vb.net equivalent

Question

Hi

What is the vb.net equivalent for the following c# snippet?

void Editor_Init(object sender, EventArgs e,WebControl baseEditor)
{
     ASPxWebControl editor = (ASPxWebControl)sender;
     editor.Init -= new EventHandler((s, args) => Editor_Init(s, args, baseEditor));
}

Thanks

Regards

Answer 1

    Private Sub Editor_Init(ByVal sender As Object, ByVal e As EventArgs, ByVal baseEditor As WebControl)
        Dim editor As ASPxWebControl = DirectCast(sender, ASPxWebControl)
        Dim lambda = Sub(s, args) Editor_Init(s, args, baseEditor)
        RemoveHandler editor.Init, lambda
    End Sub

Are you sure you need to remove the event wireup here?  It would seem to make more sense to add it.  Anyway, for some reason VB does not allow combining RemoveHandler with a lambda (this seems very odd) - you have to break it out into two statements.

If it were an event wireup instead, then you would have the following in VB:

Private Sub Editor_Init(ByVal sender As Object, ByVal e As EventArgs, ByVal baseEditor As WebControl)
	 Dim editor As ASPxWebControl = DirectCast(sender, ASPxWebControl)
	 AddHandler editor.Init, Sub(s, args) Editor_Init(s, args, baseEditor)
End Sub

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Answer 2

@Dave:

I've seen web app examples like this before.  Leaking memory via event handlers is a common problem in web apps and one solution I've seen is to blindly unregister dynamic handlers before registering them.  This covers reentrant code without actually coding for it (or tries to anyway; may or may not actually be enough to prevent the issue).

What's really strange about this particular code is the non conformant event handler signature... that baseEditor instance should be a member of a custom eventargs instance (at least, one would think).  This might be a case where a C# translation is not appropriate and the solution should be refactored into VB.

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Reed Kimble - "When you do things right, people won't be sure you've done anything at all"

Answer 3

Reed, Dave

I would think the third argument is an optional argument. The signature can then work.

----------------------------

  When using Addhandler, with the lambda in line, 

AddHandler Me.SomeEvent, Sub(s as object, e as eventargs) ...

 then, the event becomes is impossible to remove from the collection. As this code show if you try it

Public Class Form1

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        AddHandler Button1.Click, Sub(s, arg) Beep()
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim Lambda = Sub(s, arg) Beep()
        RemoveHandler Button1.Click, Lambda
    End Sub

End Class

The reason is that the lambda installed in the event collection and the lambda you created to remove it are two different objects. (Even if they look, code wise, identical)

therefore

RemoveHandler Me.SomeEvent, Sub(s as object, e as eventargs) ...

will never remove nothing.

The proper way would be to do it like this

Public Class Form1

    Private Lambda As EventHandler = Sub(s, e) Beep()

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        AddHandler Button1.Click, Lambda
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click

        RemoveHandler Button1.Click, Lambda
    End Sub

End Class

(  NOTE: This is also true in C#)

Answer 4

Thanks Crazypennie - that makes sense to me.

So the original C# code was probably not doing what it was intended to do and the VB version I gave was equivalent but also not serving the intended purpose.

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Answer 5

For some reason I thought there was a way C# handled this when VB could not, but you are right, this code just doesn't do what it is intended to do.

---

Reed Kimble - "When you do things right, people won't be sure you've done anything at all"

Answer 6

On the same lines as Crazypennie the following can not be removed

Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        AddHandler cmdClose.Click,
            Sub()
                Close()
            End Sub

        Controls.OfType(Of Control).ToList.ForEach(
            Sub(x)
                If TypeOf x Is TextBox OrElse TypeOf x Is Label Then
                    AddHandler x.Click,
                        Sub(s As System.Object, a As System.EventArgs)
                            CType(s, Control).Visible = False
                        End Sub
                End If
            End Sub)
    End Sub
End Class

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Answer 7

Well, there is always a way to remove anything

Just posting this as it is an interesting dig inside the framework 

Public Class Form1

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        AddHandler Button1.Click,
          Sub()
              Beep()
          End Sub
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim FI_EventClick As Reflection.FieldInfo = GetType(Control).GetField("EventClick", 32 Or 8) 'Non-Public Static
        Dim oClick As Object = FI_EventClick.GetValue(Button1) 'This is the Key for the Event Click in the EventHandlerList
        Dim ComponentType As Type = GetType(System.ComponentModel.Component)
        Dim FI_Events As Reflection.FieldInfo = ComponentType.GetField("events", 32 Or 4) 'Non-Public Instance
        Dim Events As System.ComponentModel.EventHandlerList 
        Events = FI_Events.GetValue(Button1) 'This is the Button1 EventHandlerList
        Dim Handlers As EventHandler = Events.Item(oClick) 'This is the Click event delegate
        Dim InvList() As [Delegate] = Handlers.GetInvocationList ' This is the delegate invocation list for the Click event
        For Each method As [Delegate] In InvList
            If method.Method.Name.Contains("Lambda$") Then
                RemoveHandler Button1.Click, method ' Remove the Lambda handler from the event
                Exit Sub
            End If
        Next
    End Sub

End Class

Answer 8

That's interesting, but I don't think you can reliably find the exact lambda you're looking for if there were more than one wired to the event.

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Answer 9

Very true and was tempted to toss a similar peice of code out that does the same thing but thought better of it.

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Answer 10

Very true Dave.

    Those lambda method will be named 

         _Lambda$__1       _Lambda$__2   etc ... 

So, it would be required to know the order they were added to the event to be able to select one if there was more than one

Answer 11

Hi

Here is the accompanying 'add handler' c## code.

void editModeTemplate_CustomCreateCellControl(object sender, DevExpress.ExpressApp.Web.Editors.CustomCreateCellControlEventArgs e)
        {
            if (e.PropertyEditor.Editor is ASPxWebControl)
            {
                e.PropertyEditor.Editor.Init += new EventHandler((s, args) => Editor_Init(s, args, e.PropertyEditor.Editor));
            }
            else if (e.PropertyEditor is ASPxLookupPropertyEditor)
            {
                ASPxLookupPropertyEditor editor = e.PropertyEditor as ASPxLookupPropertyEditor;
                editor.DropDownEdit.DropDown.Init += new EventHandler((s, args) => Editor_Init(s, args, e.PropertyEditor.Editor));
            }
            
        }

How does vb.net code work in this case especially when having to accommodate WebControl baseEditor and e.PropertyEditor.Editor parameters in remove handler and add handler respectively?

Thanks

Regards

Answer 12

See my 'AddHandler' example in my first reply.

This C# code has the problem pointed out by Crazypennie - the "-=" code to remove the event handler methods is just wrong - it will have no effect.

See Crazypennie's suggestion for correcting this code by setting a field lambda.

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Answer 13

Hi,

Since we haven't heard from you for a long time, I temporarily close this case. I mark useful reply as answer. If you have any concerns, please free feel to reopen it or submit a new question.

Thanks for your understanding.

Best regards,
Franklin

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