如何释放事件处理程序

Question

Normally, anonymous event handlers can be released as follows:

    EventHandler hdl = null;
    hdl += (ss,ee) =>
    {
       //....
       MyObj.Completed -= hdl;            
       hdl = null;
    };

    MyObj.Completed += hdl;            
    MyObj.AsyncCall();

My question is: Is hdl = null; necessary for the latest version of C#? Also are there any simpler solutions or simpler a syntax for this release?

Answer 1

A few things:

1. Simply setting hdl = null will not free your handler for garbage collection because MyObj.Completed would still hold the reference to the handler. (But MyObj.Completed -= hdl takes care of it in this case, so you should be fine.)
2. You don't have to use += on your assignment to hdl. You should just use simple assignment = .
3. If your lambda were, instead, a named method, you could use MyObj.Completed += MyCallbackName and MyObj.Completed -= MyCallbackName . This doesn't work if you need stuff captured in closure, but I don't see that from your example.

Answer 2

No, it is not required. That is basically all tied to a capture-context (compiler-generated class); once the event is unsubscribed, nothing will be keeping the capture-context in scope, so it will be eligible for garbage collection. There is no benefit (but no real harm) wiping the variable hdl (actually a field on the capture-context).