Question

Option Strict on causing Event code to fail

Asked by: p_love

Hi,

I had some code which was working fine, but now I have turned Option Strict On, I am getting a build error.

Option Strict On disallows implicit conversion from System.EventArgs to System.ComponentModel.CancelEventArgs

VB suggests I chane the line:

RaiseEvent AfterPasswordChanged(Me, EventArgs.Empty)

to

RaiseEvent AfterPasswordChanged(Me, CType(EventArgs.Empty, CancelEventArgs))

But I dont understand why this is.  CancelEventArgs inherit from EventArgs, so isnt this a narrowing conversion because there may be properties/methods in the CancelEventArgs which are not in the base class?

If anyone can explain I would appreciate it.

Regards

P

Public Event BeforeLogin As CancelEventHandler
    Public Event AfterLogin As CancelEventHandler
    Public Event BeforeUserNameChanged As UserNameChangingEventHandler
    Public Event AfterUserNameChanged As EventHandler
    Public Event BeforePasswordChanged As CancelEventHandler
    Public Event AfterPasswordChanged As CancelEventHandler
    Public Event InvalidPassword As EventHandler
 
    Public Property Password() As String
        Get
            Password = _Password
 
        End Get
        Set(ByVal value As String)
            If value Like "*[!A-Za-z]*" Or Trim(value).Length = 0 Then
                RaiseEvent InvalidPassword(Me, EventArgs.Empty)
            Else
                If value <> _Password Then
                    Dim e As New CancelEventArgs
                    RaiseEvent BeforePasswordChanged(Me, e)
                    If e.Cancel = False Then
                        _Password = value
                        RaiseEvent AfterPasswordChanged(Me, EventArgs.Empty) ' Error appearing here
                    End If
 
                End If
            End If
        End Set
    End Property

                                  
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Answers

 

by: SanclerPosted on 2008-03-12 at 17:18:00ID: 21112148

The reason you get the error is because [line 6] you declare AfterPasswordChanged As CancelEventHandler and CancelEventHandler takes as its second argument e As CancelEventArgs not e As EventArgs.  So, without Implicit conversion (which Option Strict On disallows) the signatures don't match.

But I agree that the suggestion for how to deal with it is not a particularly good one.  I imagine, however, that it's just the result of some automatic process which is based on "this is wrong, what does it need changing to, what method will make the change" rather than the sort of more principled consideration that your post raises.  The proper method to correct this is not to do a conversion, but to change the code [line 23] to

                        RaiseEvent AfterPasswordChanged(Me, New CancelEventArgs)

Roger

 

by: p_lovePosted on 2008-03-12 at 17:44:19ID: 21112264

Hi Thanks,

Is there an equivalent to EventArgs.Empty for CancelEventArgs?  What actually are we passing as EventArgs.Empty...?

 

by: TheLearnedOnePosted on 2008-03-12 at 18:43:33ID: 21112659

I don't like Option Strict, it is just a crutch that makes for a lot of nagging and complaining.

Bob

 

by: Arthur_WoodPosted on 2008-03-12 at 21:13:19ID: 21113289

Since a CancelEventArgs object inherits from EventArgs, it therefore has all of the properties and methods of an EventArgs object, so CancelEventArgs.Empty should work just fine.

AW

 

by: SanclerPosted on 2008-03-13 at 02:07:52ID: 21114338

>>
What actually are we passing as EventArgs.Empty...?
<<

Nothing, nil, zilch.  It "represents an event with no event data .... The value of Empty is a read-only instance of EventArgs equivalent to the result of calling the EventArgs constructor".

CancelEventArgs, on the other hand, "provides data for a cancelable event": that's in its .Cancel property.

I suppose the real point here is not the one specifically  highlighted by changing Option Strict from Off to On: of whether on line 23 you should be passing EventArgs.Empty or New CancelEventArgs.  It is whether, on line 6, you should be declaring AfterPasswordChanged As CancelEventHandler or As EventHandler.  Given that - as you were originally passing EventArgs.Empty and it worked - it seems you don't need AfterPasswordChanged to be "a cancelable event", perhaps I should have said that the proper method to correct this is not to do a conversion, but to change the code [line 6] to

    Public Event AfterPasswordChanged As EventHandler

But it all depends what you're after.  Code that is technically "perfect" or just code that works ;-)

Roger

 

by: p_lovePosted on 2008-03-13 at 02:24:09ID: 21114404

Hi Thanks,

So this was working before OptionStrict was set to on because EventArgs.Empty was being cast behind the scenes as a new CancelEventArgs?

 

by: SanclerPosted on 2008-03-13 at 03:33:12ID: 21114690

No, on reflection, I don't think so.  Because, as you say, it's a narrowing conversion, although Option Strict Off may have allowed that bit of code to pass when it was being compiled, I think an error would have been thrown when the code was actually hit.  A test seems to confirm that.  Neither the explicit CType conversion suggested by VB, nor (with Option Strict Off) an implicit conversion gets past it.

If, indeed, it WAS working before Option Strict was On, I don't know how.  Was this the only change highlighted when you turned Option Strict On?  Or, more pointedly, have you made any other changes to the code since it was working with Option Strict Off?  If not, I would suggest turning Option Strict Off again, and debugging with values which would cause that line to be hit and see what happens.

Roger

 

by: p_lovePosted on 2008-03-13 at 04:11:28ID: 21114969

hI,

It was working fine with an EventArgs.Empty parameter before I put Option Strict On.

Regards

SM

 

by: SanclerPosted on 2008-03-13 at 04:24:29ID: 21115021

Well I can't explain that: perhaps someone else will.

Roger

 

by: p_lovePosted on 2008-03-17 at 14:29:55ID: 21146363

Hi,

I was wrong, you were right.  The event was never being tested in code and so was compiling fine.  When I forced it to fire, it failed at runtime.  

20120131-EE-VQP-002

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