I think he's asking something else. He's asking if you can get instantiated objects in a different process using reflection. So in process A, I have...
ArrayList<string> Foo = new ArrayList<string();
Foo.Add("I'm a Foo");
... in process B, is there any way he can call Foo.Clear(), assuming that process A doesn't explicitly open a channel for him to do so.
If that's the case, I'd say the answer is an emphatic "NO". This would be a huge security issue.
If, on the other hand like other people suspect, you are trying to automate entry on another application (either for testing purposes or as part of some "helpful" app), there are approaches for those.