EventHandlerList question (1.1)

[DeveloperX]

I'm having a play with EventHandlerLis t but the documentation is a bit
ropey and I can't find any decent examples. It also doesn't seem to do
what I was led to believe it would. I was under the impression that
windows.forms controls used EventHandlerLis ts because generally most
events aren't consumed so this saves memory.
I can add a button to a form, and have

this.button9.Cl ick += new System.EventHan dler(this.butto n9_Click);
this.button9.Cl ick += new System.EventHan dler(this.butto n9b_Click);

which will successfully fire off both functions when the button is
clicked.

The EventHandlerLis t does not fire both as it keys the delegate in a
hashtable and overwrites the first reference with the second reference
(This is using AddHandler).
More confusingly in trying to get the thing running I see code all over
the internet that can't run. A VB example that used a foreach (there's
no enumerator so that can't work) and a C++ example where the
EventHandlerLis t returns an EventHandler (It only returns Delegates in
my tests)
I'm certain I'm right, I can see plenty of alternative ways of doing it
that would work, I'm just confused about the information I'm getting
off the internet.
So am I being fed porkies? Do Windows Forms controls use
EventHandlerLis t objects and if so how come their implementation allows
for += and the one in ComponentModel doesn't?

Thanks
Ian

[Dave Sexton]

Re: EventHandlerLis t question (1.1)

Hi Ian,

<snip>

I was under the impression that
windows.forms controls used EventHandlerLis ts because generally most
events aren't consumed so this saves memory.

True.

I can add a button to a form, and have
>
this.button9.Cl ick += new System.EventHan dler(this.butto n9_Click);
this.button9.Cl ick += new System.EventHan dler(this.butto n9b_Click);
>
which will successfully fire off both functions when the button is
clicked.
>
The EventHandlerLis t does not fire both as it keys the delegate in a
hashtable and overwrites the first reference with the second reference
(This is using AddHandler).

Here's an example that I'm using from a project I'm working on right now in a
class that derives from Component:

private readonly object ServiceViewChan gedEvent = new object();

public event EventHandler<Se rviceViewChange dEventArgsServi ceViewChanged
{
add
{
lock (ServiceViewCha ngedEvent)
{
Events.AddHandl er(ServiceViewC hangedEvent, value);
}
}
remove
{
lock (ServiceViewCha ngedEvent)
{
Events.RemoveHa ndler(ServiceVi ewChangedEvent, value);
}
}
}

private void OnServiceViewCh anged(ServiceVi ewChangedEventA rgs e)
{
EventHandler<Se rviceViewChange dEventArgshandl er = null;

lock (ServiceViewCha ngedEvent)
{
handler = (EventHandler<S erviceViewChang edEventArgs>)
Events[ServiceViewChan gedEvent];
}

if (handler != null)
handler(this, e);
}

More confusingly in trying to get the thing running I see code all over
the internet that can't run. A VB example that used a foreach (there's
no enumerator so that can't work) and a C++ example where the
EventHandlerLis t returns an EventHandler (It only returns Delegates in
my tests)

EventHandler is a delegate. The value returned by the EventHandlerLis t
indexer isn't type-safe, so it must be cast into the appropriate delegate
Type, as you can see in the code above.

I'm certain I'm right, I can see plenty of alternative ways of doing it
that would work, I'm just confused about the information I'm getting
off the internet.
So am I being fed porkies? Do Windows Forms controls use
EventHandlerLis t objects and if so how come their implementation allows
for += and the one in ComponentModel doesn't?

An EventHandlerLis t instance is exposed as a protected property named,
"Events" from the Component class to all derived types. The "+=" operator
acts on public events, not the "Events" property. The underlying
implementation of "add" and "remove" on these public events can use the
"Events" property, internally, to store the supplied delegate.

Yes, I'm sure WinForms controls use the Events property, though I'm not sure
if ALL events on ALL WinForms Controls are registered through the inherited
Component.Event s property. Web controls use the "Events" property as well,
but it's declared in the base Control since Control doesn't derive from
Component. It still works the same way.

