COM interface instantiation C#

Question

I am working with the CDO COM DLL for handling .eml files. When I try to instantiate the MessengerClass class, VC#2010 tells me I have to instantiate using the interface e.g.

CDO.Message msg = new CDO.Message();

instead of

CDO.Message msg = new CDO.MessageClass();

How can this be? Wont all the functionality be missing from this instance? This article says to do the same thing. What am I missing? Does an interface still contain usable functionality?

This is COM. Normal rules do not apply. – SLaks Aug 26 '12 at 22:05 — SLaks, Aug 26 '12 at 22:05

score 1 · Answer 1 · edited May 23 '17 at 12:16

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The CDO.Message class exposes the IMessage, IBodyPart and IDataSource interfaces according to MSDN

This answer has more details on some of the C#/COM behind the scenes magic.

edited May 23 '17 at 12:16

Community

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answered Aug 26 '12 at 22:13

Rob Walker

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when viewing the Message interface definition in VC#2010 it contains [CoClass(typeof(MessageClass))], and MessengerClass is defined as a bunch of virtual functions. I'm confused. – Nate Aug 26 '12 at 22:24

score 1 · Accepted Answer · answered Aug 26 '12 at 22:21

1

The compiler translates new COMInterface() into actual COM code that looks up the COM class that implements the interface.

As I understand, COM interfaces do not have multiple implementations; instead, the runtime loads the implementation from appropriate DLL specified in HKCR\Classes{guid}.
(I don't know much about COM, so this may be inaccurate.

answered Aug 26 '12 at 22:21

SLaks

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This does explain some things. – Nate Aug 26 '12 at 22:28

score 0 · Answer 3 · answered Aug 04 '14 at 11:20

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Just go to the CDO reference right click on it chose Properties and change the property of Embed Interop Types to False

answered Aug 04 '14 at 11:20

ghalib ghniem

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COM interface instantiation C#

3 Answers3