Need COM Interop Guru help please...

[Richard]

Hi,

I am coding in C# and working with COM objects that were
written in Java. The product documentation for the Java
stuff states that the MS JVM does not support interface
inheritance - and then the documentation goes on to give
VB6 examples of how to get to a base interface given a
child interface.

Well I need to get to the base interface in C#! I have an
interface called "IPersistentObj ect" and it has a base
interface of "ISysObject ". How can I get to ISysObject
given an instance of IPersistentObje ct? My code below
compiles but it's not working {pardon the spacing but the
newsgroup dooesn't do source code too well}:

{
IPersistentObje ct pobj = CreatePersisten tObject();

ISysObject sysObj = (ISysObject)
Marshal.GetType dObjectForIUnkn own
(Marshal.GetIUn knownForObject( pobj), typeof(ISysObje ct));

IntPtr punk = Marshal.GetIUnk nownForObject(s ysObj);

sysObj.setObjec tName("MyObject ");
}

When this code runs "punk" reveals a non-zero value which
I'm taking to mean that in fact the QueryInterface did
succeed; the sysObj is an RCW that referes to something
that supports IUnknown. However when I try to use the
interface, to call "setObjectName" , I get a
System.NullRefe rence exception. Any ideas?

--Richard

[Mattias Sjögren]

Re: Need COM Interop Guru help please...

Richard,
[color=blue]
>and then the documentation goes on to give
>VB6 examples of how to get to a base interface given a
>child interface.[/color]

Can you post that VB6 code?

[color=blue]
>How can I get to ISysObject
>given an instance of IPersistentObje ct?[/color]

All you should need is a cast

ISysObject sysObj = (ISysObject)pob j;



Mattias

--
Mattias Sjögren [MVP] mattias @ mvps.org

http://www.msjogren.net/dotnet/

Please reply only to the newsgroup.

[Richard]

Per Request: VB6 sample code

Yepp all they do in VB6 is cast to the other interface. I
tried that first in C# and I got a System.NullRefe rence
exception too. I dunno what's going wrong, it is a
mystery wrapped in an RCW... Here's their VB6 code:

Dim pObj As IPersistentObje ct
Dim sysObj As ISysObject

Set pObj = sess.newObject( "dm_documen t")
....
Set sysObj = pObj
....
sysObj.setObjec tName docName

--Richard

[Jeffrey Tan[MSFT]]

RE: Per Request: VB6 sample code


Hi Richard,

NullReference exception related to the object.
Before you invoke the setObjectName method of sysObj, is you
sysObj a valid reference?
If it is valid, I think your problem must be something related to your
COM self, you can create a script client to invoke the COM to determine
this.

Hope this helps.
Best regards,
Jeffrey Tan
Microsoft Online Partner Support
Get Secure! - www.microsoft.com/security
This posting is provided "as is" with no warranties and confers no rights.

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|
| Yepp all they do in VB6 is cast to the other interface. I
| tried that first in C# and I got a System.NullRefe rence
| exception too. I dunno what's going wrong, it is a
| mystery wrapped in an RCW... Here's their VB6 code:
|
| Dim pObj As IPersistentObje ct
| Dim sysObj As ISysObject
|
| Set pObj = sess.newObject( "dm_documen t")
| ...
| Set sysObj = pObj
| ...
| sysObj.setObjec tName docName
|
| --Richard
|
|

[Mattias Sjögren]

Re: Per Request: VB6 sample code

Richard,
[color=blue]
>I tried that first in C# and I got a System.NullRefe rence
>exception too.[/color]

There are a number of possible reasons that can cause a
NullReferenceEx ception. For example using the wrong thread apartment
(are you missing the STAThread attribute?) or a bug in the COM
object's IUnknown implementation.



Mattias

--
Mattias Sjögren [MVP] mattias @ mvps.org

http://www.msjogren.net/dotnet/

Please reply only to the newsgroup.