Question

Ted Faison in a podcast on event-based software design mentioned that ‘sender’ and ‘self’ objects in .NET, C++ and Java event statements such as:

private void Button_Click(object sender, RoutedEventArgs e) 

are a misnomer since e.g. in the above example ‘sender’ is not really the object which produced the event but a proxy, since you wouldn’t want to couple your applications that tightly.

Did I understand him incorrectly (since when I debug it, ‘sender’ does indeed seem to be the original object).

Or is it that common event patterns in these languages (e.g. a common click handler) are tightly coupled but they should be more decoupled, e.g. in composite applications.

He also mentioned that e.g. you shouldn’t make inherit from EventArgs since it leads to an explosion of classes, one per event, which only transport a few variables. Many times in his opinion, you can just send a string for instance. He mentioned that this opinion is the opposite of what Microsoft Patterns and Practices suggests.

Any thoughts on these areas?

Answer 1

In most cases, sender is the Button (or whatever) that raised the event. There are some occasions when this isn’t the case – such as a (perhaps lazy) pass-thru event:

class Foo {    private Bar bar;    public Foo(Bar bar) {        this.bar = bar;    }    public event EventHandler SomeEvent {        add {bar.SomeEvent += value;}        remove {bar.SomeEvent -= value;}    }    //... } 

Here, if we subscribe to foo.SomeEvent, we will actually get back the event originated by the Bar instance – so sender won’t be foo. But this is arguably because we’ve implemented Foo.SomeEvent incorrectly.

To be honest, in most cases you don’t need to check sender; the main time this is useful is when a number of controls share a handler. You should generally be able to assume the sender is the instance you subscribed to (for the purposes of reference-equality tests).

Re EventArgs – the standard pattern (when creating a new event-type) would recommend you to inherit from this. I don’t recommend deviating from this. A minor reason is that it allows you to use EventHandler<T>, but there are other variance reasons too. Besides – sometimes doing what other people expect is reason enough; people expect an EventArgs derived value.

That said – I have done non-standard events before (in MiscUtil’s Push LINQ) – but this was already in a very unusual setup, so it didn’t feel out of place.