Access violation casting IDispatch in XE2

Question

We're using some old code (ComLib.pas created by Binh Ly) so we can use the enumeration interface on an (OleVariant) object:

type
  TDispNewEnum = dispinterface
    ['{97079E31-6957-11D2-9154-0000B4552A26}'] // dummy
    property _NewEnum: IUnknown readonly dispid -4; // DISPID_NEWENUM
    function _NewEnumFunc: IUnknown; dispid -4; // DISPID_NEWENUM
  end;

procedure TEnumVariant.AttachUnknown (const Unk: IUnknown);
var
  pDisp: IDispatch;
  _NewEnumPropFailed: boolean;
  Unknown: IUnknown;
begin
  Detach;
  Unknown := Unk;
  { extract IEnumVariant }
  if (Unknown <> nil) then
  begin
    { try IEnumVariant }
    if not (Succeeded (Unknown.QueryInterface (IEnumVariant, FEnumVariant))) then
    begin
      FEnumVariant := nil;  // safety!

      { test _NewEnum prop and _NewEnum func }
      if (Succeeded (Unknown.QueryInterface (IDispatch, pDisp))) then
      begin
        _NewEnumPropFailed := False;
        try
          //property _NewEnum
          Unknown:=TDispNewEnum(pDisp)._NewEnum; // <---- RAISES EXCEPTION -----
          if not (Succeeded(Unknown.QueryInterface(IEnumVariant, FEnumVariant))) then
            FEnumVariant := nil;  // safety!
        except
          _NewEnumPropFailed := True;
        end;  { except }

This code is working on Delphi 2010 and 2007, but not with XE2. On the line marked above (with the comment "RAISES EXCEPTION"), we get an exception:

Project x.exe raised exception class $C0000005 with message 'access violation at 0xbaadf00d: read of address 0xbaadf00d'.

The object passed in does have the TDispNewEnum interface so no exception should be raised (as is the case with Delphi 2010 and 2007).

Suggestions? Thanks.

See stackoverflow.com/questions/7886116 about the root cause probable issue. Seems definitively like a regression. — Arnaud Bouchez

Arnaud Bouchez Arnaud Bouchez · Accepted Answer · 2011-10-24T12:42:18

The 0xbaadf00d memory address is a pseudo memory address, meaning "BAD FOOD" (look at the hexa chars). This is used usually by code when you ask for invalid interfaces or calls.

What if you change the line into:

pDisp: TDispNewEnum;
...
if (Succeeded (Unknown.QueryInterface (IDispatch, pDisp))) then
begin
   _NewEnumPropFailed := False;
   try
     //property _NewEnum
     Unknown:= pDisp._NewEnum; 
...

which makes better sense to me.

I've noticed some undocumented changes in the XE2 interface binding. Perhaps the previous code with a forced typecast (TDispNewEnum(pDisp)) is not working any more because of that.

Access violation casting IDispatch in XE2

1 Answers