Delphi: Why A class variable affects the result of the method?

Question

This is a full simplification of my code, but even so it doesn't work when class var ID_COUNTER is there, in this code I don't use the class var, but in my real code yes, but just the existence of this class variable makes the result of 's' different. This is the most weird I have ever seen.

Here is a simplification, but still doesn't Works, one Unit in 75 lines.

 unit Umain;
 interface
 uses
 Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs,     Vcl.StdCtrls,XMLIntf,XmlDoc,IOUtils,XMLDom,System.Generics.Collections;

type
  TStore = class
  public
   class var ID_COUNTER: Integer;
   MainNode: IDomNode;
  constructor create(node:IDomNode);
   function getNode():IDomNode;
 end;
TForm1 = class(TForm)
  Button1: TButton;
  procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
 FMain: TStore;
   function Recursive(node:IDomNode):TStore;

 end;

 var
    Form1: TForm1;

 implementation

 {$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  Doc:IXMLDocument;
  content: WideString;
  html: IDomNode;
  s: String;
 begin
    Doc := TXMLDocument.Create(Application);
    Doc.LoadFromFile('C:\temp\example.xml');
    Doc.Active := true;
    html := Doc.DOMDocument.getElementsByTagName('html').item[0];
    FMain := Recursive(html);
    s := FMain.getNode().nodeName;
 end;

 function TForm1.Recursive(node: IDOMNode):TStore;
 var
   i: Integer;
   store: TStore;
  nodeName,nodeValue:String;
 begin
   store := TStore.create(node);

  if(not node.hasChildNodes)then
    Exit(store);

   for i := 0 to node.childNodes.length-1 do
   begin
     Recursive(node.childNodes.item[i]);
   end;

  Exit(store);
end;
constructor TStore.create(node: IDOMNode);
begin
  self.MainNode := node;
end;
function TStore.getNode:IDomNode;
begin
  Result := self.MainNode;
end;

end.

Some notes:

example.xml is only a simple HTML document. Everything is broken when ID_COUNTER exists, if it is commented, everything is Ok. It happens here and in my real and wide Project.

Answer 1

The problem is that, syntactically, class var introduces a class field block rather than a single class field, meaning that if you use class var, all following field declarations in the same visibility section will be class variables too. So now, MainNode becomes a class variable too and that probably causes the problems you encounter. Reformatting your code shows this a little more clearly:

public
  class var 
    ID_COUNT: Integer;
    MainNode: IDomNode;
  constructor Create(... etc.

Your options are:

• move ID_COUNT one line down:

  public
    MainNode: IDomNode;
    class var ID_COUNTER: Integer;
    constructor Create(... etc.
  

• create a special section for MainNode:

  public
    class var ID_COUNTER: Integer;
  public
    MainNode: IDomNode;
    constructor Create(... etc.
  

• preface MainNode with the var keyword (which, likewise, introduces a block, specifically an instance field block within the current visibility section):

  public
    class var 
      ID_COUNTER: Integer;
      // any other class variables
    var 
      MainNode: IDomNode;
      // any other instance variables
    constructor Create(... etc.