Access Violation while parsing XML in Delphi XE2

Question

I'm trying to parse a MusicBraninz XML file in Delphi XE2 using the following code:

webquery := 'http://www.musicbrainz.org/ws/2/recording/?query='+escape(tracktitle)+'&artist:'+escape(ArtistTitle);

Log('WebQuery: ' + webquery, 0);

begin
  XMLDoc:= TXMLDocument.Create(nil);
  XMLDoc.FileName := webQuery;
  XMLDoc.Active := True;

  Log('Report: ' + XMLDoc.XML.Text, 0);


   StartItemNode := XMLDoc.DocumentElement.ChildNodes.First.ChildNodes.FindNode('release-list') ;

   ANode := StartItemNode;
   repeat
     Result.Album := ANode.ChildNodes['title'].Text;  <-- Access Violation
     Result.Status:= ANode.ChildNodes['status'].Text;

     ANode := ANode.NextSibling;
   until ANode = nil;

end;

The XML file is fetched correctly and looks like what's below:

<metadata xmlns="http://musicbrainz.org/ns/mmd-2.0#" xmlns:ext="http://musicbrainz.org/ns/ext#-2.0">
<recording-list offset="0" count="1">
<recording ext:score="100" id="a399eec1-d45d-4505-b475-ead0da6cad17">
<title>Mestecăniș</title>
<length>359000</length>
<artist-credit>
<name-credit>
<artist id="8fb78a16-0cba-4175-8c92-d9645dfb007d">
<name>Bucovina</name>
<sort-name>Bucovina</sort-name>
</artist>
</name-credit>
</artist-credit>
<release-list>
<release id="22b00afc-86ea-445a-8805-b6bfa33da74e">
<title>Duh</title>
<status>Official</status>
<release-group type="EP" id="4e8fb87c-3760-48c1-a3d7-88e7a2c839fa">
<primary-type>EP</primary-type>
</release-group>
<date>2010</date>
<country>RO</country>
<medium-list>
<track-count>5</track-count>
<medium>
<position>1</position>
<format>CD</format>
<track-list offset="3" count="5">
<track>
<number>4</number>
<title>Mestecăniș</title>
<length>359000</length>
</track>
</track-list>
</medium>
</medium-list>
</release>
</release-list>
</recording>
</recording-list>
</metadata>

My question is: am I doing anything wrong here? All variables are declared and initialized OK.

Thanks,

Answer 1

You have an access violation because the FindNode method is returning a nil value and you are trying to access of a invalid memory location. In order to use the FindNode method you must check the hierarchy (level) of the nodes to search and then check if the result is not nil.

Try this sample.

  XMLDoc:= LoadXMLDocument(webQuery);
   StartItemNode := XMLDoc.DocumentElement.ChildNodes.FindNode('recording-list');
   if not Assigned(StartItemNode) then exit;
   StartItemNode := StartItemNode.ChildNodes.FindNode('recording');
   if Assigned(StartItemNode) then
   begin
     StartItemNode := StartItemNode.ChildNodes.FindNode('release-list');
     if Assigned(StartItemNode) then
     begin
       StartItemNode := StartItemNode.ChildNodes.FindNode('release');
       if Assigned(StartItemNode) then
       begin
         ANode := StartItemNode;
         repeat
           Result.Album := ANode.ChildNodes['title'].Text;
           Result.Status:= ANode.ChildNodes['status'].Text;
           ANode := ANode.NextSibling;
         until ANode = nil;
       end;
     end;
   end;