interop with COM object with mutiple interfaces ?

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Lawrence Lourduraj - 08 Oct 2003 18:09 GMT

I am trying to use a COM object that implements interfaces A, B, C. A is the
default interface for the coclass in the IDL. The COM object is represented
by managed type X.

this is what I am doing in VB.NET,

Dim foo as object = new X()

Dim obj1 as object = foo.GetObj1 // this actually returns a COM object that
implements interface B

obj1.someMethod() // instead of invoking the method on interface B it uses A
and throws a MissingMemberException

Any ideas on how to force .NET to use interface B instead of the default
interface A ? Everything is via automation and "someMethod" in not in the
typelib

Thanks

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Mattias Sj?gren - 08 Oct 2003 23:36 GMT

Lawrence,

>Dim obj1 as object = foo.GetObj1 // this actually returns a COM object that
>implements interface B

So why don't you type the variable As B?

Dim obj1 As B = foo.GetObj1

or, if necessary

Dim obj1 As B = CType(foo.GetObj1, B)

Mattias

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Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/
Please reply only to the newsgroup.

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Lawrence Lourduraj - 10 Oct 2003 15:24 GMT

Hello Mattias:

The COM object (coclass) implements A, B, C all of which are derived from
IDispatch and A is the default.

I did try,

Dim obj1 as B = foo.GetObj1 and

Dim obj1 as B = CType(foo.GetObj1, B)

and got the same MissingMemberException. Now the returned object (obj1)
actually implements A, B, C but I want "someMethod" to be invoked via B not
A which is the default. This is because the COM component differentiates
between whose IDispatch is used and does different things. I did not write
it so I can explain this design decision !

The IDL contains only one coclass, so "foo" and "obj1" are instances of the
same class. The difference is "GetObj1()" returns a ptr to B after
performing the appropriate cast.

Further NONE of the methods (GetObj1(), someMethod()) are described in the
typelib.

I finally got the following to work,

GetType(B).InvokeMember("someMethod", ..., obj1,...)

I would appreciate it if you could explain why your suggested approach did
not work. I would prefer to use that as it is more intuitive.

Thanks

Lawrence

> Lawrence,
>
[quoted text clipped - 10 lines]
>
> Mattias

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interop with COM object with mutiple interfaces ?

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