If you created an object in another domain e.g. with AppDomain.CreateInstanceAndUnwrap, all you need to call the object in another domain is to call an object's method.
The simplest way to make a cross-application domain call is just to make a call directly on that object, which actually is exposed from another domain via its proxy, existing in another domain.
UPD
Unfortunately getting the host domain is not that easy. You should enumerate domains like this and find among them the host one. I suppose that your host domain is the one for which the method AppDomain.IsDefaultAppDomain
returns true.
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