How to easilly see number of event subscriptions while debugging?

Question

While debugging, can I look into textBox1.TextChanged to see the number of event subscriptions? If yes, then how do I drill to it? I need to know how many subscriptions there are at a given time for debugging because it looks like an event is triggered multiple times, but I suspect this bug is really because textBox1.TextChanged += handler is being mismanaged in the application, so there are too many subscribers.

Here is a simplified version of what I think is happening. If possible, I just want to set a breakpoint and count up the number of subscriptions to "textBox1.TextChanged":

private void textBox1_TextChanged(object sender, EventArgs e)
{
    textBox1.TextChanged += textBox1_TextChanged;
    MessageBox.Show("asdf");
    textBox1.TextChanged -= textBox1_TextChanged;        
    textBox1.Text = DateTime.Now.ToString();
    textBox1.TextChanged += textBox1_TextChanged;
}

Is that possible or is it more complicated?

Why downvoting? It's a good question. – AgentFire Jun 02 '14 at 21:39 — AgentFire, Jun 02 '14 at 21:39

score 6 · Answer 1 · answered Jun 02 '14 at 23:12

6

If you're only concerned with doing it under the debugger, rather than programmatically, then this is perhaps a simpler, non-invasive way:

class _24003458
{
    event EventHandler MyEvent;
    public void Test()
    {
        MyEvent += Handler1;
        MyEvent += Handler2;
        MyEvent(this, EventArgs.Empty);
    }
    void Handler1(object sender, EventArgs e)
    {
        throw new NotImplementedException();
    }
    void Handler2(object sender, EventArgs e)
    {
        throw new NotImplementedException();
    }
}

Put a breakpoint on either of the event handlers, and when it breaks, look at the Locals tab. The event, when expanded, will show the invocation count and event handlers:

Screenshot of program stopped at breakpoint and Locals window showing the expanded event with invocation count and handlers

answered Jun 02 '14 at 23:12

ClickRick

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Awesome response. Could I do something similar to find a ComboBox's TextChanged event (i.e., via Locals)? I looked around for a while but couldn't find the ComboBox's events when drilling. This will be helpful nonetheless, so thanks. – Madmartigan Jun 03 '14 at 19:32
If you can navigate from `this` to your ComboBox, you'll be able to get from there to its events. – ClickRick Jun 03 '14 at 19:37
In VS 2017, this event field is hidden in debugger. Looks like VS debugger changed the event visibility engine in Locals tab – andreyk2 Hohlov Jun 18 '20 at 17:32
When I put `this.FormClosing` into the watch window, I get error CS0079: The event 'Form.FormClosing' can only appear on the left hand side of += or -= – Thomas Weller Mar 03 '23 at 12:37
@ThomasWeller Indeed, which is why I started from `this` and drilled down. – ClickRick Mar 05 '23 at 16:16

score 5 · Accepted Answer · answered Jun 02 '14 at 21:57

You will have to use Reflection to get to the invocation list of the event delegate:

    textBox1.TextChanged += textBox1_TextChanged;
    MessageBox.Show("asdf");
    textBox1.TextChanged -= textBox1_TextChanged;        
    textBox1.Text = DateTime.Now.ToString();
    textBox1.TextChanged += textBox1_TextChanged;
    var eventField = textBox1.GetType().GetField("TextChanged", BindingFlags.GetField
                                                               | BindingFlags.NonPublic
                                                               | BindingFlags.Instance);

    var subscriberCount = ((EventHandler)eventField.GetValue(textBox1))
                .GetInvocationList().Length;

score 2 · Answer 3 · edited May 23 '17 at 12:33

2

~~It is not possible with an event like this (for good reason), however,~~ it is possable via reflection as Selman22 says, above) if you are using an event directly you can do so:

private event EventHandler handler;

var delegates = handler.GetInvocationList();

edited May 23 '17 at 12:33

Community

1
1

answered Jun 02 '14 at 21:52

BanksySan

27,362
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score 1 · Answer 4 · answered Feb 22 '18 at 10:41

You can create a member method which you add to the object which implements the INotifyPropertyChanged interface. It makes debugging very easy:

#if DEBUG
    public System.Delegate[] GetInvocationList()
    {
        return PropertyChanged?.GetInvocationList();
    }
#endif

How to easilly see number of event subscriptions while debugging?

4 Answers4

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