If I create a lambda callback such as this:
var someInstance = new SomeObject(); someInstance.Finished += (obj, args) => { // Do something // Deregister here }; someInstance.DoAction(); How could I deregister the callback as part of the actual handler code? This pattern would be ideal as I could ensure it is released as soon as possible.
I've seen similar questions but not one where this type of example is directly addressed.
With something like,
var someInstance = new SomeObject(); EventHandler myDelegate = null; myDelegate = (obj, args) => { // Do something // Deregister here someInstance.Finished -= myDelegate; }; someInstance.Finished += myDelegate; someInstance.DoAction(); 
delegate myDelegate = null; does not compile.
EventHandler myDelegate = null; instead, that should work.You can use MethodInfo.GetCurrentMethod inside your lambda to retrieve the MethodInfo of the lambda.
With the MethodInfo, you can use Delegate.CreateDelegate to get the properly typed delegate representing your lambda.
And with the delegate, you can unregister the lambda, all without storing your function in a variable or making it a named method.
class MyClass { public event EventHandler TheEvent; void TestIt() { TheEvent += (sender, eventargs) => { Console.WriteLine("Handled!"); // do something in the handler // get a delegate representing this anonymous function we are in var fn = (EventHandler)Delegate.CreateDelegate( typeof(EventHandler), sender, (MethodInfo)MethodInfo.GetCurrentMethod()); // unregister this lambda when it is run TheEvent -= fn; }; // first time around this will output a line to the console TheEvent(this, EventArgs.Empty); // second time around there are no handlers attached and it will throw a NullReferenceException TheEvent(this, EventArgs.Empty); } } 
If you want the ability to unregister, then it's best to define your lambda as an Action, so that you can write:
someInstance.Finished += MyCustomAction; and later
someInstance.Finished -= MyCustomAction; If you want to remove the event handler afterwards, you should use a named function:
someInstance.Finished += HandleFinished; //... void HandleFinished(object sender, EventArgs args) { someInstance.Finished -= HandleFinished; } 
MethodInfo.GetCurrentMethod), but I don't have the time at this moment to find out exactly how ... if I remember I'll come back to it but I would say don't give up too quickly.This appears to work:
EventHandler handler = null; handler = new EventHandler((sender, e) => SomethingHappened -= handler); SomethingHappened += handler; Just be sure you don't assign handler to any other value, as the closure is tied to that variable. This is what Resharper complains of as an "Access to Modified Closure."
