[Delegates and Events]

[Johnny E Jensen]

Hi
Using C#, VS2005 .NET2

I have a custom usercontrol.
Here i have a picturebox (Close) and a CheckedListBox

A public delegate void for CloseControlHan dler();, and event for that
CloseControlEve nt;
A public delegate void for AddFieldToListH andler(string tablename, object
item); and event for that AddFieldToListE vent;

When the user clicks the picture, the picturebox.clic k event fires. Here I
raises my CloseControlEve nt, this allways works fine.
When the user clicks on a item in the CheckListBox the
CheckListBox.It emCheck is fired (besause the CheckOnClick property = true),
I call a method to raise my event AddFieldToListE vent, but always i'll get
the 'Object reference not set....' Error. it seems that the
AddFieldToListE vent i allways NULL.
I do use the:
If ( AddFieldToListE vent != null ) to prevent the application to crash.

Could anyone provide with some hints.

The Code:
public delegate void AddFieldToListH andler(string tablename,
FieldDescriptio nClass description);
public event AddFieldToListH andler AddFieldToListE vent;

public delegate void CloseControlHan dler(Control ClosingControl) ;
public event CloseControlHan dler CloseControlEve nt;

private void picClose_Click( object sender, EventArgs e)
{
if ( CloseControlEve nt !=null)
{
CloseControlEve nt(this);
}
}

private void lstFields_ItemC heck(object sender, ItemCheckEventA rgs e)
{
RaiseAddEvent(_ strTablename, _objField);
}

private void RaiseAddEvent(s tring tablename, FieldDescriptio nClass
description)
{
if ( AddFieldToListE vent != null) // this AddFieldToListE vent is always
null ?????????
AddFieldToListE vent(tablename, description);
}

Kind Regards
Johnny E Jensen

[Jon Skeet [C# MVP]]

Re: [Delegates and Events]

Johnny E Jensen wrote:

<snip>

Could anyone provide with some hints.

It's not clear why you expect the event *not* to be null. Instead of
providing partial code, could you provide a short but complete program
which demonstrates the problem?

See http://www.pobox.com/~skeet/csharp/complete.html for what I mean by
that.

In addition, if you're storing a handler separately to the event, you
would normally implement the add/remove behaviour by hand to use that
handler delegate. It's possible that I've misunderstood the point of
the separate delegate/event due to only seeing part of the code though.

See http://www.pobox.com/~skeet/csharp/events.html for more details of
this.

Jon

[Jon Skeet [C# MVP]]

Re: [Delegates and Events]

Johnny E Jensen wrote:

<snip>

Could anyone provide with some hints.

It's not clear why you expect the event *not* to be null. Instead of
providing partial code, could you provide a short but complete program
which demonstrates the problem?

See http://www.pobox.com/~skeet/csharp/complete.html for what I mean by
that.

In addition, if you're storing a handler separately to the event, you
would normally implement the add/remove behaviour by hand to use that
handler delegate. It's possible that I've misunderstood the point of
the separate delegate/event due to only seeing part of the code though.

See http://www.pobox.com/~skeet/csharp/events.html for more details of
this.

Jon

[Dave Sexton]

Re: [Delegates and Events]

Hi Johnny,

You should consider following Microsoft's standards for event design:

".NET Framework Developer's Guide, Event Design"

http://msdn2.microsoft.com/en-us/library/ms229011.aspx

Also, in the 2.0 framework you no longer need to create custom delegates.
Instead, you can use the generic EventHandler<TE ventArgsdelegat e if you
need to use custom event arguments. Create your own class that derives from
System.EventArg s and use it for the TEventArgs parameter.

--
Dave Sexton

"Johnny E Jensen" <jej@winner-crm.dkwrote in message
news:elMFBAGIHH A.960@TK2MSFTNG P04.phx.gbl...

Hi
Using C#, VS2005 .NET2
>
I have a custom usercontrol.
Here i have a picturebox (Close) and a CheckedListBox
>
A public delegate void for CloseControlHan dler();, and event for that
CloseControlEve nt;
A public delegate void for AddFieldToListH andler(string tablename, object
item); and event for that AddFieldToListE vent;
>
When the user clicks the picture, the picturebox.clic k event fires. Here I
raises my CloseControlEve nt, this allways works fine.
When the user clicks on a item in the CheckListBox the
CheckListBox.It emCheck is fired (besause the CheckOnClick property =
true), I call a method to raise my event AddFieldToListE vent, but always
i'll get the 'Object reference not set....' Error. it seems that the
AddFieldToListE vent i allways NULL.
I do use the:
If ( AddFieldToListE vent != null ) to prevent the application to crash.
>
Could anyone provide with some hints.
>
The Code:
public delegate void AddFieldToListH andler(string tablename,
FieldDescriptio nClass description);
public event AddFieldToListH andler AddFieldToListE vent;
>
public delegate void CloseControlHan dler(Control ClosingControl) ;
public event CloseControlHan dler CloseControlEve nt;
>
private void picClose_Click( object sender, EventArgs e)
{
if ( CloseControlEve nt !=null)
{
CloseControlEve nt(this);
}
}
>
private void lstFields_ItemC heck(object sender, ItemCheckEventA rgs e)
{
RaiseAddEvent(_ strTablename, _objField);
}
>
private void RaiseAddEvent(s tring tablename, FieldDescriptio nClass
description)
{
if ( AddFieldToListE vent != null) // this AddFieldToListE vent is always
null ?????????
AddFieldToListE vent(tablename, description);
}
>
Kind Regards
Johnny E Jensen
>
>