Fix the delegate type system

[Banyc]

Background and Motivation

I want to clean up all the delegates that are previously assigned to a timer's Elapsed event. I don't want it to fire the old delegates that have had the old parameters. I want the timer to fire the newest delegate with the newest parameters.

From this small perspective, we can find out that there is a pitfall in the delegate type system. It might be a C# language design flaw. Based on the contributor of dotnet/runtime, it is about the ability to modify the contract of delegate and event. I hope the contributors of both repositories can have a discussion of this issue since I, as a common user, have not dived into the .NET platform.

Proposed API

Please make EventHandler as a IList that could be Clear(), Add(), ...

Just make something like:

public class EventHandler : IList<Delegate>

or

public class EventHandler : List<Delegate>

Usage Examples

timer.Elapsed.Clear();

Alternative Designs

There are a lot of complaints about this issue. One of the examples:

• https://stackoverflow.com/questions/91778/how-to-remove-all-event-handlers-from-an-event

It is not easy to remember/keep track of the old delegates that have been assigned to an event handler. Make Clear() method could make life tons' better.

As contributor stated here, it could be solved by adding clear accessor to EventHandler. But the contributor here believes the ability to modify the contract of delegate and event is the key to fix similar issues once and for all. I believe a more inclusive solution can address more problems in the future.

Risks

Might take some time to implement and test, or even refactor all the runtime library. (I am not quite sure about that since I am just a usual user).

Relative issues

• [Proposal]: make EventHandler as a IList csharplang#4535
• [Proposal]: make EventHandler as a IList #49595

From the discussion dotnet/csharplang#4536.