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December 05, 2007
DLLs with COM interface
Hello!


I was reading up on http://www.digitalmars.com/d/dll.html#com regarding 
how to call DLLs with a COM interface, since this is exactly what I am 
struggling with.

I'm trying to interface with proprietary DLLs that I know expose COM 
interfaces. Fortunately, I have the documentation for them so I know 
exactly what methods they expose.

There is, for example trtCom.dll, which exposes a trtComMgr class, which 
 has a method AboutBox(). I thought I'd start with calling that, since 
it  takes no arguments and returns nothing.

Now, it said in the article mentioned above that COM objects and D 
interfaces are virtually the same thing, so I tried this:

extern(Windows)
{
	interface MbtComMgr
	{
		void AboutBox();

	}
}

But that wasn't very popular with the D compiler. It simply said:
Error: need 'this' to access member AboutBox

What would be the proper way of accessing a COM DLL?


The article said that "The sample code includes an example COM client 
program and server DLL." What sample code is it talking about exactly? I 
looked under C:\dmd\samples\d\mydll but found nothing that looks like an 
example on this.


Cheers!

Henrik
December 05, 2007
Re: DLLs with COM interface
Henrik wrote:
> But that wasn't very popular with the D compiler. It simply said:
> Error: need 'this' to access member AboutBox

the interface is just an abstract type, you'll always need an object 
that you access as such an interface. that object needs to be an 
instance of some class that implements the interface.
usually the DLL would expose a C-style function that returns such an object.
for standard COM, the DLL exposes DllGetClassObject that returns a class 
factory object, that can instantiate all COM objects contained in the 
DLL. the class factory object (that implements IClassFactory) has a 
CreateInstance method, that will give you object pointer for a requested 
interface.
for more details, [1] is a good place to start. i also recommend the 
book Essential COM by Don Box.

> The article said that "The sample code includes an example COM client 
> program and server DLL." What sample code is it talking about exactly? I 
> looked under C:\dmd\samples\d\mydll but found nothing that looks like an 
> example on this.

you can look at my code for "seatd for scite" at [2], that plugs code 
navigation functionality into the scite editor using com objects. the 
relevant code is in src/scite/seatd_scite.dll and src/scite/scite_ext.d
i'm not following COM completely here, i.e. i'm not implementing 
IClassFactory and the method that returns the object is not called 
DllGetClassObject, but it's basically COM and it covers your problem as 
well.

[1] http://en.wikipedia.org/wiki/Component_object_model
[2] http://seatd.mainia.de/
December 05, 2007
Re: DLLs with COM interface
"Henrik" <zodiachus@gmail.com> wrote in message 
news:fj617q$15qk$1@digitalmars.com...
> Hello!
>
>
> I was reading up on http://www.digitalmars.com/d/dll.html#com regarding 
> how to call DLLs with a COM interface, since this is exactly what I am 
> struggling with.
>
> I'm trying to interface with proprietary DLLs that I know expose COM 
> interfaces. Fortunately, I have the documentation for them so I know 
> exactly what methods they expose.
>
> There is, for example trtCom.dll, which exposes a trtComMgr class, which 
> has a method AboutBox(). I thought I'd start with calling that, since it 
> takes no arguments and returns nothing.
>
> Now, it said in the article mentioned above that COM objects and D 
> interfaces are virtually the same thing, so I tried this:
>
> extern(Windows)
> {
> interface MbtComMgr
> {
> void AboutBox();
>
> }
> }
>
> But that wasn't very popular with the D compiler. It simply said:
> Error: need 'this' to access member AboutBox
>
> What would be the proper way of accessing a COM DLL?

COM interfaces have to inherit from IUnknown.  IUnknown is "magic" and the 
compiler treats it specially.

Furthermore, I think you should put the extern(Windows) on the inside of the 
interface, as I don't think putting it on the outside will affect the 
calling conventions of the things on the inside.  (Putting it on the outside 
won't do much of anything, actually, except maybe change the name mangling 
of the interface.)

