Events in C#

19 Jul 2006 5:21 AM

pSm

When I override the 'add' and 'remove' methods for an event, why can't I
invoke it ? The sample code is provided below - I have two events - 'my' and
'ev' - while invoking, my() works but ev() does not ! The compiler complains
that : The event "'Blog.EventsvsDelegates.Form1.ev' can only appear on the
left hand side of += or -="

Why ?

        public delegate void MyEvent();
    public class Form1 : System.Windows.Forms.Form
    {
        private System.Windows.Forms.Button button1;
        private event MyEvent my;

        private event MyEvent pvt_ev;
        public event MyEvent ev
        {
            add
            {
                    pvt_ev +=value;
            }

            remove
            {
                pvt_ev-=value;
            }
        }

        public Form1()
        {
            //
            // Required for Windows Form Designer support
            //
            InitializeComponent();

            my+=new MyEvent(this.Test);
            my+=new MyEvent(this.Test1);

            ev+=new MyEvent (this.Test );
        }

        protected void Test()
        {
            MessageBox.Show ("Test");
        }

        protected void Test1()
        {
            MessageBox.Show ("Test1");
        }

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.Run(new Form1());
        }

        private void button1_Click(object sender, System.EventArgs e)
        {
            my(); // WORKS
                      //ev (); //DOES NOT WORK
        }

    }

19 Jul 2006 9:12 AM

Larry Lard

pSm wrote:

> When I override the 'add' and 'remove' methods for an event, why can't I
> invoke it ? The sample code is provided below - I have two events - 'my' and
> 'ev' - while invoking, my() works but ev() does not ! The compiler complains
> that : The event "'Blog.EventsvsDelegates.Form1.ev' can only appear on the
> left hand side of += or -="
>
> Why ?

The help page for this error (CS0079) includes a sample very similar to
yours that shows you exactly what to do. I found this out by reproducing
your sample then hitting F1 with the error selected...

--
Larry Lard
larryl***@googlemail.com
The address is real, but unread - please reply to the group
For VB and C# questions - tell us which version

19 Jul 2006 12:39 PM

pSm

Thanks Larry.

But this does not really answer my question ! Why does the compiler restrict
the invocation ?

19 Jul 2006 4:34 PM

Jon Skeet [C# MVP]

pSm <p**@discussions.microsoft.com> wrote:

> When I override the 'add' and 'remove' methods for an event, why can't I
> invoke it ? The sample code is provided below - I have two events - 'my' and
> 'ev' - while invoking, my() works but ev() does not ! The compiler complains
> that : The event "'Blog.EventsvsDelegates.Form1.ev' can only appear on the
> left hand side of += or -="

And it's right. An event is really just an add/remove pair of methods.
(There's also raise, but that doesn't get exposed in C#.) When you
declare a field-like event, that's really declaring both the event
*and* a delegate. When you access the name within the class, it refers
to the delegate. When you access the name outside the class, it refers
to the event. Delegates can be invoked - events can't.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

20 Jul 2006 12:25 AM

Lloyd Dupont

> And it's right. An event is really just an add/remove pair of methods.
> (There's also raise, but that doesn't get exposed in C#.) When you

No, there is no such thing as "raise"!
Events are stored as multicast delegates and used as such.

> declare a field-like event, that's really declaring both the event
> *and* a delegate. When you access the name within the class, it refers
> to the delegate. When you access the name outside the class, it refers
> to the event. Delegates can be invoked - events can't.

Well events are just delegate, and of course they could be ".Invoke(..)", as
any other delegate.
But a semantic akin to private/protect/public prevent to do it from anywhere
but the declaring class itself.

Show quote

>
> --
> Jon Skeet - <sk***@pobox.com>
> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
> If replying to the group, please do not mail me too

20 Jul 2006 7:03 AM

Jon Skeet [C# MVP]

Lloyd Dupont wrote:

> > And it's right. An event is really just an add/remove pair of methods.
> > (There's also raise, but that doesn't get exposed in C#.) When you
>
> No, there is no such thing as "raise"!
> Events are stored as multicast delegates and used as such.

C# doesn't expose it, but it's part of the .NET event system.
See CLI spec partition 1 section 10.4.

> > declare a field-like event, that's really declaring both the event
> > *and* a delegate. When you access the name within the class, it refers
> > to the delegate. When you access the name outside the class, it refers
> > to the event. Delegates can be invoked - events can't.
>
> Well events are just delegate, and of course they could be ".Invoke(..)", as
> any other delegate.

