How do you unsubscribe to an event? - C# Discussion Boards

28-Aug-02 14:18

I sitting here coding along when I release I need to be able to unsubscribe to an event I just subscribed to. So I go to my books, and none of them talk about it, they show you how to subscribe to an event but not unsubscribe to it. So I do an internet search I get nothing. I do a Code Project search and I get nothing. I have a hard time believing that I am the first person who needed to know how to unsubscribe to an event in C#. In Java it was always straight forward with add and remove events tied to the object want to know about the event.

Well here is the way everyone teaches to subscribe to and event:

<pre lang="cs">    OnEvent += new EventHandler(HandlerMethod);</pre>So I would assume that to unsubscribe you would do the reverse:<br />
<code lang="cs"><pre lang="cs">    OnEvent -= handleToEventHandlerObject;</pre>However since this the EventHandler was created anonymously you do not have a handle/reference to it.  <br />
<br />
So my next assumption would be this:<br />
<code lang="cs"><pre lang="cs">	EventHandler handleToEventHandler = new EventHandler(HandlerMethod);
		
	/ ... /
 		
	OnEvent += handleToEventHandler;
		
	/ ... /
	
	OnEvent -= handleToEventHandler;</pre>Does this sound right.  I have not tried it yet, since I was hoping there was a better way, and also I wanted to do some gripping about C#'s auto-magical delegate system for handling events.  Which have created problems, at least for me.  Mainly when you want to wrap another object, and provide pass through event handling, you can not.  Your wrapper class has to register for the events, and then provide its own delegates for handling it.  Also when you want to do something like subscribe and unsubscribe to an event it seams like subscribing is no big deal, but unsubscribing seams like it comes with a little extra (more like annoying) work.<br />
<br />
Just to say it, I think the Java event handling system was far better and more flexible.  There was nothing going on auto-magically under the covers.  You new there was a Collection holding all the classes who subscribed, and that they are required to have a method signature that you will call when it is thrown.  There were standardized add/remove methods that all you have to do is pass the object who subscribed which was 99% of the 'this' keyword.  You always hand a handle to the object you used to subscribe to it with.<br />
<br />
Well that’s it with my gripping about C# events.  If someone can confirm that above is the correct and only way to subscribe and unsubscribe to an event, or provide a alternative solution.<br />
<br />
:laugh: Thanks for putting up with me.  ;P<br />
<br />
Aalst

Paul Riley28-Aug-02 18:17

28-Aug-02 18:17

To unsubscribe, believe it or not, you use:

<br />
OnEvent -= new EventHandler(HandlerMethod);<br />

See ms-help://MS.VSCC/MS.MSDNQTR.2002APR.1033/csref/html/vcwlkEventsTutorial.htm[^] for details.

By the way, you can probably simulate pass-through events easily by using event properties.

See ms-help://MS.VSCC/MS.MSDNQTR.2002APR.1033/cpguide/html/cpconraisingmultipleevents.htm[^] for deatils.

Paul

Rodney S. Foley28-Aug-02 19:14

28-Aug-02 19:14

Thanks, and as hard as it was to believe, and even after reading the related part in the tutorial you linked to, I believe it only because I know how stupid Microsoft can be.

Is anyone with me here on this? Confused | :confused:

You create an EventHandler object and add it to the an event
to subscribe to this event by means of the += operation.  Then
to unsubscribe to this event you have to create another, 
unrelated, EventHandler object and subtract it from the event
by means of the -= operation.  So you are saying I want to
remove an object that is not there, and it actually removes the
object you intended even though you did not reference this
object at all. (I know it must do it via the method passed as
an argument). This is the most illogical and idiotic thing.  If
this is not auto-magical behavior I don't know what is.

