Delphi - TXMLDocument created at run-time generates AV, with component on the form is working

问题:

I'm creating an instance of TXMLDocument at runtime, to load and parse a XML file. You can check the code below:

    unit Unit1;  interface  uses   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,   Dialogs, xmldom, XMLIntf, msxmldom, XMLDoc, StdCtrls;  type   Txml = class(TForm) //    XMLDocument1: TXMLDocument;     Memo1: TMemo;     procedure FormCreate(Sender: TObject);   private     { Private declarations }   public     { Public declarations }   end;  var   xml: Txml;  implementation  {$R *.dfm}  procedure Txml.FormCreate(Sender: TObject); var    i,j:integer;        aNode:IXMLNode;        ws:String;        XMLDocument1:TXMLDocument; begin  Memo1.Lines.Clear;  XMLDocument1 := TXMLDocument.Create(nil);  try   XMLDocument1.LoadFromFile('C:\a.xml');   XMLDocument1.Active := true;   aNode := XMLDocument1.ChildNodes.First;   while aNode<>nil do   begin    for i := 0 to aNode.ChildNodes.Count-1 do     begin      if aNode.ChildNodes[i].NodeName = 'Role' then       begin        Memo1.Lines.Add('Tag - '+aNode.ChildNodes[i].ChildNodes['Tag'].Text);        for j := 0 to aNode.ChildNodes[i].ChildNodes.Count-1 do         if aNode.ChildNodes[i].ChildNodes[j].HasChildNodes then          begin           ws :=  VarToStr(aNode.ChildNodes[i].ChildNodes[j].ChildValues['Tag']);           if trim(ws)<>'' then            Memo1.Lines.Add(ws);           ws :=  VarToStr(aNode.ChildNodes[i].ChildNodes[j].ChildValues['Value']);           if trim(ws)<>'' then            Memo1.Lines.Add(ws);          end;       end;     end;    aNode := aNode.NextSibling;   end;   XMLDocument1.Active := false;  finally    FreeAndNil(XMLDocument1);  end; end;  end. 

The problem is that this is generating an AV. As you probably have seen, before the component was on the form (// XMLDocument1: TXMLDocument;).

Why when the component was on the form the code was working, but when I'm creating it at run-time it generates AV?

LE: solution: based on the answers/comments and Delphi Help:

XMLDocument1 : IXMLDocument;  //not TXMLDocument  XMLDocument1 := LoadXMLDocument(...);  FreeAndNil;// must be deleted 

回答1:

From what I know you should be using interface IDoc: IXMLDocument; instead.

From docs:

When TXMLDocument is created without an Owner, it behaves like an interfaced object. That is, when all references to its interface are released, the TXMLDocument instance is automatically freed. When TXMLDocument is created with an Owner, however, it behaves like any other component, and is freed by its Owner.

In other words, when creating a TXMLDocument instance with a nil Owner, do not call Free() or FreeAndNil() on the instance, and you must assign the object to an IXMLDocument variable so its now-active reference count is managed properly.

回答2:

You need to provide an Owner to TXMLDocument when creating it in run-time.

XMLDocument1 := TXMLDocument.Create(xml); 

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