No, events aren't delegates any more than properties are variables. An
event is solely a set of appropriately decorated methods. It's possible
(although it would be very odd) to implement an event without actually
storing a delegate. You'd need to store pretty much the same
information, but the only ways in which events are intrinsically linked
to delegates are:

1) C#'s field-like event support, which declares a delegate variable
and an event
2) the signatures of event methods.

> But a semantic akin to private/protect/public prevent to do it from anywhere
> but the declaring class itself.

Events only have add/remove semantics, along with the raise as
mentioned before. Whether they are implemented by having a delegate
variable is an implementation matter. Some implementations keep the
delegates for all events in a hashtable, so that you don't need a
variable per event where there are lots of events which may not be
subscribed to at all.

Jon

20 Jul 2006 1:26 AM

Lloyd Dupont

give pvt_ev a public accessor.
otherwise it's quite obvious, if you have no access to it, how could you
hope invoke it?!?!

Show quote

"pSm" <p**@discussions.microsoft.com> wrote in message
news:8A65460C-9937-41BD-958B-50078772C822@microsoft.com...
> When I override the 'add' and 'remove' methods for an event, why can't I
> invoke it ? The sample code is provided below - I have two events - 'my'
> and
> 'ev' - while invoking, my() works but ev() does not ! The compiler
> complains
> that : The event "'Blog.EventsvsDelegates.Form1.ev' can only appear on the
> left hand side of += or -="
>
> Why ?
>
>        public delegate void MyEvent();
> public class Form1 : System.Windows.Forms.Form
> {
> private System.Windows.Forms.Button button1;
> private event MyEvent my;
>
> private event MyEvent pvt_ev;
> public event MyEvent ev
> {
> add
> {
> pvt_ev +=value;
> }
>
> remove
> {
> pvt_ev-=value;
> }
> }
>
> public Form1()
> {
> //
> // Required for Windows Form Designer support
> //
> InitializeComponent();
>
> my+=new MyEvent(this.Test);
> my+=new MyEvent(this.Test1);
>
> ev+=new MyEvent (this.Test );
> }
>
> protected void Test()
> {
> MessageBox.Show ("Test");
> }
>
> protected void Test1()
> {
> MessageBox.Show ("Test1");
> }
>
> /// <summary>
> /// The main entry point for the application.
> /// </summary>
> [STAThread]
> static void Main()
> {
> Application.Run(new Form1());
> }
>
> private void button1_Click(object sender, System.EventArgs e)
> {
>   my(); // WORKS
>                   //ev (); //DOES NOT WORK
> }
>
> }
>

20 Jul 2006 3:55 AM

pSm

Lloyd,
Why would pvt_ev() need a public accessor when I invoke it from the same
class ? In fact any 'event' that is declared as public, would be converted to
'private' by the compiler - events can never be invoked from outside it's
class.

I was checking the MSIL and noticed that when 'add' and 'remove' accessors
are declared, the ev declaration is removed, and so invoke on ev() no longer
works.

I had posted this question on GOTDOTNET discussion forum too and I think
Alan came close to answering why the compiler does not allow invoking ev().

You can read the whole story here : <a
href="http://blogs.ittoolbox.com/visualbasic/operating/archives/events-vs-delegates-contd-10596">Events vs Delegates </a>

Show quote

"Lloyd Dupont" wrote:

> give pvt_ev a public accessor.
> otherwise it's quite obvious, if you have no access to it, how could you
> hope invoke it?!?!
>
> "pSm" <p**@discussions.microsoft.com> wrote in message
> news:8A65460C-9937-41BD-958B-50078772C822@microsoft.com...
> > When I override the 'add' and 'remove' methods for an event, why can't I
> > invoke it ? The sample code is provided below - I have two events - 'my'
> > and
> > 'ev' - while invoking, my() works but ev() does not ! The compiler
> > complains
> > that : The event "'Blog.EventsvsDelegates.Form1.ev' can only appear on the
> > left hand side of += or -="
> >
> > Why ?
> >
> >        public delegate void MyEvent();
> > public class Form1 : System.Windows.Forms.Form
> > {
> > private System.Windows.Forms.Button button1;
> > private event MyEvent my;
> >
> > private event MyEvent pvt_ev;
> > public event MyEvent ev
> > {
> > add
> > {
> > pvt_ev +=value;
> > }
> >
> > remove
> > {
> > pvt_ev-=value;
> > }
> > }
> >
> > public Form1()
> > {
> > //
> > // Required for Windows Form Designer support
> > //
> > InitializeComponent();
> >
> > my+=new MyEvent(this.Test);
> > my+=new MyEvent(this.Test1);
> >
> > ev+=new MyEvent (this.Test );
> > }
> >
> > protected void Test()
> > {
> > MessageBox.Show ("Test");
> > }
> >
> > protected void Test1()
> > {
> > MessageBox.Show ("Test1");
> > }
> >
> > /// <summary>
> > /// The main entry point for the application.
> > /// </summary>
> > [STAThread]
> > static void Main()
> > {
> > Application.Run(new Form1());
> > }
> >
> > private void button1_Click(object sender, System.EventArgs e)
> > {
> >   my(); // WORKS
> >                   //ev (); //DOES NOT WORK
> > }
> >
> > }
> >
>
>
>

