I would like to pass a CORBA interface-type as parameter and test for equality, So, for example, I have

{$mode objfpc}
{$interfaces corba}
type
	IMyInterface = Interface
		[  '{34aad6d0-5884-4143-97c2-b6e330305ae3}']
		Function MyFunc : Integer;
	end;

Now I can pass IMyInterface to TObject.GetInterface as the first parameter, which is of type TGuid in the declaration.

However, if I use the same TGuid parameter type in my own procedure

	procedure TestInterfaceParameter
	      ( const theInterface            : TGuid);
	    begin
	      if IsEqualGUID
	       ( theInterface, ITestInterface1)
	        then writeln
	           ( 'theInterface = ITestInterface1')
	    end;

then I run into the problem that IMyInterface is accepted as actual parameter of TestInterfaceParameter and of IsEqualGUID only if IMyInterface inherits from IUnknown (which I certainly don't want, as it is reference counted). Strangely, TObject.GetInterface does accept IMyInterface as a parameter. So where is the difference ? Is there some compiler magic involved in TObject.GetInterface ?

Regards,

Adriaan van Os

If you need to know if an instance implement certain interface, you can use is operator
if anInstance is IInterface1 thenbegin     end;

Zamrony P. Juhara

On Sat, Dec 21, 2019 at 0:25, Adriaan van Os wrote: I would like to pass a CORBA interface-type as parameter and test for equality, So, for example, I have

{$mode objfpc}
{$interfaces corba}

type
    IMyInterface = Interface
   
        [  '{34aad6d0-5884-4143-97c2-b6e330305ae3}']

        Function MyFunc : Integer;
    end;

Now I can pass IMyInterface to TObject.GetInterface as the first parameter, which is of type TGuid in the declaration.

However, if I use the same TGuid parameter type in my own procedure

    procedure TestInterfaceParameter
          ( const theInterface            : TGuid);         begin
          if IsEqualGUID
          ( theInterface, ITestInterface1)
            then writeln
              ( 'theInterface = ITestInterface1')         end;

then I run into the problem that IMyInterface is accepted as actual parameter of TestInterfaceParameter and of IsEqualGUID only if IMyInterface inherits from IUnknown (which I certainly don't want, as it is reference counted). Strangely, TObject.GetInterface does accept IMyInterface as a parameter. So where is the difference ? Is there some compiler magic involved in TObject.GetInterface ?

Regards,

Adriaan van Os

Thanks for the reply, but that is not what I meant.

Regards,

Adriaan van Os

The important difference in this regard between COM and CORBA interfaces is that for CORBA interfaces the "guid" is a ShortString, not a TGUID. Thus the compiler allows to convert a CORBA interface to a ShortString. For TObject.GetInterface this works, because it has a overload for ShortString. ;) (this is document here:
https://www.freepascal.org/docs-html/current/ref/refse47.html#x100-1220007.6 )

Regards,
Sven

Thanks, I hadn't seen that. Yes, this solves the problem. And this clarifies that the CORBA interface-identifying string doesn't need to be in GUID form, it can be something as simple as [ 'IMyInterface' ].

I will note that the interface-identifier comparison below (in procedure TestInterfaceParameter) doesn't work without the explicit typecast from interface-identifier to "shortstring". Apparently, that typecast is implicit when passing an interface-identifier as actual parameter to a "shortstring" formal parameter.

type
     InterfaceType                           = string[ 255];

procedure TestInterfaceParameter

       ( const theInterface            : InterfaceType);
     begin
       if theInterface = InterfaceType( ITestInterface1)
         then writeln
            ( 'theInterface = ITestInterface1')
     end;

begin
   TestInterfaceParameter
     ( ITestInterface1)
end.

Regards,

Adriaan van Os

That is because the compiler only implements the *assignment* of an interface to a ShortString (or TGUID for COM interfaces), not the comparison.

Regards,
Sven