There are some cases when Nothing should be passed as an event argument, for example when the AccountChanged event is fired with actPositionsReloaded type, the position parameter of the event should be Nothing (Null). If the user application is an ATL Client, an exception will be thrown in such cases.
Starting from v 3.0, CQG API proposes a workaround for this problem: if you use it from an ATL Client, you can notify the CQG API about it via UsedFromATLClient property of CQG API Configuration. In that case, a special instance of corresponding object will be passed as an event argument. For example, in case of CQGPosition: a special not valid instance of CQGPosition will be passed.
Such objects can be differentiated via IsValid Method of CQGCEL.
Note: If you try to access any property/method of such instances, an exception will be thrown.