How do I test whether a certain XML node exists?

xml delphi

12,410

Solution 1

You can use XPath, try this sample.

uses
  MSXML;


Var
  XMLDOMDocument  : IXMLDOMDocument;
  XMLDOMNode      : IXMLDOMNode;
begin
  XMLDOMDocument:=CoDOMDocument.Create;
  XMLDOMDocument.loadXML(XmlStr);
  XMLDOMNode := XMLDOMDocument.selectSingleNode('//Antenna/Mount/BirdBathMount');
  if XMLDOMNode<>nil then
    Writeln('BirdBathMount node Exist')
  else
  begin
    XMLDOMNode := XMLDOMDocument.selectSingleNode('//Antenna/Mount/AzEl');
    if XMLDOMNode<>nil then
      Writeln('AzEl node Exist');
  end;
end;

Solution 2

Use .FindNode. It returns nil, if the node doesn't exist.

e.g.

xmlNode := MountNode.ChildNodes.FindNode('AzEl');
if Assigned(xmlNode) then
...

Solution 3

I have tested it successfully. with this code. It is somewhat more complicated and I need a root element .

XmlFile

<ThisIsTheDocumentElement>
<Antenna >
  <Mount Model="text" Manufacture="text">
   <BirdBathMount/>
  </Mount>
</Antenna>
<Antenna >
  <Mount Model="text" Manufacture="text">
   <AzEl/>
  </Mount>
</Antenna>
</ThisIsTheDocumentElement>

Delphi2010.pas

procedure TForm1.RetrieveDocument;
var
  LDocument: IXMLDocument;
  LNodeElement, LNode,BNode,CNode : IXMLNode;
  I: Integer;
begin
  LDocument := TXMLDocument.Create(nil);
  LDocument.LoadFromFile(XmlFile);
  LNodeElement := LDocument.ChildNodes.FindNode('ThisIsTheDocumentElement');
  if (LNodeElement <> nil) then
  begin
    for I := 0 to LNodeElement.ChildNodes.Count - 1 do
    begin
      LNode := LNodeElement.ChildNodes.Get(I);
       if (LNode <> Nil) AND (LNode.NodeName='Antenna') then begin
          Memo1.lines.Add('Node name: ' + LNode.NodeName);
          BNode:=LNode.ChildNodes.FindNode('Mount');
         if (BNode <> Nil) then CNode:=BNode.ChildNodes.FindNode('AzEl');
            if (CNode <> Nil) then Memo1.lines.Add('found: '+CNode.NodeName) else continue;
       end;
    end;
  end;
end;

OUTPUT:

Node name: Antenna
Node name: Antenna
found: AzEl

12,410

Seti Net

Building the only operational SETI Station in the US

Updated on September 16, 2022

Comments

Seti Net over 1 year
What is the proper way to test for the existance of an optional node? A snipped of my XML is:
```
<Antenna >
  <Mount Model="text" Manufacture="text">
   <BirdBathMount/>
  </Mount>
</Antenna>
```
But it could also be:
```
<Antenna >
  <Mount Model="text" Manufacture="text">
   <AzEl/>
  </Mount>
</Antenna>
```
The child of Antenna could either be BirdBath or AzEl but not both...

In Delphi XE I have tried:
```
 if (MountNode.ChildNodes.Nodes['AzEl'] <> unassigned then //Does not work
 if (MountNode.ChildNodes['BirdBathMount'].NodeValue <> null) then // Does not work
 if (MountNode.BirdBathMount.NodeValue <> null) then  // Does not work
```
I use XMLSpy to create the schema and the example XML and they parse correctly. I use Delphi XE to create the bindings and it works on most other combinations fine.

This must have a simple answer that I have just overlooked - but what? Thanks...... Jim
- paulsm4 almost 12 years
  
  Have you tried "VarIsNull()" from unit "Variants"?
Warren P almost 12 years

+1 for XPath. Since the document already would exist in any real world test, the above code sample could really be the selectSingleNode call.
Remy Lebeau almost 12 years

Note that this is Microsoft-specific. You can use XPath with TXMLDocument/IXMLDocument if the DOMVendor is set to MSXML (the default on Windows).
Seti Net almost 12 years

This is the one that finally worked for me. {code} if (MountNode.ChildNodes.FindNode('AzEl') <> nil) then {code}
Seti Net almost 12 years

What worked for me was:'code'if (MountNode.ChildNodes.FindNode('AzEl') <> nil) then 'code'
moskito-x almost 12 years

In My example. BNode == MountNode
moskito-x almost 12 years

In My example. BNode == MountNode
RRUZ almost 12 years

@moskito-x the XPath sentences //Antenna/Mount/BirdBathMount and //Antenna/Mount/AzEl works with the xml posted by the OP, maybe you are trying with another xml.
moskito-x almost 12 years

@RRUZ I've tried it with a TMemo. The file "XMLFile.xml" (from my answer) loaded with load file and the String "xmlstr" xmlstr:= Memo1.Text; passed. With and without "<ThisIsTheDocumentElement> "
RRUZ almost 12 years

@moskito-x, try with the XML posted by the OP, if yuu want try with you xml version try using //ThisIsTheDocumentElement/Antenna/Mount/BirdBathMount instead
moskito-x almost 12 years

YOU ARE RIGHT.!!! In The Question I overlooked "The child of Antenna could either be BirdBath or AzEl but not both..." . Is It OK for you,if I delete my comments ??