Sending an event from a class to a form

Question

Hi Can someone help please... I'm trying to do the following: create a class (ClassA) that raises an event on a form (FormB) (if it's not open just don't send) not knowing the Form itself. My problem is i create the definition of the event on the Class A with the delegates events definition. On the ClassA when it enters a function it should raise an event on the FormB (which I don't what the ClassA to know about. Just send to the active delegates. The namespace of the ClassA is completely different from the one from the client and that lib doesn't know the existance of the client form). I always get the message: Object reference not set to an object instance. I'm I doing this completely wrong? The only thing I managed on ClassA was to close the ActiveForm (and like this I have to have FormB as active since if I change the focus i will get the message Object reference not set to an object instance again. Furthermore ClassA is MarshalByRef from a server (and defined on client/server)... which I'm not sure affects something on this. I tried to explain the best... Thx Rui

richard.ryerson · Answer

Look into the Observer Design pattern. What this pattern specifies that objects (forms controls business objects etc) can be observed or observe others. When an event in fired in an Observable object this object will raise then event for all the Observers. For example: You can a class implementing the IObservable interface ClassA. This class has an event called Changed it is of type EventHandler (most basic delegate definition). This class also contains a method Register which registers delegates in say an arraylist or some collection type object. It also uses AddHandler to assign the delegate to the Changed event When an instantiated object of ClassA has it's state changed it would raise the event Changed. In its OnChanged method it would call RaiseEvent which since we added delegates to the event will fire the event and will be caught by all the Observers. You should also implement IDisposable in your class and call the RemoveHandler to remove all the delegates in listed in the ArrayList of delegates when the object is destroyed. I hope this helps a bit. Best regards Rick

macrui · Answer

Forget the last post. The trick was singleton. I used a Singleton that broadcasts the messages to registered clients and i'm not using events and delegates. Now: ClassA receives a message from a server Forms register under a Broadcast class (singleton - static) ClassA calls the Broadcast that in turn calls the Listener on the form (registered on the Broadcaster). Thank you so much. I owe you big. Now i'm going to try to unregister in case of un-disposed form clients of the Broadcaster. Rui.

macrui · Answer

hi again ClassA is in the lib (shared by Server and Client) but formB isn't. Anyway they belong to the same application. ClassA is called async by a server (MarshalByRef) and runs on the client. The problem is that I want this class (ClassA) to raise an event on FormB. I do not want FormB to call anything on ClassA since ClassA receives the messages from the server and from that point on it should send a message to FormB (FormC FormD...etc). That's why I was thinking in events and delegates but I'm not able to call the delegate on FormB from ClassA since I always get an error Object reference not set.... How can I get the (from ClassA) a reference to FormB? Sorry for all this questions and probably i'm not doing much sense but I'm stuck on this Thx Rui.

Mykonine · Answer

If ClassA and FormB are in different programs you will have to do it via Com Interop using Windows API calls. This is assuming that ClassA and all the Forms are in the same application. You can initiate a singleton (static field) something like ClassA.activeInstance at startup. This way your forms can make a call to ClassA.activeInstance to access the class instead of worrying about how to get a reference to it much like System.out in Java.

macrui · Answer

Thx. But i will still have a problem... Since ClassA is called async from the server how to make FormB aware of the instance from ClassA since FormB only resides on the client and I want to use ClassA instance to be the only instance on the client serving many different forms. ClassA is a class that handles messages from the server and depending on the message (message parser) it should raise an event on the corresponding form that has a delegate of the sort. I was thinking that I could do this on one of two different ways: - Make ClassA an interface and all the forms inherit from it. This way I would have to change the function that handles messages on ClassA virtual and each function on FormX use a definition of it's own. The drawback I would have to use reflection on ClassA and turn my messaging system passing objects that especifi forms understand - Create a static hashtable that holds the forms open on the client and ClassA searches for the corresponding forms that it wants to raise an event and raise it Can I ask for your opinion on what isthe best practice? (forgot to say in first post that i'm doing my first steps in C# and i don't know the best ways to do it) Thx again Rui.

Mykonine · Answer

If ClassA is not aware of FormB you cannot have ClassA raise an event on FormB. You will have to have a FormB instance attach a delegate to an event in the instance of ClassA. When ClassA raises its own event code in FormB will execute.