C# – “Delegate subtraction has unpredictable result” in ReSharper/C#

ccompiler-warningsdelegateseventsresharper

When using myDelegate -= eventHandler ReSharper (version 6) issues:

Delegate subtraction has unpredictable result

The rational behind this is explained by JetBrains here. The explanation makes sense and, after reading it, I'm doubting all my uses of - on delegates.

How then,

  • can I write a non-auto event without making ReSharper grumpy?
  • or, is there a better and/or "correct" way to implement this?
  • or, can I just ignore ReSharper?

Here is simplified code:

public delegate void MyHandler (object sender);

MyHandler _myEvent;

public event MyHandler MyEvent
{
    add
    {
        _myEvent += value;
        DoSomethingElse();
    }
    remove
    {
        _myEvent -= value; // <-- ReSharper warning here
    }
}

Best Answer

Don't be afraid! The first part of ReSharper's warning only applies to removing lists of delegates. In your code, you're always removing a single delegate. The second part talks about ordering of delegates after a duplicate delegate was removed. An event doesn't guarantee an order of execution for its subscribers, so it doesn't really affect you either.

Since the above mechanics can lead to unpredictable results, ReSharper issues a warning whenever it encounters a delegate subtraction operator.

ReSharper is issuing this warning because multicast delegate subtraction can have gotchas, it isn't condemning that language feature entirely. Luckily those gotchas are in fringe cases and you are unlikely to encounter them if you're just instrumenting simple events. There is no better way to implement your own add/remove handlers, you just gotta take notice.

I'd suggest downgrading ReSharper's warning level for that message to "Hint" so that you don't get desensitized to their warnings, which are usually useful.

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