How can I clear event subscriptions in C

Question

Take the following C  class   c1    event EventHandler someEvent      If there are a lot of subscriptions to c1 s someEvent event and I want to clear them all  what is the best way to achieve this  Also consider that subscriptions to this event could be are lambdas anonymous delegates   Currently my solution is to add a ResetSubscriptions   method to c1 that sets someEvent to null  I don t know if this has any unseen consequences

User · Answer

You can achieve this by using the Delegate Remove or Delegate RemoveAll methods

User · Answer

Instead of adding and removing callbacks manually and having a bunch of delegate types declared everywhere     The hard way public delegate void ObjectCallback ObjectType broadcaster    public class Object       public event ObjectCallback m ObjectCallback           void SetupListener                 ObjectCallback callback   null          callback    ObjectType broadcaster    gt                           one time logic here             broadcaster m ObjectCallback -  callback                     m ObjectCallback    callback                  void BroadcastEvent                 m ObjectCallback  Invoke this            You could try this generic approach  public class Object       public Broadcast lt Object gt  m EventToBroadcast   new Broadcast lt Object gt          void SetupListener                 m EventToBroadcast SubscribeOnce  ObjectType broadcaster    gt                   one time logic here                         Object                 m EventToBroadcast Dispose            m EventToBroadcast   null             void BroadcastEvent                 m EventToBroadcast Broadcast this             public delegate void ObjectDelegate lt T gt  T broadcaster   public class Broadcast lt T gt    IDisposable       private event ObjectDelegate lt T gt  m Event      private List lt ObjectDelegate lt T gt  gt  m SingleSubscribers   new List lt ObjectDelegate lt T gt  gt           Broadcast                 Dispose               public void Dispose                 Clear            System GC SuppressFinalize this              public void Clear                 m SingleSubscribers Clear            m Event   delegate                    add a one shot to this delegate that is removed after first broadcast     public void SubscribeOnce ObjectDelegate lt T gt  del                m Event    del          m SingleSubscribers Add del                 add a recurring delegate that gets called each time     public void Subscribe ObjectDelegate lt T gt  del                m Event    del             public void Unsubscribe ObjectDelegate lt T gt  del                m Event -  del             public void Broadcast T broadcaster                m Event  Invoke broadcaster           for  int i   0  i  lt  m SingleSubscribers Count    i                        Unsubscribe m SingleSubscribers i                      m SingleSubscribers Clear

User · Answer

Setting the event to null inside the class works  When you dispose a class you should always set the event to null  the GC has problems with events and may not clean up the disposed class if it has dangling events

User · Answer

Remove all events  assume the event is an  Action  type   Delegate   dary   TermCheckScore GetInvocationList     if   dary    null         foreach   Delegate del in dary                 TermCheckScore -    Action   del

User · Answer

From within the class  you can set the  hidden  variable to null  A null reference is the canonical way of representing an empty invocation list  effectively   From outside the class  you can t do this - events basically expose  subscribe  and  unsubscribe  and that s it   It s worth being aware of what field-like events are actually doing - they re creating a variable and an event at the same time  Within the class  you end up referencing the variable  From outside  you reference the event   See my article on events and delegates for more information

User · Answer

This is my solution   public class Foo   IDisposable       private event EventHandler  statusChanged      public event EventHandler StatusChanged               add                        statusChanged    value                    remove                        statusChanged -  value                       public void Dispose                  statusChanged   null            You need to call Dispose   or use using new Foo             pattern to unsubscribe all members of invocation list

User · Answer

The best practice to clear all subscribers is to set the someEvent to null by adding another public method if you want to expose this functionality to outside  This has no unseen consequences  The precondition is to remember to declare SomeEvent with the keyword  event   Please see the book - C  4 0 in the nutshell  page 125  Some one here proposed to use Delegate RemoveAll method  If you use it  the sample code could follow the below form  But it is really stupid  Why not just SomeEvent null inside the ClearSubscribers   function  public void ClearSubscribers         SomeEvent    EventHandler  Delegate RemoveAll SomeEvent  SomeEvent         Then you will find SomeEvent is set to null

User · Answer

Conceptual extended boring comment   I rather use the word  event handler  instead of  event  or  delegate   And used the word  event  for other stuff  In some programming languages  VB NET  Object Pascal  Objective-C    event  is called a  message  or  signal   and even have a  message  keyword  and specific sugar syntax   const   WM Paint   998       lt --  question  can be done by several talkers   WM Clear   546   type   MyWindowClass   class Window      procedure NotEventHandlerMethod 1      procedure NotEventHandlerMethod 17       procedure DoPaintEventHandler  message WM Paint      lt --  answer  by this listener     procedure DoClearEventHandler  message WM Clear    end    And  in order to respond to that  message   a  event handler  respond  whether is a single delegate or multiple delegates   Summary   Event  is the  question    event handler  s   are the answer  s

User · Answer

Add a method to c1 that will set  someEvent  to null  public class c1       event EventHandler someEvent      public ResetSubscriptions     gt  someEvent   null

User · Answer

class c1       event EventHandler someEvent      ResetSubscriptions     gt  someEvent   delegate         It is better to use delegate     than null to avoid the null ref exception

[c#] How can I clear event subscriptions in C#?

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