IDE bug fixed in VS 2015? - vb.net

Question

This appears to be an IDE bug specific to VB in VS 2013. The following code produces a compiler warning:
The Task returned from this Async Function will be dropped, and any exceptions in it ignored. Consider changing it to an Async Sub so its exceptions are propagated.
Well, clearly it already is an Async Sub (void in C#). While the general rule is to avoid Async Subs, it's perfectly acceptable—and even advised—to do so with top-level event handlers such as this.
That's why I'm calling this an IDE bug. It only manifests when we set an event handler in VB (using either AddHandler or the Handles keyword). I'm not finding other reports of it, which frankly is odd... I can't imagine I'm the only one seeing it.
But to the question: can anyone report whether this nuisance has been fixed in VS 2015? If it has, I'll have further justification for upgrading sooner rather than later. (I have other reasons holding me back for the time being that aren't appropriate for discussion here.)

Answer 1

The source of the problem is a relaxed delegate without an implicit widening conversion.
The Click event accepts an Object as the first parameter, but the handler accepts Button which makes it a relaxed delegate.
Because the handler's signature is more restrictive than the event signature, the code as is will only compile with Option Strict Off, and generate the seemingly spurious warning about a dropped async task.
Note that the problem does not occur when a widening conversion exists (e.g. where the event has Button and the handler has Object).
To fix the problem, you can:
Change the handler's signature to match the event signature exactly:
Private Async Sub cmdStart_Click(sender As Object, e As RoutedEventArgs)
Wrap the AddressOf into a delegate constructor:
AddHandler cmdStart.Click, New RoutedEventHandler(AddressOf cmdStart_Click)
Ditch the AddressOf and wrap the call to the handler into a lambda:
AddHandler cmdStart.Click, Sub(sender As Object, e As RoutedEventArgs)
Me.cmdStart_Click(sender, e)
End Sub
As for why this is happening and whose fault that is (yours, IDE's or the compiler's), I cannot tell you for sure.
However there is one thing I can see.
These two lines of code:
AddHandler cmdStart.Click, AddressOf RelaxedHandler
AddHandler cmdStart.Click, Sub(a0 As Object, a1 As EventArgs)
Me.RelaxedHandler(a0, a1)
End Sub
produce identical IL code, as far as the ILSpy is concerned:
this.cmdStart.Click += delegate(object a0, EventArgs a1)
{
this.RelaxedHandler((Button)a0, a1);
}
;
this.cmdStart.Click += delegate(object a0, EventArgs a1)
{
this.RelaxedHandler((Button)a0, a1);
}
;
but the first line generates a warning and the second line does not.