Man, Microsoft went off the deep end on that one. No wonder no publisher prints this, everyone would think it was a type-o. Wink | ;)

As for using event properties to "simulate" pass-through, that looks like more overhead than the way I described. The method I mention also "simulates" pass-through behavior. Regardless, the point is that you cannot actually do any pass-through of event's short of some complicated reflection. All because Microsoft has a few screws loose.

Don't get me wrong, I complain and grip this much about Java and C++ all the time. I am an Equal Opportunity Gripper (EOG).

Big Grin | :-D

C# is new, so I have some catching up to do. Big Grin | :-D

Adios,

Aalst

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Paul Riley28-Aug-02 19:25

28-Aug-02 19:25

All they've done is run into the same problem you were griping about in the first place and come up with an easy-to-use (if a little auto-magical) solution, now you're griping about that Wink | ;)

.

The basic idea is that creating the EventHandler object creates enough information for either += or -= to operate, so it's easy enough to understand how it works.

You're not far wrong about pass-through behavior, but the point is there are several ways to achieve it, depending on the purpose.

For example, another way to achieve it is to make the object containing the event public and "handle" it an extra level up.

ie.
this.MyPassThroughClass.MyEventingClass.MyEvent += new EventHandler...

Paul

Rodney S. Foley28-Aug-02 19:50

28-Aug-02 19:50

I am not clear on the problem I was gripping about that is the same problem "they" ran into.
Because if a solution is logical, I would only grip about it's extra steps or complexity over a simpler solution. You may be talking about the example I gave in the original post of how I thought it may work, and how I gripped about it have extra steps. If it was the solution it would have been at least a logical solution.

They could have done away with EventHanlders all together, and just have the event use the += operation on the method directly, and -= on the method directly.

public event OnEvent(object, EventArgs);

/ ... /

private void eventHandler(object sender, EventArgs e) {  / ... / }

/ ... /

OnEvent += eventHandler;

/ ... /

OnEvent -= eventHandler;

It gains the exact same results with out all the HandlerObjects in the framework, and at least makes more logical sense as to how to use it. I still have not seen anything better and more robust that the way Java handles events.

Regardless its very easy to go around and around on subjects like these. Since nothing we can say can change the fact that the way it is, is the way it is. I easily adjust to stupid and illogical things, all programming languages have there faults.

It is my opinion we are kick the crap out of a dead horse here, I would recommend we just leave it be that no matter how much justification is provided I will not accept the fact that it is the way it is, I will just have to deal with it, and that no matter how much I grip about it and say how much it sucks, and this way or that way is better it will not change the fact that that way or this will will never come to be and that I have to deal with it.

Aalst

leppie28-Aug-02 22:02

leppie

28-Aug-02 22:02

Aalst wrote:
They could have done away with EventHanlders all together, and just have the event use the += operation on the method directly, and -= on the method directly.

But an eventhandler is a delegate which in turn is really a function pointer, thats why u need new EventHandler(MyHandlingfunction) to create a pointer to the function handling the event.

Im not gonna make a big point but, C# is not MS Java (it mite look and feel like it), so there is no point comparing, if you dont like the way C# works use Java or even J# (all of .Net plus most of Java) Smile | :)

MYrc : A .NET IRC client with C# Plugin Capabilities. See
http://sourceforge.net/projects/myrc for more info. Big Grin | :-D

am i dreaming

imran_rafique28-Aug-02 11:58

28-Aug-02 11:58

i am tring to develop an application which can install any type of software on any platform with out user interaction in dot net.
First of all my application will learn all events and messages at least one time then it will perform the task on the network on any computer.
is it possible?
if yes how?
can some one provide the helping metrial and links about that?

r00d0034@yahoo.com

Rodney S. Foley28-Aug-02 14:46

28-Aug-02 14:46

First of all, make sure your not re-inventing the wheel.

There are currently some commercial products:

InstallAnywhere Zero G (Java based)
JExpress from DeNova (Java based)

And an open source projects:

InstallBase MPI (C & TCL based)

I am sure there are more of these in each area, this is all I could remember quickly.