So you'll have:

import std.c.windows.com;

interface MbtComMgr : IUnknown
{
extern(Windows):
   void AboutBox();
}

:)
December 06, 2007
Re: DLLs with COM interface
have a look at :
http://www.dsource.org/projects/juno/wiki/TypeLibraryImporter

you'll find worthfull COM information and a nice tool to automate this task.
HTH Bjoern

Henrik schrieb:
> Hello!
> 
> 
> I was reading up on http://www.digitalmars.com/d/dll.html#com regarding 
> how to call DLLs with a COM interface, since this is exactly what I am 
> struggling with.
> 
> I'm trying to interface with proprietary DLLs that I know expose COM 
> interfaces. Fortunately, I have the documentation for them so I know 
> exactly what methods they expose.
> 
> There is, for example trtCom.dll, which exposes a trtComMgr class, which 
>  has a method AboutBox(). I thought I'd start with calling that, since 
> it  takes no arguments and returns nothing.
> 
> Now, it said in the article mentioned above that COM objects and D 
> interfaces are virtually the same thing, so I tried this:
> 
> extern(Windows)
> {
>     interface MbtComMgr
>     {
>         void AboutBox();
> 
>     }
> }
> 
> But that wasn't very popular with the D compiler. It simply said:
> Error: need 'this' to access member AboutBox
> 
> What would be the proper way of accessing a COM DLL?
> 
> 
> The article said that "The sample code includes an example COM client 
> program and server DLL." What sample code is it talking about exactly? I 
> looked under C:\dmd\samples\d\mydll but found nothing that looks like an 
> example on this.
> 
> 
> Cheers!
> 
> Henrik
December 06, 2007
Re: DLLs with COM interface
On Wed, 05 Dec 2007 12:15:10 +0100, Henrik <zodiachus@gmail.com> wrote:

> Hello!
>
>
> I was reading up on http://www.digitalmars.com/d/dll.html#com regarding  
> how to call DLLs with a COM interface, since this is exactly what I am  
> struggling with.
>
> I'm trying to interface with proprietary DLLs that I know expose COM  
> interfaces. Fortunately, I have the documentation for them so I know  
> exactly what methods they expose.
>
> There is, for example trtCom.dll, which exposes a trtComMgr class, which  
>   has a method AboutBox(). I thought I'd start with calling that, since  
> it  takes no arguments and returns nothing.
>
> Now, it said in the article mentioned above that COM objects and D  
> interfaces are virtually the same thing, so I tried this:
>
> extern(Windows)
> {
> 	interface MbtComMgr
> 	{
> 		void AboutBox();
>
> 	}
> }
>
> But that wasn't very popular with the D compiler. It simply said:
> Error: need 'this' to access member AboutBox
>
> What would be the proper way of accessing a COM DLL?

An interface on its own is just a contract saying how the actual instance  
will be layed out in memory and how it will be called (usually extern  
(Windows) for COM). You need an actual instance, which you can get from  
Windows with the CoCreateInstance call:

extern (Windows) IFooBar : IUnknown
{
    ...
}

CoInitialize(null);              // initialize COM before getting the  
instance
scope (exit) CoUninitialize();   // close COM after we're finished

IFooBar instance; // this just declares a reference to the interface  
IFooBar which is null at first

CLSID clsid = ...;     // you need to know the DLL's class id first!
CLSCTX clsctx = ...;   // the context, normally CLSCTX_INPROC_SERVER

auto rc = CoCreateInstance(clsid, null, clsctx, clsid,  
cast(void**)&instance);

if (rc != S_OK) throw new Exception("Couldn't create instance!");

/* now you can call the IFooBar instance */

instance.Release(); // release the instance

The important part is to get the instance pointer from COM with  
CoCreateInstance. It creates the instance and stores the pointer to it in  
instance (that's why there's a cast(void**) in there). Deriving from  
IUnknown is important too 'cause then the housekeeping is done  
automatically and you just use the instance as if you'd created it with  
new. Don't forget to release the instance after you're done with it, don't  
delete it!

The definitions for CLSID etc. are in phobos and Tango, just search for  
them. If it doesn't work (like it did for me) I can recommend downloading  
the schooner project from dsource.org and using its win32 files - they  
have every definition and they work.

-Mike

-- 
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
December 17, 2007
Re: DLLs with COM interface
That is a very nifty little tool! After resolving the interdependencies 
between the DLLs, the stuff actually runs! Excellent, thanks.




BLS wrote:
> have a look at :
> http://www.dsource.org/projects/juno/wiki/TypeLibraryImporter
> 
> you'll find worthfull COM information and a nice tool to automate this 
> task.
> HTH Bjoern
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