I'm seeing some strange behavior where the RunWorkerCompleted event for one of two threads I start isn't being called depending on how I call them. Check out the code below, and the two methods of firing the threads, good() and bad().
Public Class Form1
Private WithEvents bw As System.ComponentModel.BackgroundWorker
Private WithEvents bw2 As System.ComponentModel.BackgroundWorker
Private starts As Integer = 0
Private Sub bw_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bw.DoWork
starts += 1
Threading.Thread.Sleep(e.Argument)
End Sub
Private Sub bw_Completed(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles bw.RunWorkerCompleted
MessageBox.Show("Ending " + starts.ToString())
End Sub
Private Sub bad()
bw = New System.ComponentModel.BackgroundWorker()
bw.RunWorkerAsync(5000)
Threading.Thread.Sleep(500)
bw = New System.ComponentModel.BackgroundWorker()
bw.RunWorkerAsync(5)
End Sub
Private Sub good()
bw2 = New System.ComponentModel.BackgroundWorker()
AddHandler bw2.DoWork, AddressOf bw_DoWork
AddHandler bw2.RunWorkerCompleted, AddressOf bw_Completed
bw2.RunWorkerAsync(500)
bw2 = New System.ComponentModel.BackgroundWorker()
AddHandler bw2.DoWork, AddressOf bw_DoWork
AddHandler bw2.RunWorkerCompleted, AddressOf bw_Completed
bw2.RunWorkerAsync(5)
End Sub
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
'good()
bad()
End Sub
End Class
In both cases the DoWork event is called for both threads. But in the bad() case, only the second thread fires the RunWorkerCompleted event. This is obviously due to the two different ways I'm using VB to handle events here. I'm looking for an explanation of this behavior, preferably with a link to some documentation that could help me understand how these events are being handled in VB better. It seems strange to me that just reusing a variable name here seems to either dispose of the thread before it's done or else just make it stop firing events.
Where is this automatic unsubscribing documented?
In the Visual Basic Language Specification, a document you can download from Microsoft. Chapter 9.6.2 "WithEvents Variables" says this:
The implicit property created by a WithEvents declaration takes care of hooking and unhooking the relevant event handlers. When a value is assigned to the variable, the property first calls the remove method for the event on the instance currently in the variable (unhooking the existing event handler, if any). Next the assignment is made, and the property calls the add method for the event on the new instance in the variable (hooking up the new event handler).
The bolded phrase describes the behavior you see. It is rather important it works that way. If it didn't then you could never unsubscribe from an event and the event subscriptions would pile on without limit.
Related
I have a form used to display options about processes.
When options are applyed :
frmOptions
For Each ltvi As ListViewItem In ltvProcesses.CheckedItems
Dim proc As Process = CType(ltvi.Tag, Process)
targeted_processes.Add(proc)
AddHandler proc.Exited, AddressOf frmAET.a_target_process_has_been_exited
proc.EnableRaisingEvents = True
Next
And in a tools module :
Public Sub a_target_process_has_been_exited(sender As Object, e As EventArgs)
frmAET.btnStatus.ForeColor = Color.Red
msgbox("OK")
End Sub
And... the messagebox displays its message but the color doesn't change.
After some tries, the problem is when a_target_process_has_been_exited is actived by the handler.
If I do this (Button1 belongs to frmAET, like btnStatus) :
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
a_target_process_has_been_exited()
End Sub
It works ! But not when I really want (when a process is ended).
So, the problem is when the sub is called by the process end event.
And when I try to specify this (maybe a frmAET's sub can modify its controls) :
AddHandler leproc.Exited, AddressOf frmAET.a_target_process_has_been_exited
Error : Reference to a non-shared member requires an objet reference
Could you help me ?
Your AddHandler seems to use AddressOf frmAET.a_target_process_has_been_exited, that means method in frmAET form itself. Not tools module as you stated.
Let's consider your frmOptions is correct and frmAET is containing this (with removed explicit reference to frmAET, since it's local)
Public Sub a_target_process_has_been_exited(sender As Object, e As EventArgs)
btnStatus.ForeColor = Color.Red
MsgBox("OK")
End Sub
As comments already explained, your event handler is called in another thread and you need to sync yourself to main UI thread. For example like this:
Public Sub a_target_process_has_been_exited(sender As Object, e As EventArgs)
Me.BeginInvoke(Sub() HandleProcessExit())
End Sub
Public Sub HandleProcessExit
btnStatus.ForeColor = Color.Red
MsgBox("OK")
End Sub
This version will block main UI thread until you click on the MsgBox button.
You should add some Try/Catch block. Exception in another threads are difficult to detect otherwise.
This code depends on implicit form instances that VB.NET creates for you. I expect your frmAET is actually My.Forms.frmAET instance to make this work.
I'm hooking an arcobjects map event to a vb.net form to listen for map selection changes. This all works fine but users are reporting this error occassionally when opening the form. I can't see any pattern to reproduce the error and it seems to be random.
"COM object that has been separated from its underlying RCW cannot be used"
It originates from the form Load() method where I am hooking the event.
