Event Handling in C#

[Nishant Gaurav]

Hi All,

I want to create a event on a static variable of the class.
The intention is to notify the class whenever the static variable value is changed, so that it can do some processings.

Please help me do this.
Thanks & Regards
Nishant Guarav

[Aimee Bailey]

Have a look at properties, you can build the event raiser from the set block. eg:

private static int _ANumber = 5;
 
public static int ANumber
{
  get { return _ANumber; }
  set 
  {
    _ANumber = value;
    //Event would be fired here.
  }
}

[Nishant Gaurav]

thanks AmzBee for the response, i did the same thing but was not able to get the event handler respond to that, when the static value was changed in some other class.
It will be of great help if u can show it with a example how this can be done.

Thanks & Regards
Nishant Gaurav

[Christian Binder]

Hi!

Please provide the code you've actually written, so I'd be able to find out what you've done wrong.

Who do you want to notify when the property's value is changed? The static class or any instance of this class?

[Aimee Bailey]

Be aware that firing events outside of a form will most likely cause a cross-thread exception, im taking a stab in the dark that your trying to update a form?

Aimee.

[Nishant Gaurav]

yes AmzBee, u r spot on on this.
Let me clear the scenario once again.

namespace asd
{
class abc //my main class running the form is running
{
  // the variable for which i want to create a listener.
  public static int i;
 
  // function that i want to call once the change in value is notified by event listener
  public void fn()
  {
  }
 
}
}
 
// this is a library which is called by some other class
using asd;
namespace dsa
{
Class lib
{
  public static setvalue()
  {
    // will be able to change the value of abc class variable i as it is static.
    abc.i =10;
    /// once this value is set i want to notify abc class about the change. Which inturn will call the function fn().
  }
}
 
}
 

any Help on this thing will be of great help.
If there is some another way of doing this, i want to know that too..

i want the most efficient way of doing this thing.

Thanks & Regards
Nishant Gaurav

[Aimee Bailey]

To be honest this area is a quite clouded one, and ive found many people find it hard to deal with events in c#, id reccomend checking out MDSN on the topic of events and delegates, however as im in the giving mood hehe, here is the most elegant way to handle events within static classes:

This is the external static class:

public static class clsTest
    {
        public delegate void MyEventHandler(string message, DateTime when);
        public static event MyEventHandler MyEvent;
 
        static clsTest()
        {
            // Adds our internal subscriber.
            MyEvent += new MyEventHandler(clsTest_MyEvent);
        }
 
        static void clsTest_MyEvent(string message, DateTime when)
        {
            // Events require atleast 1 subscriber.
        }
 
        public static void FireEvent(string message)
        {
            MyEvent(message, DateTime.Now);
        }
    }
 

And here is a form i created:

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            clsTest.MyEvent += new clsTest.MyEventHandler(clsTest_MyEvent);
        }
 
        void clsTest_MyEvent(string message, DateTime when)
        {
            this.BeginInvoke(  new clsTest.MyEventHandler(HandleEvent), message, when );
        }
 
        void HandleEvent(string message, DateTime when)
        {
            string msg = string.Format("Message: {0} \r\n At: {1}", message, when);
            MessageBox.Show(msg);
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            clsTest.FireEvent("hello");
        }
 
    }
 

Make note that i actually subscribe to the event twice, this is because an event is required to have atleast one subscriber on initialization.

Also, to raise the event like in visual basic, all you need do is call the event and pass it the arguments specified in the delegate.

I know this is a bit of a verbose answer, and il probably be told off for it, but hey if it helps!

[Nishant Gaurav]

Hi AmzBee/ChBinder,
Sorry but i didn't get the info that i intended for.
Or may be i am not able to understand in what way it is potraying the scenario i informed about.

For that case let me again specify the need.

namespace abc
{
    class asd
    {
        public static int x;
        // want an eventlistener or a way to call a function asd
        public asd()
        {
            //having code to access non static variable so can not be static.
        }
        public static void Main()
        {
            xyz.setValue();
        }
    }
 
    class xyz
    {
        public statis void setValue()
        {
            asd.x = 10; // At this instant of time the event should notify 
            // class asd about the change in value.
        }
    }
}
 

Please let me know if i am able to convey u my requirement this time :)

[Christian Binder]

class asd {
    public static event Action<int> XChanged;
 
    private static int _x;
 
    public static int x {
      get { return _x; }
      set {
        if (value != x) {
          _x = value;
          if (XChanged != null)
            XChanged(x);
        }
      }
    }
 
    // Constructor
    public asd() {
      XChanged += EventHandlingMethod;
    }
 
    void EventHandlingMethod(int newValue) {
      // Do something
    }
  }
 
  class xyz {
    public void setValue() {
      asd.x = 1; // the event should be fired after setting x to 1
    }
  }

[Nishant Gaurav]

@ChBinder
Hi chBinder,
Thanks for the above code, but
the above code gives error at XChanged(x);
The error is:
System.NullReferenceException: Object reference not set to an instance of an object.

