<s>
IDispatch	B-Application
is	O
the	O
interface	B-Application
that	O
exposes	O
the	O
OLE	B-Application
Automation	I-Application
protocol	O
.	O
</s>
<s>
Extending	O
IUnknown	B-Application
,	O
it	O
is	O
one	O
of	O
the	O
standard	O
interfaces	B-Application
that	O
can	O
be	O
exposed	O
by	O
COM	B-Application
objects	I-Application
.	O
</s>
<s>
COM	B-Application
distinguishes	O
between	O
three	O
interface	B-Application
types	O
:	O
custom	O
that	O
are	O
VTABLE-based	O
IUnknown	B-Application
interfaces	B-Application
,	O
dispatch	O
that	O
are	O
IDispatch	B-Application
interfaces	B-Application
supporting	O
introspection	O
,	O
and	O
dual	O
interfaces	B-Application
supporting	O
both	O
types	O
.	O
</s>
<s>
The	O
Automation	O
(	O
IDispatch	B-Application
)	O
interface	B-Application
allows	O
a	O
client	O
application	O
to	O
find	O
out	O
what	O
properties	O
and	O
methods	O
are	O
supported	O
by	O
an	O
object	O
at	O
run-time	O
,	O
i.e.	O
</s>
<s>
implements	O
the	O
concept	O
of	O
RTTI	B-Application
.	O
</s>
<s>
This	O
allows	O
COM	B-Application
and	O
ActiveX	O
objects	O
to	O
be	O
called	O
by	O
scripting	O
programs	O
platforms	O
such	O
as	O
the	O
ASP	B-General_Concept
server	O
and	O
JavaScript	B-Language
on	O
Internet	B-Application
Explorer	I-Application
,	O
where	O
calling	O
conventions	O
were	O
not	O
known	O
at	O
the	O
time	O
IIS	B-Application
or	O
IE	O
were	O
built	O
.	O
</s>
<s>
A	O
script	O
writer	O
can	O
ask	O
the	O
COM	B-Application
object	I-Application
for	O
a	O
method	O
or	O
property	O
it	O
already	O
knows	O
about	O
from	O
documentation	O
.	O
</s>
<s>
Then	O
,	O
the	O
client	O
can	O
execute	O
the	O
function	O
with	O
Invoke	O
provided	O
by	O
the	O
IDispatch	B-Application
interface	B-Application
,	O
a	O
form	O
of	O
late-binding	O
.	O
</s>
<s>
This	O
sort	O
of	O
capability	O
was	O
also	O
supported	O
by	O
Dynamic	B-Operating_System
Data	I-Operating_System
Exchange	I-Operating_System
(	O
DDE	O
)	O
,	O
which	O
never	O
became	O
popular	O
due	O
to	O
being	O
too	O
low-level	O
.	O
</s>
<s>
Dispatch	O
interfaces	B-Application
are	O
flexible	O
,	O
but	O
suffer	O
from	O
additional	O
introspection	O
and	O
invocation	O
overhead	O
compared	O
to	O
custom	O
interfaces	B-Application
.	O
</s>
<s>
It	O
is	O
therefore	O
often	O
a	O
good	O
idea	O
to	O
support	O
both	O
interface	B-Application
types	O
with	O
dual	O
interfaces	B-Application
.	O
</s>
<s>
That	O
way	O
,	O
clients	O
supporting	O
VTABLE-based	O
invocation	O
can	O
use	O
the	O
custom	O
interface	B-Application
instead	O
of	O
the	O
dispatch	O
counterpart	O
.	O
</s>
<s>
Each	O
property	O
and	O
method	O
implemented	O
by	O
an	O
object	O
that	O
supports	O
the	O
IDispatch	B-Application
interface	B-Application
has	O
what	O
is	O
called	O
a	O
Dispatch	O
ID	O
,	O
which	O
is	O
often	O
abbreviated	O
DISPID	O
.	O
</s>
<s>
IDispatch	B-Application
derives	O
from	O
IUnknown	B-Application
and	O
extends	O
it	O
with	O
four	O
additional	O
methods	O
:	O
</s>
<s>
The	O
IDispatch	B-Application
interface	B-Application
ID	O
is	O
defined	O
as	O
a	O
GUID	O
with	O
the	O
value	O
of	O
 { 00020400-0000-0000-C000-000000000046 } 	O
.	O
</s>