In my example, the public event is "ServiceViewCha nged". The "add" and
"remove" implementations of that event, which correspond to the "+=" and "-="
operators, respectively, add or remove the supplied delegate to or from the
underlying EventHandlerLis t by calling either Events.AddHandl er or
Events.RemoveHa ndler. Both of these methods require the same key used in the
indexer when you attempt to retrieve the delegate reference from the list.

If things still aren't clear, let me know.

--
Dave Sexton

[Dave Sexton]

Re: EventHandlerLis t question (1.1)

Hi Ian,

Sorry about using C# generics - I just realized you wrote (1.1) in the title.

To be honest, I don't have a code snippet without generics on hand anyway. If
you need me to rewrite my example without generics I will - just let me know.

--
Dave Sexton

"Dave Sexton" <dave@jwa[remove.this]online.comwrote in message
news:%23xyoZP4A HHA.3380@TK2MSF TNGP04.phx.gbl. ..

Hi Ian,
>
<snip>
>

>I was under the impression that
>windows.form s controls used EventHandlerLis ts because generally most
>events aren't consumed so this saves memory.

>
True.
>

>I can add a button to a form, and have
>>
>this.button9.C lick += new System.EventHan dler(this.butto n9_Click);
>this.button9.C lick += new System.EventHan dler(this.butto n9b_Click);
>>
>which will successfully fire off both functions when the button is
>clicked.
>>
>The EventHandlerLis t does not fire both as it keys the delegate in a
>hashtable and overwrites the first reference with the second reference
>(This is using AddHandler).

>
Here's an example that I'm using from a project I'm working on right now in
a class that derives from Component:
>
private readonly object ServiceViewChan gedEvent = new object();
>
public event EventHandler<Se rviceViewChange dEventArgsServi ceViewChanged
{
add
{
lock (ServiceViewCha ngedEvent)
{
Events.AddHandl er(ServiceViewC hangedEvent, value);
}
}
remove
{
lock (ServiceViewCha ngedEvent)
{
Events.RemoveHa ndler(ServiceVi ewChangedEvent, value);
}
}
}
>
private void OnServiceViewCh anged(ServiceVi ewChangedEventA rgs e)
{
EventHandler<Se rviceViewChange dEventArgshandl er = null;
>
lock (ServiceViewCha ngedEvent)
{
handler = (EventHandler<S erviceViewChang edEventArgs>)
Events[ServiceViewChan gedEvent];
}
>
if (handler != null)
handler(this, e);
}
>

>More confusingly in trying to get the thing running I see code all over
>the internet that can't run. A VB example that used a foreach (there's
>no enumerator so that can't work) and a C++ example where the
>EventHandlerLi st returns an EventHandler (It only returns Delegates in
>my tests)

>
EventHandler is a delegate. The value returned by the EventHandlerLis t
indexer isn't type-safe, so it must be cast into the appropriate delegate
Type, as you can see in the code above.
>

>I'm certain I'm right, I can see plenty of alternative ways of doing it
>that would work, I'm just confused about the information I'm getting
>off the internet.
>So am I being fed porkies? Do Windows Forms controls use
>EventHandlerLi st objects and if so how come their implementation allows
>for += and the one in ComponentModel doesn't?

>
An EventHandlerLis t instance is exposed as a protected property named,
"Events" from the Component class to all derived types. The "+=" operator
acts on public events, not the "Events" property. The underlying
implementation of "add" and "remove" on these public events can use the
"Events" property, internally, to store the supplied delegate.
>
Yes, I'm sure WinForms controls use the Events property, though I'm not sure
if ALL events on ALL WinForms Controls are registered through the inherited
Component.Event s property. Web controls use the "Events" property as well,
but it's declared in the base Control since Control doesn't derive from
Component. It still works the same way.
>
In my example, the public event is "ServiceViewCha nged". The "add" and
"remove" implementations of that event, which correspond to the "+=" and
"-=" operators, respectively, add or remove the supplied delegate to or from
the underlying EventHandlerLis t by calling either Events.AddHandl er or
Events.RemoveHa ndler. Both of these methods require the same key used in
the indexer when you attempt to retrieve the delegate reference from the
list.
>
If things still aren't clear, let me know.
>
--
Dave Sexton
>
>

