</xsl:stylesheet>';

var
  Doc, XSL, Res: IXMLDocument;
  UTF8: UTF8String;
begin
   try
     Doc := LoadXMLData(XMLString);
     XSL := LoadXMLData(cRemoveNSTransform);
     Res := NewXMLDocument;
     Doc.Node.TransformNode(XSL.Node,Res);  // Param types IXMLNode, IXMLDocument
     Res.SaveToXML(Utf8);      // This ensures that the encoding remains utf-8
     Result := String(UTF8);
   except
     on E:Exception do Result := E.Message;
   end;
end; { RemoveNameSpaces }

(TXMLHelper is a helper class that I have with some useful XML handling functions)

Answer 5

Do not try to include namespaces in your XPath query. If all you want is the text of the SomeResult node, then you can use '//SomeResult' as query. For some reason the default xml implementation (msxml) barfs on the default namespace xmlns="http://someurl" on the SomeResponse parentnode. However, using OmniXML as the DOMVendor (= Crossplatform and valid from XE7 - thanks to @gabr) this works:

program Project3;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  Xml.XmlIntf,
  Xml.XMLDoc,
  Xml.XMLDom,
  Xml.omnixmldom,
  System.SysUtils;

const
 xml = '<?xml version="1.0" encoding="utf-8"?>'+#13#10+
        '<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"'+#13#10+
        'xmlns:xsd="http://www.w3.org/2001/XMLSchema"'+#13#10+
        'xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">'+#13#10+
        ' <soap:Body>'+#13#10+
        '  <SomeResponse xmlns="http://tempuri.org">'+#13#10+
        '   <SomeResult>1</SomeResult>'+#13#10+
        '  </SomeResponse>'+#13#10+
        ' </soap:Body>'+#13#10+
        '</soap:Envelope>';

function selectNode(xnRoot: IXmlNode; const nodePath: WideString): IXmlNode;
var
  intfSelect : IDomNodeSelect;
  dnResult : IDomNode;
  intfDocAccess : IXmlDocumentAccess;
  doc: TXmlDocument;
begin
  Result := nil;
  if not Assigned(xnRoot) or not Supports(xnRoot.DOMNode, IDomNodeSelect, intfSelect) then
    Exit;
  dnResult := intfSelect.selectNode(nodePath);
  if Assigned(dnResult) then
  begin
    if Supports(xnRoot.OwnerDocument, IXmlDocumentAccess, intfDocAccess) then
      doc := intfDocAccess.DocumentObject
    else
      doc := nil;
    Result := TXmlNode.Create(dnResult, nil, doc);
  end;
end;

function XPathQuery(Doc : IXMLDocument; Query : String) : String;

var
 Node : IXMLNode;

begin
 Result := '';
 Node := SelectNode(Doc.DocumentElement, Query);
 if Assigned(Node) then
  Result := Node.Text
end;

var
 Doc : IXMLDocument;

begin
 DefaultDOMVendor := sOmniXmlVendor;
 Doc := TXMLDocument.Create(nil);
 try
  Doc.LoadFromXML(Xml);
  Writeln(Doc.XML.Text);
  Writeln(XPathQuery(Doc, '//SomeResult'));
 except
  on E: Exception do
   Writeln(E.ClassName, ': ', E.Message);
 end;
 Doc := nil;
 Readln;
end.