Custom Event Throwing Null When Called?

Abydosgater

Hi, Im trying to define some custom events within one of my classes. I have set the delegate and the event as follows: public delegate void SegmentProgressUpdate(); public event SegmentProgressUpdate ProgressUpdate; That is just above my constructor. It compiles, But at runtime when the event gets called I get the "Object reference not set to an instance of an object." Error. Im sure i am doing something wrong, But i cant see what. I have read a dozen tutorials on custom events online, that all only have what i have above. And i have googled the error related to events but cant find a solution that works. Does anyone have any idea?

Gareth H

Abydosgater, I guessing you don't listen to the event. When you fire the event, check its not null.

if (ProgressUpdate != null)
ProgressUpdate();

Also, its best to try and stick to .net conventions when possible, so your delegate should take 2 params.

public delegate void SegmentProgressUpdate(object sender, EventArgs e);

Sender is the object calling the event, and the event args is the class with the information you'd like to pass, if you don't pass anything, just do a: EventArgs.Empty

Regards, Gareth. (FKA gareth111)

Abydosgater

Thanks for the quick reply Gareth. I have tried this, and it still doesnt work. It doesnt crash with the error but as you will see: SegmentOne.ProgressUpdate += new Segment.SegmentProgressUpdate(SegmentOne_ProgressUpdate); Im listening to the event, Thats in my main application, And i have changed it to this: if (ProgressUpdate != null) { ProgressUpdate(this, System.EventArgs.Empty); } else System.Windows.Forms.MessageBox.Show("error segment.cs:109"); But now all it does is throw the error message box.

Gareth H

Abydosgater, When do you listen to the event?, once you've created SegmentOne? or...?

Regards, Gareth. (FKA gareth111)

Abydosgater

Yes once i have created the SegmentOne Object. SegmentOne = new Segment(SegmentOneStart, SegmentOneEnd, this._webRequest); SegmentOne.ProgressUpdate += new Segment.SegmentProgressUpdate(SegmentOne_ProgressUpdate); Hmm

DaveyM69

You've got something screwy going on somewhere in your code. This is a simple sample that has 2 events - one with custom event args.

public class TestA
{
    public event EventHandler Property1Changed;
    public event EventHandler<TestEventArgs> Property2Changed;

    public int Property1
    {
        set { OnProperty1Changed(); }
    }
    public int Property2
    {
        set { OnProperty2Changed(value); }
    }
    protected virtual void OnProperty1Changed()
    {
        if(Property1Changed!=null)
            Property1Changed(this, EventArgs.Empty);
    }
    protected virtual void OnProperty2Changed(int value)
    {
        if (Property2Changed != null)
            Property2Changed(this, new TestEventArgs(value));
    }
}
public class TestEventArgs : EventArgs
{
    public TestEventArgs(int value)
    {
        m\_Value = value;
    }
    private int m\_Value;
    public int Value
    {
        get { return m\_Value; }
    }
}

You can instanciate, subscribe to the events and set the properties to raise the events like:

TestA testA = new TestA();
testA.Property1Changed += new EventHandler(testA_Property1Changed);
testA.Property2Changed += new EventHandler<TestEventArgs>(testA_Property2Changed);
testA.Property1 = 1;
testA.Property2 = 2;

then the methods called above

    void testA\_Property2Changed(object sender, TestEventArgs e)
    {
        Console.WriteLine("Property 2 Changed to: " + e.Value);
    }

    void testA\_Property1Changed(object sender, EventArgs e)
    {
        Console.WriteLine("Property 1 Changed.");
    }

Dave
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)

Abydosgater

Ive taken a look at that example, and i have added a full new test event to try it using the code above as an example. //Above my constructor public event EventHandler TestEvent; //within my constructor (in a loop) if (TestEvent != null) TestEvent(this, System.EventArgs.Empty); else System.Windows.Forms.MessageBox.Show("Event returned null"); //And in my main app when i use the class: SegmentOne = new Segment(SegmentOneStart, SegmentOneEnd, this._webRequest); SegmentOne.TestEvent += new EventHandler(SegmentOne_TestEvent); But as before, its just returning that the event is null and not calling it. even though i am listening for it.

DaveyM69

Abydosgater wrote:

within my constructor

If it's in the constructor - then the object instance may not yet created, and even if it is, the listener SegmentOne.TestEvent += new ... has not yet been attached so it will be always be null at that point.

Dave
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)

Giorgi Dalakishvili

C# Event Implementation Fundamentals, Best Practices and Conventions[^]

Giorgi Dalakishvili #region signature My Articles / My Latest Article[^] / My blog[^] #endregion

DaveyM69

I think the only way you can raise an event in the constructor that can be listened to is using a static event.

public class TestA
{
public static event EventHandler ConstructorCompleted;
public TestA()
{
OnConstructorCompleted(this);
}
protected static void OnConstructorCompleted(TestA instance)
{
if (ConstructorCompleted != null)
ConstructorCompleted(instance, EventArgs.Empty);
}
}

TestA.ConstructorCompleted += new EventHandler(TestA_ConstructorCompleted);
TestA testA = new TestA();

void TestA_ConstructorCompleted(object sender, EventArgs e)
{
if (sender is TestA)
{
Console.WriteLine("OK");
TestA.ConstructorCompleted -= TestA_ConstructorCompleted;
}
}

Dave
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)

Abydosgater

Thanks to Davie and Giorgi Dalakishvili i figured out the issue. As Davie said above, It was a loop in the constructor so when it was getting called it had not yet been set to listen to. So i moved the loop to its own Start() and it now works as below: SegmentOne = new Segment(SegmentOneStart, SegmentOneEnd, this._webRequest); SegmentOne.ProgressUpdate += new Segment.SegmentProgressUpdate(SegmentOne_ProgressUpdate); SegmentOne.Start(); Thanks guys