Handles, AddHandler and RemoveHandler in VB.NET

Posted by Peter van Ooijen on August 3, 2005

After a crash course comes sinking in. This post is a rewrite of yesterdays one
on events in VB.NET. I will concentrate on VB.NET include Daniel and
Chi’s comment and after that I will return to my beloved C#.

There are several ways to declare events in VB.NET. The usual one is using the handles keyword. As Daniel pointed out this can link an event handling method to multiple events.

    Private Sub MyClick(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click, Button2.Click
        ListBox1.Items.Add(String.Format(“You clicked {0}”, CType(sender, Control).Name))
    End Sub
 

This code will run on a click of button1 as well as a click of
button2. The coupling of the events and handler is declared in the
method signature.

You can also add an eventhandler dynamically form code.

    Private Sub MyOtherClick(ByVal sender As Object, ByVal e As EventArgs)
        ListBox1.Items.Add(“That other click”)
    End Sub

    Private Sub ButtonAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonAdd.Click
        AddHandler Button2.Click, AddressOf MyOtherClick
    End Sub
 

Every time ButtonAdd is clicked it will add another event handler to
Button2. When ButtonAdd is clicked 4 times a click of button2 will
result in MyClick being executed first after which MyOtherClick is executed four times in a row.

You can also dynamically remove eventhandlers. This is not limited
to handlers been set dynamically. This removes the event handler which
was set with the handles keyword.

    Private Sub ButtonRemove_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonRemove.Click
        RemoveHandler Button2.Click, AddressOf MyClick
    End Sub
 

This is no big surprise as reflector had already shown that the Handles keyword translates to an addhandler statement.

Chid pointed out that you do not need to use the WithEvents keyword on an object when it’s eventhandlers are set using AddHandler.
Reflector shows that this does not make any difference at all. Just
like handles, withevents is nowhere to be found in the actual generated
code. In fact it all boils down to AddHandler and RemoveHandler which
map to CLR functions. In this post I’ve shown that you can freely mix
Handles and AddHandler. In last post Reflector showed that there are
some subtltties behind the scenes. These subtleties are enough to make or break the sinking of COM events.
I’m gratefull for VB.NET fixing that for me. And a little sad for C#
not doing what I had hoped to do. After all it’s eventhandling syntax
is imho far less confusing.

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  • Quan

    I have a problem as follow and i think you can help because i have a feeling that it is similar to the topic that you post.

    I use For …Next to generate ImageButtons with IDs are Items’ IDs loaded from Database. These buttons are AddToCart buttons.

    Thus, I dont know how to make them handle events AddItemToCart because they are created at run time. I can’t use them to write a Sub as i can do with Buttons dragged and dropped. Examples : if i have a button dragged and dropped with ID “Button1″ , all i have to do is writing a Sub “Button1_Click”

    So How can I write a sub handled by button whose ID is assigned at Run Time ?

  • Quan

    I have a problem as follow and i think you can help because i have a feeling that it is similar to the topic that you post.

    I use For …Next to generate ImageButtons with IDs are Items’ IDs loaded from Database. These buttons are AddToCart buttons.

    Thus, I dont know how to make them handle events AddItemToCart because they are created at run time. I can’t use them to write a Sub as i can do with Buttons dragged and dropped. Examples : if i have a button dragged and dropped with ID “Button1″ , all i have to do is writing a Sub “Button1_Click”

    So How can I write a sub handled by button whose ID is assigned at Run Time ?

    Could you please email solution to my Email : Tobenumb@yahoo.com .

    Thanks for your help

  • http://codebetter.com/blogs/peter.van.ooijen/ pvanooijen

    You cannot add eventhandler by declaring a handles. You have to add them from code. As explained in

    http://codebetter.com/blogs/peter.van.ooijen/archive/2005/08/03/130245.aspx

    What I would do is write one handling method. It can check which button triggered the event by inspecting the sender.

  • Priyanka

    I too faced a similar problem..
    Thanks for your help
    :-)

  • http://www.ournewbusiness.com Marvin Fray

    If it possible to find out what eventHandlers are handling an event e.g. …

    let say I have a function which is passed an object of type button. When I recieve the button object I want to remove any eventhandlers that are currently handling the Click event. Is there a way of finding out what handlers have been added to this event.

    Thanks in advance for your help.

  • http://codebetter.com/blogs/peter.van.ooijen pvanooijen

    There is a way to find out.

    The delegate has methods to inspect the list of eventhandlers subscribed. You can remove handlers using -=

    I havn’t done any of that myself but Jeffrey Richter has written some good stuff on it.

    But most of that is hardcore C#

  • Thomas

    Thank you for your example.
    Helped me.

  • zubairTechy

    Gr8 its really very useful for me…
    thnx

  • Emre Meral93

     owwww !!! very very very very thank youuu !!!!!

  • dusdiscuss

    i always wanna do this! Really great!