[DeveloperX]

Re: EventHandlerLis t question (1.1)

Thanks Dave, no problem on the Generics, I use 2 at home, 1.1 at work.
There are some changes in 2 that affect Events in VB.net, so I
specified 1.1 in case those changes were part of some underlying change
in 2. I could of made that much much clearer :)

Give me a while to digest the info and I'll let you know if I have any
joy
Cheers

Dave Sexton wrote:

Hi Ian,
>
Sorry about using C# generics - I just realized you wrote (1.1) in the title.
>
To be honest, I don't have a code snippet without generics on hand anyway. If
you need me to rewrite my example without generics I will - just let me know.
>
--
Dave Sexton
>
"Dave Sexton" <dave@jwa[remove.this]online.comwrote in message
news:%23xyoZP4A HHA.3380@TK2MSF TNGP04.phx.gbl. ..

Hi Ian,

<snip>

I was under the impression that
windows.forms controls used EventHandlerLis ts because generally most
events aren't consumed so this saves memory.

True.

I can add a button to a form, and have
>
this.button9.Cl ick += new System.EventHan dler(this.butto n9_Click);
this.button9.Cl ick += new System.EventHan dler(this.butto n9b_Click);
>
which will successfully fire off both functions when the button is
clicked.
>
The EventHandlerLis t does not fire both as it keys the delegate in a
hashtable and overwrites the first reference with the second reference
(This is using AddHandler).

Here's an example that I'm using from a project I'm working on right now in
a class that derives from Component:

private readonly object ServiceViewChan gedEvent = new object();

public event EventHandler<Se rviceViewChange dEventArgsServi ceViewChanged
{
add
{
lock (ServiceViewCha ngedEvent)
{
Events.AddHandl er(ServiceViewC hangedEvent, value);
}
}
remove
{
lock (ServiceViewCha ngedEvent)
{
Events.RemoveHa ndler(ServiceVi ewChangedEvent, value);
}
}
}

private void OnServiceViewCh anged(ServiceVi ewChangedEventA rgs e)
{
EventHandler<Se rviceViewChange dEventArgshandl er = null;

lock (ServiceViewCha ngedEvent)
{
handler = (EventHandler<S erviceViewChang edEventArgs>)
Events[ServiceViewChan gedEvent];
}

if (handler != null)
handler(this, e);
}

More confusingly in trying to get the thing running I see code all over
the internet that can't run. A VB example that used a foreach (there's
no enumerator so that can't work) and a C++ example where the
EventHandlerLis t returns an EventHandler (It only returns Delegates in
my tests)

EventHandler is a delegate. The value returned by the EventHandlerLis t
indexer isn't type-safe, so it must be cast into the appropriate delegate
Type, as you can see in the code above.

I'm certain I'm right, I can see plenty of alternative ways of doing it
that would work, I'm just confused about the information I'm getting
off the internet.
So am I being fed porkies? Do Windows Forms controls use
EventHandlerLis t objects and if so how come their implementation allows
for += and the one in ComponentModel doesn't?

An EventHandlerLis t instance is exposed as a protected property named,
"Events" from the Component class to all derived types. The "+=" operator
acts on public events, not the "Events" property. The underlying
implementation of "add" and "remove" on these public events can use the
"Events" property, internally, to store the supplied delegate.

Yes, I'm sure WinForms controls use the Events property, though I'm not sure
if ALL events on ALL WinForms Controls are registered through the inherited
Component.Event s property. Web controls use the "Events" property as well,
but it's declared in the base Control since Control doesn't derive from
Component. It still works the same way.

In my example, the public event is "ServiceViewCha nged". The "add" and
"remove" implementations of that event, which correspond to the "+=" and
"-=" operators, respectively, add or remove the supplied delegate to or from
the underlying EventHandlerLis t by calling either Events.AddHandl er or
Events.RemoveHa ndler. Both of these methods require the same key used in
the indexer when you attempt to retrieve the delegate reference from the
list.

If things still aren't clear, let me know.

