Changing XML node attribute value gives "Namespace error"

This is a bug. A pretty major one too. With the current implementation of the ADOM XML vendor, if you create an attribute in the null namespace, you cannot change its value.

Here is the offending code from the AdomCore_4_3 unit, as bundled with Delphi 2010.

procedure TDomAttr.SetPrefix(const Value: WideString);
begin
  if IsReadonly then
    raise ENo_Modification_Allowed_Err.Create('No modification allowed error.');

  if NodeName = 'xmlns' then
    raise ENamespace_Err.Create('Namespace error.');

  if NamespaceURI = 'http://www.w3.org/2000/xmlns/' then begin
    if Value <> 'xmlns' then
      raise ENamespace_Err.Create('Namespace error.');
  end else if NamespaceURI = 'http://www.w3.org/XML/1998/namespace' then begin
    if Value <> 'xml' then
      raise ENamespace_Err.Create('Namespace error.');
  end else begin
    if NamespaceURI = '' then
      raise ENamespace_Err.Create('Namespace error.');
    if Value = 'xml' then
      raise ENamespace_Err.Create('Namespace error.');
    if Value = 'xmlns' then
      raise ENamespace_Err.Create('Namespace error.');
  end;

  if Value = '' then begin
    if (NamespaceURI = 'http://www.w3.org/2000/xmlns/') then
      raise ENamespace_Err.Create('Namespace error.');
    FPrefix := '';
    FNodeName := LocalName;
    Exit;
  end;

  if not IsXmlName(Value) then
    raise EInvalid_Character_Err.Create('Invalid character error.');
  if not IsXmlPrefix(Value) then
    raise ENamespace_Err.Create('Namespace error.');

  FPrefix := Value;
  FNodeName := Concat(Value, ':', LocalName);
end;

In the above, the issue can be isolated to here...

    if NamespaceURI = '' then
      raise ENamespace_Err.Create('Namespace error.');

Work-around 1

I have no idea what the author's intent was, but as it stands, this test is nonsense. To fix, remove this test and recompile.

Work-around 2

As an alternative, you could delete the attribute before setting like this ...

procedure TForm6.Button1Click(Sender: TObject);
var
  XML: TXMLDocument;
  XMLNode, XMLSubNode: IXMLNode;
  OldAttrib: IXMLNode;
begin
  XML := TXMLDocument.Create(nil);
  //XML.DOMVendor := GetDOMVendor('MSXML'); // Works using MSXML
  XML.DOMVendor := GetDOMVendor('ADOM XML v4');
  XML.Active := True;
  XMLNode := XML.AddChild('test');
  XMLNode.Attributes['state'] := 1;
  OldAttrib := XMLNode.AttributeNodes.FindNode('state');
  if assigned( OldAttrib) then
    XMLNode.AttributeNodes.Remove( OldAttrib);
  XMLNode.Attributes['state'] := 0; 
end;