Can anyone help me understand what I've done wrong? I'm unhooking the map selection event in the FormClosing() event which I think is the correct approach.
Public Class MyForm
Private _activeViewEvents As IActiveViewEvents_Event
Private Sub FormLoad(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
_activeViewEvents = TryCast(pMxDoc.ActiveView.FocusMap, IActiveViewEvents_Event)
AddHandler _activeViewEvents.SelectionChanged, AddressOf SelectionChanged
End Sub
Private Sub SelectionChanged
'do something when selection is changed
End Sub
Private Sub FormClosing(sender As Object, e As FormClosingEventArgs) Handles MyBase.FormClosing
RemoveHandler _activeViewEvents.SelectionChanged, AddressOf SelectionChanged
End Sub
End Class
The approach you are taking to creating and destroying your handlers are valid. You can receive a RCW COM Exception when the map document is changed while your form is open. Since you are using the FocusMap to create the handles, when the document is changed, so is the FocusMap, which means you need to re-create your handlers for the new map document.
Ok so I think i've resolved this via use of the ActiveViewChanged event. Instead of rehooking the event on each form load or new document event, I tried listening for when the ActiveViewChanged event was fired and rehooking the SelectionChanged event each time. Turns out this is fired more than once each time a new document is opened (not sure why). Anyway, problem seems to have gone. Here's some example code:
Public Class MyForm
Private _activeViewEvents As IActiveViewEvents_Event
Private _docEvents As IDocumentEvents_Event
Private Sub FormLoad(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
AddHandler _docEvents.ActiveViewChanged, AddressOf ActiveViewChanged
End Sub
Private Sub ActiveViewChanged()
Dim maps = pMxDoc.Maps
For i = 0 to maps.Count - 1 'remove handlers from all maps
RemoveActiveViewEvents(maps.Item(i))
Next
SetupActiveViewEvent(pMxDoc.ActiveView.FocusMap) 'only add handler to active map
End Sub
Private Sub RemoveActiveViewEvents(map As IMap)
_activeViewEvents = CType(map, IActiveViewEvents_Event)
RemoveHandler _activeViewEvents.SelectionChanged, AddressOf SelectionChanged
End Sub
Private Sub SetupActiveViewEvents(map As IMap)
_activeViewEvents = CType(map, IActiveViewEvents_Event)
AddHandler _activeViewEvents.SelectionChanged, AddressOf SelectionChanged
End Sub
Private Sub SelectionChanged
'do something when selection is changed
End Sub
End Class
I have a form declared as s property WithEvents. If I add Handles formServers.FormClosing to a Sub declaration it works fine, but when I want to handle an event of a control within formServers I get the following error -
'Handles' in classes must specify a 'WithEvents' variable.
How do I correctly set this up? Thanks.
Private WithEvents formServers As New formServers
Private Sub txtServers_Closing(ByVal Sender As Object,
ByVal e As EventArgs) Handles formServers.txtServers.LostFocus
Me.SetServers()
If Me.ServersError Then
Dim Ex As New Exception("Error validating Servers.")
Dim ErrorForm = New formError(Ex, 101)
End If
End Sub
The error message is fairly misleading. The Handles keyword has several restrictions, it cannot work across different classes, it needs an object reference. You must use the more universal AddHandler keyword instead.
There are some additional problems in your scenario. Never use the LostFocus event, use Leave instead. And it is very important that you subscribe the event for the specific instance of the form, using As New gets you into trouble when you display the form multiple times, an ObjectDisposedException will be the outcome. Correct code looks like this:
Private formInstance As FormServers
Private Sub DisplayFormServer()
formInstance = new FormServers
AddHandler formInstance.txtServers.Leave, AddressOf txtServers_Closing
AddHandler formInstance.FormClosed, _
Sub()
formInstance = Nothing
End Sub
formInstance.Show()
End Sub
A much more elegant approach is to expose the event explicitly in your FormServers class. Make that look like this:
Public Class FormServers
Public Event ServersLeave As EventHandler
Private Sub txtServers_Leave(sender As Object, e As EventArgs) Handles txtServers.Leave
RaiseEvent ServersLeave(Me, EventArgs.Empty)
End Sub
End Class
The problem is that you do are not specifying WithEvents on the TextBox. Rather, you are specifying WithEvents on the Form. You can only use Handles on variables which you have declared directly with the WithEvents keyword. With the WithEvents being on the form, you will only be able to use Handles to handle events that are raised directly by the form itself. You will not be able to do so for events raised by any of its controls.
You can fix this in one of two ways. Either you can use AddHandler to register your event handler (rather than using the Handles keyword), or you can create a TextBox variable WithEvents and then set it to the appropriate TextBox object on the form, like this.
Private formInstance As New FormServers
Private WithEvents txtServers As TextBox
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
txtServers = formServers.txtServers
End Sub
Private Sub txtServers_LostFocus(Sender As Object, e As EventArgs) Handles txtServers.LostFocus
' ...
End Sub
The advantage of the latter approach, besides the more consistent, and possibly more elegant syntax, is that you don't have to remember to call RemoveHandler.
I have a form that has a start button (to allow users to run the processes over and over if they wish), and I want to send a btnStart.Click event when the form loads, so that the processes start automatically.
