如何测试某个XML节点是否存在?

What is the proper way to test for the existance of an optional node? A snipped of my XML is:

测试可选节点是否存在的正确方法是什么?我的XML片段是:

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

But it could also be:

但也可能是:

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

The child of Antenna could either be BirdBath or AzEl but not both...

天线之子可以是BirdBath或AzEl，但不是两者都…

In Delphi XE I have tried:

在Delphi XE中，我尝试过:

 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.

我使用XMLSpy创建模式和示例XML，并正确地解析它们。我使用Delphi XE创建绑定，它适用于大多数其他组合。

This must have a simple answer that I have just overlooked - but what? Thanks...... Jim

这一定有一个我刚刚忽略的简单答案——但什么呢?谢谢……吉姆

4 个解决方案

#1

You can use XPath, try this sample.

可以使用XPath，请尝试此示例。

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;

#2

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

使用.FindNode。如果节点不存在，则返回nil。

e.g.

如。

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

#3

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

我已经测试成功了。这段代码。它有点复杂，我需要一个根元素。

XmlFile

XmlFile

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

Delphi2010.pas

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:

输出:

Node name: Antenna
Node name: Antenna
found: AzEl

#4

What worked for me was:

对我起作用的是:

if (MountNode.ChildNodes.FindNode('AzEl') <> nil) then

Its not clear to me how the nil responds to the options setting on TXMLDocumnet like doAttrNull but it works.

我不清楚nil如何响应TXMLDocumnet上的doAttrNull之类的选项设置，但它确实有效。

#1