So if you’re using the “implements” keyword, the object being targeted by “implements” really should descend from TAggregatedObject instead of TInterfacedObject. From the comments in System.pas:

TAggregatedObject and TContainedObject are suitable base classes for interfaced objects intended to be aggregated or contained in an outer controlling object. When using the “implements” syntax on an interface property in an outer object class declaration, use these types to implement the inner object.

Interfaces implemented by aggregated objects on behalf of the controller should not be distinguishable from other interfaces provided by the controller. … TAggregatedObject simply reflects QueryInterface calls to its controller.

However Microsoft’s COM guidelines are very specific that casting an object to a given interface, directly or indirectly, should always return the same pointer. In the above case you should override QueryInterface on the subobject to return the parent’s IUnknown.


uses classes;

var
intf1: IStreamPersist;
intf2: IInterfaceList;
obj: TImplementor;
begin
obj := TImplementor.Create;
try
intf1 := obj;
intf2 := obj;
Assert((intf1 as IInterface) = (intf2 as IInterface), 'interfaces not equal');
finally
obj.Free;
end;
end;


http://hallvards.blogspot.com/2004/07/hack-7-interface-to-object-in-delphi.html

>In Delphi for Win32, you can’t necessarily use the “equals” operator to tell whether two >interfaces both point to the same object.

And you offer the solution, that is to cast both interfaces to IInterface

>Assert(Foo as IInterface = Bar as IInterface);

My sample show that although both Interfaces (First IInterface and SomeA) use the same object
(you may find out this by inspecting the hidden compiler generated stub that fix EAX(in register calling convention), i.e. self param of interface Method

But casting both Interfaces to Iunknown fails because of overridden QueryInterface method of SomeA Interface, but both interfaces still use the same object.

To force your solution fail you may

1. create new instance, i.e. delegate implementation to an other object.
2. delegate implementation to inner(aggregated) object.
3. suppress returning any interfaces from QueryInterface and get exception.

Best regards Sergey Antonov.

This sample shows that if QueryInterface is override

And what about the next case?

SomeA=interface
[’{008F39E4-9C7F-4ED8-AD56-56CD7FAD6477}’]
procedure Method1;
end;

TABC=class(TInterfacedObject,SomeA)
protected
function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
procedure Method1;
public
end;

procedure TForm1.Button1Click(Sender: TObject);
var a:IUnknown;
begin
a:=TABC.create;
if not (A=a as someA as IUnknown) then showmessage(’Failed’);
end;

procedure TABC.Method1;
begin
//
end;

function TABC.QueryInterface(const IID: TGUID; out Obj): HResult;
var A:TInterfacedObject;
begin
Iunknown(Obj):=TInterfacedObject.Create;
Result:=S_OK;
end;