How to practically use Events?

Question

Can anyone place some clear examples of using Events in C# (or any .NET language)? There are plenty of such on the internet, but most of them are complicated and they don't apply to every situation. Make it universal if possible please.

Answer 1

From your post, it appears that you want an "example" which would "apply to every situation" while being "universal" and "practical". I don't see how that is possible.

If you simply want to understand events better, start here. You could find something shorter, but it might not be clearer.

Answer 2

• Giving progress updates and completion on tasks.
• Reacting to user input
• Timers

Answer 3

Events are the basis of event-driven programming (obviously). So an event is appropriate any time you want to take action(s) based on something else occuring. What that something else could be a myriad of things - a user clicking a button, a timer elapsing, a window being closed, etc. It's hard to make it really universal, since there are so many different scenarios.

One example would be to fill a label with text when the user clicks on a button:

btnMyButton.Click += (o, ev) => { SetTextLabel(label1, "You clicked the button"); };

Note that SetTextLabel is a function that you would provide, and it should be ThreadSafe (otherwise I would get pounded by commenters saying my code wasn't so.)

Answer 4

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public class EXMAPLE 
    {
        public event EventHandler Changed;
        protected string _content;

        public string Content{
            get { return _content; }
            set 
            {
                _content = value;
                OnChanged(EventArgs.Empty);
            }
        }

        // Invoke the Changed event; called whenever list changes:
        protected virtual void OnChanged(EventArgs e)
        {
            if (Changed != null)
                Changed(this, e);
        }
    }


    class Program
    {
        private static void EXAMPLE_CONTENT_CHANGED(object sender, EventArgs e)
        {
            EXMAPLE ex = (EXMAPLE)sender;
            Console.WriteLine(ex.Content);
        }

        private static void INNA_REAKCJA(object sender, EventArgs e)
        {
            Console.WriteLine("The Content of EXAMPLE was changed");
        }


        static void Main(string[] args)
        {
            EXMAPLE ex1 = new EXMAPLE();
            EXMAPLE ex2 = new EXMAPLE();

            //add event ;->
            ex1.Changed += new EventHandler(EXAMPLE_CONTENT_CHANGED);

            ex2.Changed += new EventHandler(EXAMPLE_CONTENT_CHANGED);
            ex2.Changed += new EventHandler(INNA_REAKCJA);  

            //test
            ex1.Content = "value 1";
            ex2.Content = "value 2";

            System.Console.ReadKey();
        }
    }
}

That is only simple example - not universal ;p

Answer 5

The most simple example is when your creating a form which contains a clickable control such as a button. When you double click on the button, Visual Studio will generate an event in the code behind pointing to a method that will be called when the event gets fired off.

this.MyButton.Click += new System.EventHandler(this.MyButton_Click);

In general events are basically messages sent by an object to notify other objects that an action has occurred.

Answer 6

Events is the current way of handling user interface interaction with the rest of the application. Microsoft future into event driven development depends on the future of this new technology called RX Extensions, though (seems very promising). Java has the Actionlistener component (coming from the observer pattern), if you know it and can evaluate the usability of Events (in UI). Events in general is the mechanism with which notifications are provided to the referring object when something happens.

Check the relevant tutorial from msdn also.

Example:

    List.Changed -= new ChangedEventHandler(ListChanged);

Here the developer hooked up the method which will be called with the rising of the Changed event of the List container. So, when the list will change (added, or removed an item), this method will be called.