Also C#/.NET really is not the best language to write cross-platform software yet. Unless you are talking across multiple Windows platforms since Windows is the only OS that has full support for the .NET Framework at this time. There is a UNIX/Linux port in process called the Mono Project. However they have a lot of work to do still before it is finished. I have not heard anything about a MAC .NET Framework yet.

Right now the only true "write once, and run 'almost' anywhere" is Java. C/C++ is also good if you stick to ANSI standards, and keep any platform specific code modular. Then you have pretty much "write once, and compile many times, then run on those compiled platforms". Other than C/C++ or Java there is not popular cross-platform language right now.

So if you have looked at those products and your idea has something they done, either open source versions of the commercial ones, or some newer or better than the existing open source ones. I would say go for it in Java or C/C++.

If not, then you may want to just use one of the ones listed or if you do some searching you may find more that are even better than those.

And remember -- Anything is possible, if you don't give up.

Aalst

Nnamdi Onyeyiri28-Aug-02 21:48

Nnamdi Onyeyiri

28-Aug-02 21:48

dont forget OnTechnology CCM Big Grin | :-D

Suspicious | :suss: Email: theeclypse@hotmail.com URL: http://www.onyeyiri.co.uk
"All programmers are playwrights and all computers are lousy actors."

Paul Riley28-Aug-02 17:56

28-Aug-02 17:56

imran_rafique wrote:
i am tring to develop an application which can install any type of software on any platform with out user interaction in dot net.

Have fun. Wink | ;)

What is the end purpose here? Are you looking to create a marketable product or just for personal use?

If marketable, you have a lot of competition out there. Microsoft's SMS allows a fair amount of remote installing (sit in one console and install all over a network, regardless of OSs). PC-Duo Enterprise (formerly LANutil) has been getting some rave reviews lately as a cheaper, more powerful alternative (I should know, I work for the company that writes it) and will soon be available as an MMC plug-in.

Then there's a LOT of cheap alternatives for smaller companies and individual use (that is, products that do the job you're talking about without the inventory and other functionality offered by PCDE or SMS), several listed by the previous poster.

Also: The first thing we're bearing in mind before considering a .NET conversion is that .NET doesn't work on Win 95, so you've already discounted that option once you look at C#.

Finally, pay a lot of attention to Windows Installer which is doing the job automatically and an increasing number of companies are already picking up on it (we are, despite the fact that it threatens to drive us out of business in the long term!).

Just a few comments for you there, not to discourage you (we're not afraid of healthy competition) but to say "ask yourself the question: why are you doing it? Then research the available products (some of which will be converting to .NET a lot quicker than you can start from scratch) because the effort will be huge and you're not the first to think of it!"

If it's just a personal project, for learning C#, think of something more simple to do first Wink | ;)

.

Paul

imran_rafique1-Sep-02 12:15

1-Sep-02 12:15

thanks
let me tell u why i am doing this.i am doing mine final project of MCS.
can u provide the links related to these type of application and available products so that i can study them.
Because i want to do it as fast as possible.
plz provide links if u could?

Paul Riley2-Sep-02 5:03

2-Sep-02 5:03

You can find PC-Duo Enterprise at
http://www.vector-networks.com[^]

There's a 30-day trial download which includes all sorts of different things, if you look at the product properly you might even find something easier to emulate. Software Distribution is the bit you're looking for though.

I'm sure you can find SMS at Microsoft's site, but I don't know any more than that. You can also find info on Windows Installer there.

Not sure where you'll find the smaller products but I would advise looking for them, there's probably something much more simple to base your ideas on.

Looking at
http://www.installshield.com
http://www.installsite.com
may be useful to you also.

Paul

imran_rafique1-Sep-02 12:50

1-Sep-02 12:50

thanks
i am doing it as mine final project of MCS?

r00d0034@yahoo.com

who to use it

imran_rafique28-Aug-02 11:22