--
Dave Sexton

[DeveloperX]

Re: EventHandlerLis t question (1.1)


Dave Sexton wrote:

Hi Ian,
>
Sorry about using C# generics - I just realized you wrote (1.1) in the title.
>
To be honest, I don't have a code snippet without generics on hand anyway. If
you need me to rewrite my example without generics I will - just let me know.
>
--
Dave Sexton
>
"Dave Sexton" <dave@jwa[remove.this]online.comwrote in message
news:%23xyoZP4A HHA.3380@TK2MSF TNGP04.phx.gbl. ..

Hi Ian,

<snip>

I was under the impression that
windows.forms controls used EventHandlerLis ts because generally most
events aren't consumed so this saves memory.

True.

I can add a button to a form, and have
>
this.button9.Cl ick += new System.EventHan dler(this.butto n9_Click);
this.button9.Cl ick += new System.EventHan dler(this.butto n9b_Click);
>
which will successfully fire off both functions when the button is
clicked.
>
The EventHandlerLis t does not fire both as it keys the delegate in a
hashtable and overwrites the first reference with the second reference
(This is using AddHandler).

Here's an example that I'm using from a project I'm working on right now in
a class that derives from Component:

private readonly object ServiceViewChan gedEvent = new object();

public event EventHandler<Se rviceViewChange dEventArgsServi ceViewChanged
{
add
{
lock (ServiceViewCha ngedEvent)
{
Events.AddHandl er(ServiceViewC hangedEvent, value);
}
}
remove
{
lock (ServiceViewCha ngedEvent)
{
Events.RemoveHa ndler(ServiceVi ewChangedEvent, value);
}
}
}

private void OnServiceViewCh anged(ServiceVi ewChangedEventA rgs e)
{
EventHandler<Se rviceViewChange dEventArgshandl er = null;

lock (ServiceViewCha ngedEvent)
{
handler = (EventHandler<S erviceViewChang edEventArgs>)
Events[ServiceViewChan gedEvent];
}

if (handler != null)
handler(this, e);
}

More confusingly in trying to get the thing running I see code all over
the internet that can't run. A VB example that used a foreach (there's
no enumerator so that can't work) and a C++ example where the
EventHandlerLis t returns an EventHandler (It only returns Delegates in
my tests)

EventHandler is a delegate. The value returned by the EventHandlerLis t
indexer isn't type-safe, so it must be cast into the appropriate delegate
Type, as you can see in the code above.

I'm certain I'm right, I can see plenty of alternative ways of doing it
that would work, I'm just confused about the information I'm getting
off the internet.
So am I being fed porkies? Do Windows Forms controls use
EventHandlerLis t objects and if so how come their implementation allows
for += and the one in ComponentModel doesn't?

An EventHandlerLis t instance is exposed as a protected property named,
"Events" from the Component class to all derived types. The "+=" operator
acts on public events, not the "Events" property. The underlying
implementation of "add" and "remove" on these public events can use the
"Events" property, internally, to store the supplied delegate.

Yes, I'm sure WinForms controls use the Events property, though I'm not sure
if ALL events on ALL WinForms Controls are registered through the inherited
Component.Event s property. Web controls use the "Events" property as well,
but it's declared in the base Control since Control doesn't derive from
Component. It still works the same way.

In my example, the public event is "ServiceViewCha nged". The "add" and
"remove" implementations of that event, which correspond to the "+=" and
"-=" operators, respectively, add or remove the supplied delegate to or from
the underlying EventHandlerLis t by calling either Events.AddHandl er or
Events.RemoveHa ndler. Both of these methods require the same key used in
the indexer when you attempt to retrieve the delegate reference from the
list.

If things still aren't clear, let me know.

--
Dave Sexton

You were spot on with the assertion that I needed to cast the Delegate
to the correct type. I had previously attempted to cast it to an
EventHandler with didn't work. The reason being I was using an example
borrowed from MS which uses MouseEventHandl er (Their code only went as
far as adding the delegate to the list not firing it).

I now have it working both by deriving from Component and using Events,
and with a EventHandlerLis t declared in the class.

Thanks for your help
Ian