How do I get the number of event subscribers in the add and remove blocks?

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If I use the explicit syntax for declaring an event, is it possible to get the number of subscribers inside the add {} and remove {} code blocks? For example:

C#

public event EventHandler MyEvent
{
    add
    {
        MyEvent += value;
        // Can I get a count of subscribers here?
    }
    remove
    {
        MyEvent -= value;
        // And also here?
    }
}

What I have tried:

I have found a way to do this in other parts of the code, using...

C#

MyEvent.GetInvocationList().Count

...but I can't figure out how or even if this syntax can be used inside the event declaration code.

Posted 13-Mar-17 23:49pm

Patrick Skelton

Updated 14-Mar-17 0:33am

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Richard MacCutchan 14-Mar-17 5:56am

What happens when you try it?

Patrick Skelton 14-Mar-17 6:03am

It won't compile. It says something like 'MyEvent can only appear on the left hand side of += or -='.

Bernhard Hiller 14-Mar-17 6:16am

What about this.GetInvocationList()?

Patrick Skelton 14-Mar-17 6:19am

The 'this' keyword appears to refer to the enclosing class, so I get a message saying that the class does not contain a method called GetInvocationList().

I'm guessing maybe it isn't possible because what I am trying to do is get a reference to the class instance that the C# compiler generates automatically in response to the 'event' keyword.

Tomas Takac 14-Mar-17 6:22am

Why? What is your use case? I don't think you can do this from inside the add/remove accessors. If you have custom accessors the you can have a counter which you increment or decrement on add/remove. Or you can drop the custom accessors and get the count via GetInvocationList() anywhere inside your class. Depends on what you are trying to achieve.

Patrick Skelton 14-Mar-17 6:34am

I am simply trying to log the count in the Debug build, to keep an eye on potential memory leaks. I will use a separate count variable as you suggest. It does seem a shame, though, to be introducing a variable and associated code that will only exist in the Debug build and will therefore have to be wrapped in #if DEBUG ... #endif.

Thank you all.

Tomas Takac 14-Mar-17 6:38am

Look at solution #2. I think that solves your problem.

2 solutions

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User 7407470 · Accepted Answer · 2017-03-14T00:33:00

Solution 2

How about this?

C#

private EventHandler _myEvent;
private int _myEventSubscribersCount;

public event EventHandler MyEvent
{
    add
    {
        _myEvent += value;
        _myEventSubscribersCount = _myEvent.GetInvocationList().Length;
    }

    remove
    {
        _myEvent -= value;
        _myEventSubscribersCount = _myEvent.GetInvocationList().Length;
    }
}

Posted 14-Mar-17 0:33am

User 7407470

Comments

Patrick Skelton 14-Mar-17 13:49pm

Excellent. Actually, since all I am doing is a bit of debug logging inside the 'add' and 'remove', I don't even need the count-variable. Which is nice because it means I don't need a single #if DEBUG, and my final release code is almost the same as if I had used the simpler property-like event declaration syntax.

Thank you very much.

[no name] 14-Mar-17 16:21pm

You're welcome. Thanks for the reply.
Nice 'problem', I needed this construction to start and stop a timer driven option...
Happy coding, Peter

OriginalGriff · Accepted Answer · 2017-03-14T00:32:00

You can only do it from within the class that creates the event:

public class MyClass
    {
    public event EventHandler MyEvent;
    protected virtual void OnMyEvent(EventArgs e)
        {
        EventHandler eh = MyEvent;
        if (eh != null)
            {
            eh(this, e);
            }
        }
    public int CountEvents()
        {
        return MyEvent.GetInvocationList().Length;
        }
    }
...
private void MyButton_Click(object sender, EventArgs e)
    {
    MyClass mc = new MyClass();
    mc.MyEvent += mc_MyEvent;
    Console.WriteLine(mc.CountEvents());
    mc.MyEvent += mc_MyEvent;
    Console.WriteLine(mc.CountEvents());

How do I get the number of event subscribers in the add and remove blocks?

2 solutions

Solution 2

Solution 1

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How do I get the number of event subscribers in the add and remove blocks?

2 solutions

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Solution 1

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