It will be really helpful if u tell me what has gone wrong in this case.

[Nishant Gaurav]

Hi chBinder,
Thanks for the above code, but
the above code gives error at XChanged(x);
The error is:
System.NullReferenceException: Object reference not set to an instance of an object.

It will be really helpful if u tell me what has gone wrong in this case.
As per the code Xchanged is a public static variab;e so there is no need to create the instance.

i am putting here the code that i tested with ur code.

 
namespace StaticEvent
{
    public class VuserData
    {
        public static event Action<int> Xchanged;
 
        private static int _x;
 
        public static int x
        {
            get
            {
                return _x;
            }
            set
            {
                if (value != x)
                    _x = value;
                Xchanged(x);
            }
        }
 
        public VuserData()
        {
            Xchanged += new Action<int>(VuserData_Xchanged);
        }
 
        void VuserData_Xchanged(int obj)
        {
            Console.WriteLine("{0}", obj);
        }
        public static void Main()
        {
            Test.sMain();
        }
    }
 
    class Test
    {
        public static void sMain()
        {
            VuserData.x = 1;
        }
    }
}
 
 

[Christian Binder]

On line 20:
Xchanged(x);
You've to check first, if Xchanged isn't null!

if(Xchanged != null)
  Xchanged(x);
 

[Nishant Gaurav]

@ChBinder
In that case the Xchanged(x) is never called.
What am i missing over here.

[Christian Binder]

You never create an object of type VuserData like

VuserData obj1 = new VuserData();
 

At least when you create an object of type VuserData it will be attached to the Changed-Event

[Aimee Bailey]

Read my last reply, give's the valid way of firing events inside of a class.

[Nishant Gaurav]

@AmzBee
Hi AmzBee,

It will be great if u can help me understand how the method u told is more valid and accurate.

p.s i am a beginner and really appreicaite if u can help me out. :)

[Christian Binder]

I think the main difference to AmzBee's code is, that I'm not using a static constructor.
You could also use a static constructor and attach to the event. So it will be handled although there isn't an instance of the object.
You can also use a static constructor and a non-static constructor.

It's depending on what you want to achieve at last.

[Nishant Gaurav]

@ChBinder
Thanks ChBinder,
The thing is with a static constructor, it will require a static method to be invoked which means that the method VuserData_Xchanged at Line No 29 in the above example should be static. which i dont want to be.

what i want to know is if there is any difference in terms of performance with the above to sited examples.
And if it is there then which is the best approach.

[Christian Binder]

It is difficult to tell you the best approach without knowing the overall picture.

What do you want to do when the event comes?
How many instances of VuserData you'll create?
What is the whole thing intended to do?

There are many "best approaches", they all depend on what your program is going to do.

[Nishant Gaurav]

@ChBinder
The thing is there will be only one instance of vuser.
the other class is actually a library referencing this vuser class.
And as the variable is static i will not be required to create any other instance for that.
this library is inturn being referenced by the vuser class in other way.

please let me know if u want any other information.

[Christian Binder]

Okay, so you've got only one instance of VuserData, why do you need a static variable, you could also use a non-static for it as long as there's only 1 instance.

Which (event-handler-)method do you want to call? My first code-suggestion shows how to do it with a non-static method of VuserData-class. Is this what you've wanted?

[Nishant Gaurav]

@ChBinder
The static variable is required because that is being modified by the 2nd class which a library and not the same class.
If i would have to use the non static variable that will mean that i have to create an instance of the class first and then update the value.

[Christian Binder]

Ok, I understand. But what will you do, when the event is fired?
You've got one instance of VuserData-class, do you want to execute code of a method of this instance?

[Nishant Gaurav]

@ChBinder
yes thats the whole idea...
the only instance created for vuser has some private non static members too, which i will be using in the member of that class.

Basically,
u can see the approach as a client server one.
In which vuser instance is the server running.
and the other class which is modifying the static variable of the vuser class is a client.
and for that reason i dont want the client to create a instance of the server class in this case the vuser.

[Christian Binder]

How do you separate server and client?
If you've got two different processes (starting the program (exe) twice or starting different programs), your approach won't work because the static variable is per process.

[Nishant Gaurav]

@ChBinder
there are 2 application exe's. 1 server and 1 client.
there is a dll which is common to both server and clients.

Basically I want to bridge the 2 exe's and i am not getting a workaround.

i want to send some information from client to server.
i was working in that direction itself in order to achieve the client server communication.
but now i dont know what to do.
It will be great if u can help me in this.

[Christian Binder]

Good, so the static variable will be unuseable when there are two instances running.
The most simple way (for me) to let two programs talk to each other ist unsing files or databases which you've to poll and check for changes - it's also the most dirty way I think.

Better ways are using Named Pipes or WCF-technology. But these are greater constructs, so you've to dig into these concepts to be able to use them.

[Nishant Gaurav]

@ChBinder
Thanks both of u for ur help on event handling.
i really appreciate ur help.
I will mark this thread as completed. As now what i need is completely different from the context set for this discussion.
Thanks once again :)