Question

I’am having a very annoying problem in my code, when I try to send a string from Form B to form a. I get the error message:

Object reference not set to an instance of an object.

I’am familiar with this error and normally I know how to solve this problem, but this one is different.

I need to send a Clockname from one form to the main form, I’am trying to achieve this using the code below:

delegate void ClockClocknameReceivedEventHandler(object sender, Clock.ClocknameReceivedEventArgs e);

    internal class ClocknameReceivedEventArgs : EventArgs
    {
        string _clockname;

        public string Clockname
        {
            get { return _clockname; }
        }

        public ClocknameReceivedEventArgs(string clockname)
        {
            _clockname = clockname;
        }
    }

    // An event that clients can use to be notified whenever the
    // elements of the list change.
    public event ClockClocknameReceivedEventHandler ClocknameReceived;

    // Invoke the Changed event; called whenever list changes
    protected void OnClocknameReceived(Clock.ClocknameReceivedEventArgs e)
    {
        ClocknameReceived(this, e);
    }

And the following code gets fired when pressing a button, the form will close after that:

OnClocknameReceived(new Clock.ClocknameReceivedEventArgs(ClockName));

The error(Object reference not set to an instance of an object.) I receive occurs at

ClocknameReceived(this, e);

I’am using the exact same code, from another class to the main form to send a byte array which works fine, but this one give me that error.

Anyone any ideas?

Thanks in advance!

Answer 1

The delegate can be null. Invoke it only if it’s not null:

protected void OnClocknameReceived(Clock.ClocknameReceivedEventArgs e)
{
    ClockClocknameReceivedEventHandler handler = ClocknameReceived;
    if (handler != null)
    {
        handler(this, e);
    }
}

---

The delegate is null when you haven’t subscribed an event handler to the event yet. Subscribe an event handler:

formB.ClocknameReceived += FormB_ClocknameReceived;

with

void FormB_ClocknameReceived(object sender, Clock.ClocknameReceivedEventArgs e)
{
    MessageBox.Show(e.Clockname);
}