How to correctly fire an event in C#

Question

I'm trying to fire an event when button is clicked. I click the button and nothing happens.

The problem is I always get null as EventA in OnEventA():

namespace eventsC
{
    public partial class Form1 : Form
    {
        public event EventHandler EventA;

        protected void OnEventA()
        {
            if (EventA != null)
                //never arrives here as EventA is always = null
                EventA(this, EventArgs.Empty);
        }

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
        }

        private void button1_Click(object sender, EventArgs e)
        {
            OnEventA();
        }
    }
}

EDIT:

Based on link from Henk Holterman I tested this code also:

public delegate void Eventhandler(object sender, Eventargs args);  

// your publishing class
class Foo  
{
    public event EventHandler Changed;    // the Event

    protected virtual void OnChanged()    // the Trigger method, called to raise the event
    {
        // make a copy to be more thread-safe
        EventHandler handler = Changed;   

        if (handler != null)
        {
            // invoke the subscribed event-handler(s)
            handler(this, EventArgs.Empty);  
        }
    }

    // an example of raising the event
    void SomeMethod()
    {
       if (...)        // on some condition
         OnChanged();  // raise the event
    }
}

I call OnChanged() when I click a button but the result is still always the same: EventA = null.

I wrote up the full picture [here](https://stackoverflow.com/a/2448530/60761) — H H, May 01 '19 at 06:55
@PeterBons: i need to receive the event in another VB.NET project in the same solution. But never arrives due to the EventA = null. Can i fire the event from the same class like in the code i show? — PeterPam, May 01 '19 at 07:11
Yes you can but it only works if there are subscribers. Once you add one you will see it is not null anymore. — Peter Bons, May 01 '19 at 07:22
@PeterBons: thanks for your response.Can you please tell me how to do this in the same class? — PeterPam, May 01 '19 at 07:23
I think Henk Holterman gave the best link for that, otherwise I would only repeat his answer — Peter Bons, May 01 '19 at 07:26
i have tested code from this link and the problem is the same... — PeterPam, May 01 '19 at 07:29
You do not provide enough information here for an answer - where is your line with `+=` ? — H H, May 01 '19 at 07:38
Yes, you must provide all of the relevant code - it must be a [mcve]. — Enigmativity, May 01 '19 at 07:39
Re " i do not have" Then you do not have a subcriber and in that case EventA is supposed to be null. — H H, May 01 '19 at 07:40

PeterPam · Accepted Answer · 2019-05-01T08:09:02.817

I solved my problem in this way:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace eventsC
{    

    public partial class Form1 : Form
    {

        public static event EventHandler<EventArgs> myEvent;

        protected void OnMyEvent()
        {
            if (myEvent != null)
                myEvent(this, EventArgs.Empty);
        }

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            myEvent += Handler;

            //call all methods that have been added to the event
            myEvent(this, EventArgs.Empty);

        }

        private void button1_Click(object sender, EventArgs e)
        {
            OnMyEvent();
        }


        static void Handler(object sender, EventArgs args)
        {
            Console.WriteLine("Event Handled!");
        }

    }
}

This doesn't appear to answer your question. Your question seems to ask about consuming your event from a different class. Is that right? — Enigmativity, May 01 '19 at 08:04
It doesn't really make sense to override `OnMyEvent` and also handle the event in your own class. The event handler should be done in the subscribing class, not in `Form1` — Chris Dunaway, May 01 '19 at 14:48

score 0 · Answer 2 · edited May 01 '19 at 08:02

I think they have correctly answered that it does not have any subscriber.

namespace TestConsoleApp
{
    public delegate void Eventhandler(object sender, EventArgs args);

    // your publishing class
    class Foo
    {
        public event EventHandler Changed;    // the Event

        protected virtual void OnChanged()    // the Trigger method, called to raise the event
        {
            // make a copy to be more thread-safe
            EventHandler handler = Changed;

            if (handler != null)
            {
                // invoke the subscribed event-handler(s)
                handler(this, EventArgs.Empty);
            }
        }

        // an example of raising the event
        public void SomeMethod()
        {

                OnChanged();  // raise the event
        }
    }

    public class Program
    {

        static void Main(string[] args)
        {
            Foo objFoo = new Foo();
            objFoo.Changed += ObjFoo_Changed;
            objFoo.SomeMethod();
            Console.ReadLine();
        }

        private static void ObjFoo_Changed(object sender, EventArgs e)
        {
            Console.WriteLine("Event fired and changed");
        }
    }
  }

How to correctly fire an event in C#

2 Answers2

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