Strange memory overwrite problem in instance of my class

Question

This problem is related to this question, which I've asked earlier. The code provided by @RRUZ is working but it seems that not quite correctly or I am doing something wrong.

After executing GetSharedFiles strange thing is happening in instance of TMyObject. The field FMyEvent which was (and it should be) nil points to some random data.

What I've discovered just 5 minutes ago is that if I turn off the optimization in compiler options it works fine after rebuild. So maybe this is some compiler bug?

Here is a code snapshot (Delphi 2009 Windows 7 64 bit):

unit Unit17;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm17 = class(TForm)
    btnetst: TButton;
    procedure btnTestClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

type
  TMyEvent = procedure(Sender: TObject) of object;

type
  TMyObject = class(TObject)
  private
    FMyEvent: TMyEvent;
    function GetSharedFiles: TStringList;
  public
    property OnEvent: TMyEvent read FMyEvent write FMyEvent;
    procedure DoSomething;
  end;

var
  Form17: TForm17;

implementation

uses
  ActiveDs_TLB,
  ActiveX;

function ADsGetObject(lpszPathName:WideString; const riid:TGUID; out ppObject):HRESULT; safecall; external 'activeds.dll';

{$R *.dfm}

procedure TForm17.btnTestClick(Sender: TObject);
var
  MyObject: TMyObject;
begin
  MyObject := TMyObject.Create;
  try
    MyObject.DoSomething;
  finally
    if Assigned(MyObject) then
      MyObject.Free;
  end;
end;

{ TMyObject }

procedure TMyObject.DoSomething;
var
  TmpList: TStringList;
begin
  try

    TmpList := GetSharedFiles; //something is overwritting the memory in object and puts random data to FMyEvent?
    if Assigned(FMyEvent) then
      ShowMessage('WTF'); //this should not be called, and if you comment out GetSharedFiles it won't.

  finally
    if Assigned(TmpList) then
      TmpList.Free;
  end;
end;


function TMyObject.GetSharedFiles: TStringList;
var
  FSO           : IADsFileServiceOperations;
  Resources     : IADsCollection;
  Resource      : OleVariant;
  pceltFetched  : Cardinal;
  oEnum         : IEnumvariant;
begin
  Result := TStringList.Create;
  //establish the connection to ADSI
  if ADsGetObject('WinNT://./lanmanserver', IADsFileServiceOperations, FSO) = S_OK then
  begin
    //get the resources interface
    Resources := FSO.Resources;
    //get the enumerator
    oEnum:= IUnknown(Resources._NewEnum) as IEnumVariant;
    while oEnum.Next(1, Resource, pceltFetched) = 0 do
    begin
      Result.Add(LowerCase(Format('%s%s%s',[Resource.Path,#9,Resource.User])));
      Resource:=Unassigned;
    end;
  end;
end;    
end.

Any ideas what is going wrong? Thanks for your time.

Answer 1

The calling convention on this should probably be stdcall, not safecall:

function ADsGetObject(lpszPathName:WideString; const riid:TGUID; out ppObject):HRESULT; safecall; external 'activeds.dll';

---

Recap

Typical COM functions return a HRESULT result; They use it to pass an error code or S_OK if everything went fine. Using this type of function, you'd usually have this kind of code:

if CallComFunction(parameters) = S_OK then
  begin
    // Normal processing goes here
  end
else
  begin
    // Error condition needs to be dealt with here.
  end

Since error conditions can't usually be dealt with, Delphi provides us with the safecall pseudo-calling-convention. It's not a true calling convention because in fact it uses stdcall behind the scenes. What it does is to automatically generate the test for S_OK and, on failure, raises an error. So the typical COM method can be declared as either one of this:

function TypicalComFunction(Parameters): HRESULT; stdcall;
procedure TypicalComFunction(Parameters); safecall;

If you don't intend to deal with any potential errors use the second form (with safecall) and simply ignore the potential exception. If an error does occur, Delphi will raise an Exception, and that exception will bubble-up until it reaches a point in the application that can deal with the error. Or it bubbles up until it reaches Application's exception handler, and that's used to display the error for the user.

Using safecall, the typical code above looks like this:

TypicalComFunction(Parameters); // raises exception on error    
// Normal processing goes here

On the other hand if you do need the HRESUL even if it's different from S_OK, then use the stdcall variant.