I have the following function for the btnStart.Click event, but how do I actually tell Visual Basic 'Pretend someone has clicked the button and fire this event'?
I've tried going very simple, which essentially works. However, Visual Studio gives me a warning Variable 'sender' is used before it has been assigned a value, so I'm guessing this is not really the way to do it:
Dim sender As Object
btnStart_Click(sender, New EventArgs())
I have also tried using RaiseEvent btnStart.Click, but that gives the following error:
'btnStart' is not an event of 'MyProject.MyFormClass
Code
Imports System.ComponentModel
Partial Public Class frmProgress
Private bw As BackgroundWorker = New BackgroundWorker
Public Sub New()
InitializeComponent()
' Set up the BackgroundWorker
bw.WorkerReportsProgress = True
bw.WorkerSupportsCancellation = True
AddHandler bw.DoWork, AddressOf bw_DoWork
AddHandler bw.ProgressChanged, AddressOf bw_ProgressChanged
AddHandler bw.RunWorkerCompleted, AddressOf bw_RunWorkerCompleted
' Fire the 'btnStart.click' event when the form loads
Dim sender As Object
btnStart_Click(sender, New EventArgs())
End Sub
Private Sub btnStart_Click(sender As Object, e As EventArgs) Handles btnStart.Click
If Not bw.IsBusy = True Then
' Enable the 'More >>' button on the form, as there will now be details for users to view
Me.btnMore.Enabled = True
' Update the form control settings so that they correctly formatted when the processing starts
set_form_on_start()
bw.RunWorkerAsync()
End If
End Sub
' Other functions exist here
End Class
You should send a button as sender into the event handler:
btnStart_Click(btnStart, New EventArgs())
Steps in involved in raising an event is as follows,
Public Event ForceManualStep As EventHandler
RaiseEvent ForceManualStep(Me, EventArgs.Empty)
AddHandler ForceManualStep, AddressOf ManualStepCompletion
Private Sub ManualStepCompletion(sender As Object, e As EventArgs)
End Sub
So in your case, it should be as below,
btnStart_Click(btnStart, EventArgs.Empty)
Just Call
btnStart.PerformClick()
You are trying to implement a bad idea. Actually, you have to make a subroutine to accomplish these kind of tasks.
Private Sub btnStart_Click(sender As Object, e As EventArgs) Handles btnStart.Click
call SeparateSubroutine()
End Sub
private sub SeparateSubroutine()
'Your code here.
End Sub
And then whereever you want to call the btnStart's click event, just call that SeparateSubroutine. This should be a correct way in your case.
You can subclass the button and make its OnClick Method public as I described here.
I've searched for the difference about the use of the keyword Handles instead of AddHandler, in VB.NET, but I'm unable to explain why this code doesn't work..
Imports System.Threading
Public Class MyClass_EventArgs
Inherits System.EventArgs
End Class
Public Class MyClass
Public Event MainThreadFinished(ByVal sender As Object, ByVal e As MyClass_EventArgs)
Private WithEvents MyEvents As MyClass
Private trd As Thread
Public Sub New()
'AddHandler MainThreadFinished, AddressOf Me.MyEvents_ThreadFinished
trd = New Thread(AddressOf mainThread)
trd.IsBackground = True
trd.Start()
RaiseEvent MainThreadFinished(Me, Nothing)
End Sub
Protected Overrides Sub Finalize()
trd.Abort()
End Sub
Protected Sub MyEvents_ThreadFinished(ByVal sender As Object, ByVal e As MyClass_EventArgs) _
Handles MyEvents.MainThreadFinished
MessageBox.Show("AAA")
End Sub
Private Sub mainThread()
RaiseEvent MainThreadFinished(Me, Nothing)
End Sub
End Class
Well, this code never respond to the events, but if I uncomment the followin line, the code works and the messagebox appear...
'AddHandler MainThreadFinished, AddressOf Me.MyEvents_ThreadFinished
Why does this happen?
It looks like you've made a fine discovery! Per Microsoft documentation, RaiseEvent Statement,
Non-shared events should not be raised within the constructor of the
class in which they are declared. Although such events do not cause
run-time errors, they may fail to be caught by associated event
handlers. Use the Shared modifier to create a shared event if you need
to raise an event from a constructor.
In other words, Microsoft says you shouldn't be doing what you're doing - and if you must, to use shared events.
In looking through other sources, I would say that the difference between AddHandler and Handles is a matter of syntactic sugar. You may want to look into how events are done in C# for more insight (such as in C# Events). Handles is used in conjunction with WithEvents as a way for an instance of a class to automatically subscribe to events (which is otherwise explicitly done with += in C# and with AddHander in VB.NET).
It would seem that your explicit AddHandler ensures that the event hookups are in place before the RaiseEvent, and so then it works as you wanted. I can only guess that without that, those event hookups weren't yet done - that is, it didn't work because of however the compiler inserts the code that performs the equivalent of AddHandler behind the scenes, by whatever design pattern the compiler writers deemed as appropriate. It would seem that the designers were well aware of this possible consequence, given their warning about this.