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AS. since closing related questions - more examples added below. The below simple code (which finds a top-level Ie window and enumerates its children) works Ok with a '32-bit Windows' target platform. There's no problem with earlier versions of Delphi as well: procedure TForm1.Button1Click(Sender: TObject); function EnumChildren(hwnd: HWND; lParam: LPARAM): BOOL; stdcall; const Server = 'Internet Explorer_Server'; var ClassName: array[0..24] of Char; begin Assert(IsWindow(hwnd)); // <- Assertion fails with 64-bit GetClassName(hwnd, ClassName, Length(ClassName)); Result := ClassName <> Server; if not Result then PUINT_PTR(lParam)^ := hwnd; end; var Wnd, WndChild: HWND; begin Wnd := FindWindow('IEFrame', nil); // top level IE if Wnd <> 0 then begin WndChild := 0; EnumChildWindows(Wnd, @EnumChildren, UINT_PTR(@WndChild)); if WndChild <> 0 then .. end; I've inserted an Assert to indicate where it fails with a '64-bit Windows' target platform. There's no problem with the code if I un-nest the callback. I'm not sure if the erroneous values passed with the parameters are just garbage or are due to some mis-placed memory addresses (calling convention?). Is nesting callbacks infact something that I should never do in the first place? Or is this just a defect that I have to live with? edit: In response to David's answer, the same code having EnumChildWindows declared with a typed callback. Works fine with 32-bit: (edit: The below does not really test what David says since I still used the '@' operator. It works fine with the operator, but if I remove it, it indeed does not compile unless I un-nest the callback) type TFNEnumChild = function(hwnd: HWND; lParam: LPARAM): Bool; stdcall; function TypedEnu

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