Problems with custom events

Browsing through some online tutorials, I've been tryingto add a custom event
to an app I'm developing but I'm having a little problem.

Simplifying the code down to the following (a form with a button that fires
the event):

namespace CustomEvent
{
public delegate void MyEventHandler(object sender, MyEventArgs e);

/// <summary>
/// Summary description for Form1.
/// </summary>
public class Form1 : System.Windows.Forms.Form
{
public event MyEventHandler MyEvent;

protected virtual void OnMyEvent(MyEventArgs e)
{
MyEvent(this, e);
}

<< snip template form code >>


private void button1_Click(object sender, System.EventArgs e)
{
OnMyEvent(new MyEventArgs("Fred"));
}
}

public class MyEventArgs: EventArgs
{
private string name;
public MyEventArgs(string name)
{
this.name = name;
}
public string Name
{
get {return name;}
}
}
}

The problem I'm having is that the MyEvent(this, e); call fails in
OnMyEvent(...) with the following error:

"An unhandled exception of type 'System.NullReferenceException' occurred in
CustomEvent.exe

Additional information: Object reference not set to an instance of an object."

Can anyone explain what I'm doing wrong?

Cheers in advance
Ant

Claudio
Thu Mar 31 08:18:49 CST 2005

On Thu, 31 Mar 2005 14:32:32 GMT, Anthony Hughes wrote:

> Simplifying the code down to the following (a form with a button that fires
> the event):
>
> namespace CustomEvent
> {
> public delegate void MyEventHandler(object sender, MyEventArgs e);
>
> /// <summary>
> /// Summary description for Form1.
> /// </summary>
> public class Form1 : System.Windows.Forms.Form
> {
> public event MyEventHandler MyEvent;
>
> protected virtual void OnMyEvent(MyEventArgs e)
> {
> MyEvent(this, e);
> }
>
> << snip template form code >>
>
> private void button1_Click(object sender, System.EventArgs e)
> {
> OnMyEvent(new MyEventArgs("Fred"));
> }
> }
>
> public class MyEventArgs: EventArgs
> {
> private string name;
> public MyEventArgs(string name)
> {
> this.name = name;
> }
> public string Name
> {
> get {return name;}
> }
> }
> }
>
> The problem I'm having is that the MyEvent(this, e); call fails in
> OnMyEvent(...) with the following error:
>
> "An unhandled exception of type 'System.NullReferenceException' occurred in
> CustomEvent.exe
>
> Additional information: Object reference not set to an instance of an object."
>

You can only fire an event, if someone is listening to it. So if nowhere is
some code that subscribed to your event like:
form1.MyEvent += new Eventhandler(dosomething);
you can't fire your event. How do you no, if someone subscribed to your
event? Simply by checking for null:

protected virtual void OnMyEvent(MyEventArgs e)
{
if (MyEvent != null)
MyEvent(this, e);
}

hth

--
Claudio Grazioli
http://www.grazioli.ch

ahughes
Fri Apr 01 02:32:01 CST 2005

In article <1ohfzn4quf9g3$.139hez51wykwi$.dlg@40tude.net>, Claudio Grazioli <newsgroups@gmx-ist-cool.de> wrote:
>On Thu, 31 Mar 2005 14:32:32 GMT, Anthony Hughes wrote:
>
>> Simplifying the code down to the following (a form with a button that fires
>> the event):
>>
>> namespace CustomEvent
>> {
>> public delegate void MyEventHandler(object sender, MyEventArgs e);
>>
>> /// <summary>
>> /// Summary description for Form1.
>> /// </summary>
>> public class Form1 : System.Windows.Forms.Form
>> {
>> public event MyEventHandler MyEvent;
>>
>> protected virtual void OnMyEvent(MyEventArgs e)
>> {
>> MyEvent(this, e);
>> }
>>
>> << snip template form code >>
>>
>> private void button1_Click(object sender, System.EventArgs e)
>> {
>> OnMyEvent(new MyEventArgs("Fred"));
>> }
>> }
>>
>> public class MyEventArgs: EventArgs
>> {
>> private string name;
>> public MyEventArgs(string name)
>> {
>> this.name = name;
>> }
>> public string Name
>> {
>> get {return name;}
>> }
>> }
>> }
>>
>> The problem I'm having is that the MyEvent(this, e); call fails in
>> OnMyEvent(...) with the following error:
>>
>> "An unhandled exception of type 'System.NullReferenceException' occurred in
>> CustomEvent.exe
>>
>> Additional information: Object reference not set to an instance of an
> object."
>>
>
>You can only fire an event, if someone is listening to it. So if nowhere is
>some code that subscribed to your event like:
> form1.MyEvent += new Eventhandler(dosomething);
>you can't fire your event. How do you no, if someone subscribed to your
>event? Simply by checking for null:
>
>protected virtual void OnMyEvent(MyEventArgs e)
>{
> if (MyEvent != null)
> MyEvent(this, e);
>}
>
>hth
>

Thanks Claudio - seems so obvious to me now! Doh!

Ant