One possible approach would be to use the
Event Aggregator. However, you must keep in mind that a published event may be captured by more than one subscriber (if there are more than one instance of the subscriber class) and there can be also more than one publisher for an event.
For an example, you may have a look at the
Event Aggregation QuickStart.
For more information, you may check the following topics:
How to: Create and Publish Events
How to: Subscribe and Unsubscribe to Events
Another possible approach would be to declare an interface for the class (whose methods you want to invoke) in a common module (referenced by Module A and Module B) and then register an instance
of that class in the container (e.g.: in Module A), so that the other module (e.g.: in Module B) can retrieve it through its interface and invoke its methods.
You may also use services for the same purpose.
Hope it helps.
Ignacio Baumann Fonay