I don't think you don't usually talk in terms of exposing an application to ActiveX - usually one 'writes an (ActiveX) component that can be used by other programs'
Thinking further on this, I guess you could mean 'how do I expose an interface from my program so something else can interact with it' .. so I think you need to go back to your question and edit it, to resolve/clear up the ambiguity - what exactly are you trying to achieve ?
So if you wish to use .Net and write an activeX component, I'd start with
Create ActiveX in .NET Step by Step[
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