Has an event handler already been added

Question

Is there a way to tell if an event handler has been added to an object   I m serializing a list of objects into out of session state so we can use SQL based session state    When an object in the list has a property changed it needs to be flagged  which the event handler took care of properly before   However now when the objects are deserialized it isn t getting the event handler   In an fit of mild annoyance  I just added the event handler to the Get property that accesses the object   It s getting called now which is great  except that it s getting called like 5 times so I think the handler just keeps getting added every time the object is accessed   It s really safe enough to just ignore  but I d rather make it that much cleaner by checking to see if the handler has already been added so I only do so once   Is that possible   EDIT  I don t necessarily have full control of what event handlers are added  so just checking for null isn t good enough

User · Answer

The only way that worked for me is creating a Boolean variable that I set to true when I add the event. Then I ask: If the variable is false, I add the event.

bool alreadyAdded = false;

This variable can be global.

if(!alreadyAdded)
{
    myClass.MyEvent += MyHandler;
    alreadyAdded = true;
}

User · Answer

This example shows how to use the method GetInvocationList   to retrieve delegates to all the handlers that have been added   If you are looking to see if a specific handler  function  has been added then you can use array   public class MyClass     event Action MyEvent          MyClass myClass   new MyClass    myClass MyEvent    SomeFunction        Action   handlers   myClass MyEvent GetInvocationList      this will be an array of 1 in this example  Console WriteLine handlers 0  Method Name    prints the name of the method   You can examine various properties on the Method property of the delegate to see if a specific function has been added   If you are looking to see if there is just one attached  you can just test for null

User · Answer

I recently came to a similar situation where I needed to register a handler for an event only once  I found that you can safely unregister first  and then register again  even if the handler is not registered at all   myClass MyEvent -  MyHandler  myClass MyEvent    MyHandler    Note that doing this every time you register your handler will ensure that your handler is registered only once  Sounds like a pretty good practice to me

User · Answer

i agree with alf s answer but little modification to it is   to use              try                               control name Click -  event Click                  main browser Document Click    Document Click                            catch Exception exce                                main browser Document Click    Document Click

User · Answer

EventHandler GetInvocationList   Length  gt  0

User · Answer

From outside the defining class  as  Telos mentions  you can only use EventHandler on the left-hand side of a    or a -   So  if you have the ability to modify the defining class  you could provide a method to perform the check by checking if the event handler is null - if so  then no event handler has been added  If not  then maybe and you can loop through the values in  Delegate GetInvocationList  If one is equal to the delegate that you want to add as event handler  then you know it s there   public bool IsEventHandlerRegistered Delegate prospectiveHandler           if   this EventHandler    null                 foreach   Delegate existingHandler in this EventHandler GetInvocationList                           if   existingHandler    prospectiveHandler                                 return true                                    return false      And this could easily be modified to become  add the handler if it s not there   If you don t have access to the innards of the class that s exposing the event  you may need to explore -  and     as suggested by  Lou Franco    However  you may be better off reexamining the way you re commissioning and decommissioning these objects  to see if you can t find a way to track this information yourself

User · Answer

If I understand your problem correctly you may have bigger issues  You said that other objects may subscribe to these events  When the object is serialized and deserialized the other objects  the ones that you don t have control of  will lose their event handlers     If you re not worried about that then keeping a reference to your event handler should be good enough  If you are worried about the side-effects of other objects losing their event handlers  then you may want to rethink your caching strategy

User · Answer

If this is the only handler  you can check to see if the event is null  if it isn t  the handler has been added   I think you can safely call -  on the event with your handler even if it s not added  if not  you could catch it  -- to make sure it isn t in there before adding

[c#] Has an event handler already been added?

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