"Clearing" all chained methods in delegate c#

Question

I have a situation where I need to chain methods to a delegate D using the anonymous method syntax (D += delegate(...){}). However I need to make sure that other chained methods are removed before adding a new one.

I know I can do something like D -= a, D += b, but since I'm using an anonymous method, I'm not sure whether the -= will work (since I don't have an explicit name.).

If my reasoning is correct is there a way I can remove all chained methods using the anonymous syntax ?

Thanks in advance.

Why not simply set the delegate directly to what you want, `D = () => { whatever }` or similar. — SAJ14SAJ, Nov 23 '12 at 13:30
@SAJ14SAJ, He needs to avoid the "older" subscriptions to fire — Diego, Nov 23 '12 at 13:31
no it's not mine and setting it with D = ... throws an exception can only appear on the left hand side of += or -= — Jonny, Nov 23 '12 at 13:32
You need to show more context then, please... D is not a full delegate variable, or pure assignment would be permitted. — SAJ14SAJ, Nov 23 '12 at 13:33
You can check this [link](http://stackoverflow.com/questions/1362204/c-how-to-remove-a-lambda-event-handler) — Nikola Davidovic, Nov 23 '12 at 13:33
@Jonny: That's not an exception, that's a compiler error. You get that, because `D` is declared as an event. — Daniel Hilgarth, Nov 23 '12 at 13:42
I've edited my answer, so that it should fit all constraints given here. — Honza Brestan, Nov 23 '12 at 14:15

score 2 · Answer 1 · edited May 23 '17 at 10:24

I had to retract my original answer, because it was so wrong and comments (under both question and my answer) provided some additional info.

For both event and delegate, you can simply assign null (or a new listener) with = to get rid of all other listeners - but only in the same class as the event/delegate is defined.

I have found out that it's not possible by regular means to remove all listeners from an event that is not declared in the same class, but you can do it with Reflection by finding the event/delegate and assigning null to it with FieldInfo.SetValue.

Here's a link to an older SO question, which might have had similar constraints to yours. This particular answer worked for me: https://stackoverflow.com/a/8108103/1659828

It is not necessary to manually remove the other delegates if `D` is not an event but a simple delegate. You can simply assign your delegate, overwriting all previous assignments. No need for `GetInvocationList` — Daniel Hilgarth, Nov 23 '12 at 13:46

score 0 · Answer 2 · answered Nov 25 '12 at 09:06

Delegate types are immutable types. If D is an instance of a delegate type, it consists of a fixed-length invocation list with one or more items.

So you can't "clear" the list. You also cannot create a new instance with an empty invocation list because a null reference (not an instance) is used for the case where no methods are present.

Example (MyMethod and JonnysMethod are method groups, not delegate variables):

Action D = null;   // no object, no invocation list

D += MyMethod;     // null and new Action(MyMethod) are combined; result is assigned to D
// D is an instance, invocation list length is 1
D += JonnysMethod; // two instances are combined; D is assigned to new instance; original instances are no longer referenced and will "soon" be garbage collected
// D is an instance, invocation list length is 2

D -= MyMethod;     // D is assigned to a new instance again
D -= JonnysMethod; // D is assigned to null; no new instance

Take care: If D is really an event, be careful. If you're outisde the class/struct where D is defined, += and -= are calls to the accessors of the event. So they are not immediate delegate combinations in this case. If you're inside the class/struct that defines D, and if D is a "field-like" event, D can be used in two senses. As a delegate variable (the "hidden" backing field of the event), and as an event. The meaning of += and -= can the be interpreted in two ways. Until C# 3, it was interpreted as delegate combinantion directly on the backing field. In C# 4 and C# 5, the semantic is changed and += and -= are calls to the event accessors in this situation.

"Clearing" all chained methods in delegate c#

2 Answers2