NullReferenceError when raising an event

Question

I have a small user control having a textbox and a button in it. I have a label in a page which holds the usercontrol. On a button click event of the user control I want to find out the area of user input value and want to display that area in a label on the main page.

UserControl

public partial class WebUserControl1 : System.Web.UI.UserControl
{
   public delegate void myeventhandler(object sender,MyEventArgs e);       

    public event myeventhandler MyEvent;        

     protected void Button1_Click(object sender, EventArgs e)
     {
       int radious= Convert.ToInt32(TextBox1.Text);
       double area = 3.14 * radious * radious;          
       MyEventArgs myeventargs = new MyEventArgs();
       myeventargs.Area = area;
       MyEvent(this, myeventargs);        

    }        
}

public class MyEventArgs : EventArgs
{
    public double Area { set; get; }
}

Main page

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        WebUserControl1 objuc = new WebUserControl1();           
        objuc.MyEvent += new WebUserControl1.myeventhandler(DisplayArea);              
    }

    public void DisplayArea(object sender,MyEventArgs e) 
    {
         Label1.Text   = e.Area.ToString();               
    }
}

But I am getting a NullReferenceError on line

MyEvent(this, myeventargs);

It's recommended to tag the question with the programming language you're using as well as any other helpful tag — Nir Alfasi, May 31 '14 at 22:41
http://meta.stackexchange.com/questions/10647/how-do-i-write-a-good-title — Soner Gönül, May 31 '14 at 22:46
It would be helpful if you could provide an actual title and a clearer description of your problem — Mauvai, May 31 '14 at 22:47

score 1 · Answer 1 · answered May 31 '14 at 22:47

1

You must check that MyEvent is not null before triggering it.

if(null != MyEvent)
    MyEvent(this, myeventargs);

answered May 31 '14 at 22:47

quantdev

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I checked it and found out that MyEvent is null so getting the NullReferenceError so if Iuse if condition then MyEvent(this, myeventargs) is not getting call,I want to call it,how to do it?? – RakeshTheStar May 31 '14 at 22:52
1

It is null because no handler has been attached to the event. Make sure that the button on which you click is the one to which you attached an handler. – quantdev May 31 '14 at 22:57
I want to attach the handler in the main page which consuming the UserControl and not in the ButtonClick – RakeshTheStar May 31 '14 at 23:01
You will need to use a debugger to find out why. Guess: make sure that `Page_Load()` is being called. – quantdev May 31 '14 at 23:06

NullReferenceError when raising an event

1 Answers1