20 Jul 2006 7:50 AM

Lloyd Dupont

> Why would pvt_ev() need a public accessor when I invoke it from the same
> class ?

Sorry, I misread your sample.
just change:

>> > private void button1_Click(object sender, System.EventArgs e)
>> > {
>> > //ev (); //DOES NOT WORK
>> > }
to

private void button1_Click(object sender, System.EventArgs e)
{
pvt_ev ();
}

Show quote

> In fact any 'event' that is declared as public, would be converted to
> 'private' by the compiler - events can never be invoked from outside it's
> class.
>
> I was checking the MSIL and noticed that when 'add' and 'remove' accessors
> are declared, the ev declaration is removed, and so invoke on ev() no
> longer
> works.
>
> I had posted this question on GOTDOTNET discussion forum too and I think
> Alan came close to answering why the compiler does not allow invoking
> ev().
>
> You can read the whole story here : <a
> href="http://blogs.ittoolbox.com/visualbasic/operating/archives/events-vs-delegates-contd-10596">Events
> vs Delegates </a>
>
> "Lloyd Dupont" wrote:
>
>> give pvt_ev a public accessor.
>> otherwise it's quite obvious, if you have no access to it, how could you
>> hope invoke it?!?!
>>
>> "pSm" <p**@discussions.microsoft.com> wrote in message
>> news:8A65460C-9937-41BD-958B-50078772C822@microsoft.com...
>> > When I override the 'add' and 'remove' methods for an event, why can't
>> > I
>> > invoke it ? The sample code is provided below - I have two events -
>> > 'my'
>> > and
>> > 'ev' - while invoking, my() works but ev() does not ! The compiler
>> > complains
>> > that : The event "'Blog.EventsvsDelegates.Form1.ev' can only appear on
>> > the
>> > left hand side of += or -="
>> >
>> > Why ?
>> >
>> >        public delegate void MyEvent();
>> > public class Form1 : System.Windows.Forms.Form
>> > {
>> > private System.Windows.Forms.Button button1;
>> > private event MyEvent my;
>> >
>> > private event MyEvent pvt_ev;
>> > public event MyEvent ev
>> > {
>> > add
>> > {
>> > pvt_ev +=value;
>> > }
>> >
>> > remove
>> > {
>> > pvt_ev-=value;
>> > }
>> > }
>> >
>> > public Form1()
>> > {
>> > //
>> > // Required for Windows Form Designer support
>> > //
>> > InitializeComponent();
>> >
>> > my+=new MyEvent(this.Test);
>> > my+=new MyEvent(this.Test1);
>> >
>> > ev+=new MyEvent (this.Test );
>> > }
>> >
>> > protected void Test()
>> > {
>> > MessageBox.Show ("Test");
>> > }
>> >
>> > protected void Test1()
>> > {
>> > MessageBox.Show ("Test1");
>> > }
>> >
>> > /// <summary>
>> > /// The main entry point for the application.
>> > /// </summary>
>> > [STAThread]
>> > static void Main()
>> > {
>> > Application.Run(new Form1());
>> > }
>> >
>> > private void button1_Click(object sender, System.EventArgs e)
>> > {
>> >   my(); // WORKS
>> >                   //ev (); //DOES NOT WORK
>> > }
>> >
>> > }
>> >
>>
>>
>>

20 Jul 2006 7:52 AM

Lloyd Dupont

Oops,
I think I misread you anyway.
It was more of a phylosophical debate rather than a practical question.
Dimiss my previous answer....