In VB.NET, what is the syntax to start a new thread using a parameterless Action?
"what is the syntax to start a new thread using a parameterless Action"
Here are two examples:
Example 1:
Private Sub MyAction()
[... Code goes here]
End Sub
Public Sub Test()
Dim t As New Thread(AddressOf Me.MyAction)
t.Start()
End Sub
Example 2:
Dim t As New Thread(Sub() [... Code goes here])
t.Start()
Kind of years too late but...use ThreadStart
Private _calculateThread As Thread
Private Sub CreateAndStartThread(ByVal threadDelegate As ThreadStart)
_calculateThread = New Thread(threadDelegate)
_calculateThread.Start()
End Sub
and somewhere else in your code
CreateAndStartThread(New ThreadStart(AddressOf Calculate))
Private Sub Calculate()
'... do your thread actions
end Sub
Related
Public Class Class1
Dim thread As New System.Threading.Thread(AddressOf AMethod)
thread.Start()
Public Sub AMethod()
Console.writeline("Thread start")
End Sub
End Class
The "thread.Start()" is the problematic line, according to vb.
Declaring the thread in a sub gives an overload error, whatever that means.
If you didn't understand what #Plutonix meant by "executable code floating around" (I love that phrase) here is an example:
Public Class Class1
Dim thread As New System.Threading.Thread(AddressOf AMethod)
Public Sub StartingThread()
thread.Start()
End Sub
Public Sub AMethod()
Console.WriteLine("Thread start")
End Sub
End Class
See, the executable code thread.Start() is now inside a method. I, myself, am steering clear of threading until I know much more about it.
In order to keep responsiveness in the UI, I use a separate thread to execute various process, for example some FTP download.
Private Sub Button11_Click(sender As Object, e As EventArgs) Handles Button11.Click
Dim ThreadResync As System.Threading.Thread
ThreadResync = New System.Threading.Thread(AddressOf Bodacc_ResyncFTP)
ThreadResync.Start()
End Sub
Sub Bodacc_ResyncFTP()
Dim MyBodacc As bodacc_data = New bodacc_data
MyBodacc.Label_Status = Form1.Label1
MyBodacc.ResyncFTP()
End Sub
A way to update the UI with threading is the Delegate thingy, so in the bodacc_data I had to
Public Class bodacc_data
......
Delegate Sub UpdateLabelDelg(text As String, ThisLabel As Label)
Public Delegate_label As UpdateLabelDelg = New UpdateLabelDelg(AddressOf set_label)
Public Label_Status = Label
......
Sub set_label(stext As String, ThisLabel As Label)
ThisLabel.Text = stext
End Sub
.....
Sub ResyncFTP()
//says hello
If Label_Status.InvokeRequired = True Then
Label_Status.Invoke(Delegate_label, New Object() {"Working...", Label_Status})
Else
Label_Status.Text = "Working..."
End If
//do stuff
End Sub
End Class
It works like a charm. But I have many class doing more or less the same (disk update, database update, FTP update) and having to copy/past all the delegate / external label declaration / mini sub / invoke sound silly.
So I created a class to handle those UI update / delegate in order to have a quick access
Public Class Form_UI
Delegate Sub UpdateLabelDelg(text As String, ThisLabel As Label)
Public Delegate_label As UpdateLabelDelg = New UpdateLabelDelg(AddressOf set_label)
Private Labels(2) As Label
Sub New()
Labels(0) = Form1.Label1
Labels(1) = Form1.Label2
Labels(2) = Form1.Label3
End Sub
Sub set_label(stext As String, ThisLabel As Label)
ThisLabel.Text = stext
End Sub
Public Sub ChangeLabel(ByVal LabelNum As Integer, nText As String)
LabelNum = LabelNum - 1
If Labels(LabelNum).InvokeRequired Then
Labels(LabelNum).Invoke(Delegate_label, New Object() {nText, Labels(LabelNum)})
Else
Labels(LabelNum).Text = nText
Labels(LabelNum).Update()
End If
End Sub
End Class
So, now in the revamped bodacc_data and all others processing class I have only :
Public Class bodacc_data
......
Private MyUI as Form_UI
.....
Sub New()
MyUI = New Form_UI()
End Sub
Sub ResyncFTP()
//says hello
MyUI.ChangeLabel(1, "Working...")
//do stuff
End Sub
End Class
Question Why is MyUI.ChangeLabel not updating when the ResyncFTP is called in a thread, but works if called in the main thread (As in the code sample below)
Private Sub Button11_Click(sender As Object, e As EventArgs) Handles Button11.Click
Dim MyBodacc As bodacc_data = New bodacc_data
MyBodacc.ResyncFTP()
End Sub
Note that there is no error thrown. The notable weirdness is that <Form_UI>.ChangeLabel() never goes the .Invoke route but the normal update route. I strongly suspect a scope issue or insight issue.
When you create a windows forms app you set up a UI thread that is meant to be the owner of all the UI. The UI thread contains the message pump that is used to update all of the UI.
But what you're doing in Button11_Click is creating a new thread that goes and calls Dim MyBodacc As bodacc_data = New bodacc_data which, in turn, calls MyUI = New Form_UI().
You are creating a form on a non-UI thread. There is no message pump and therefore the UI doesn't update.
I'm looking at a console app for vb.net. I'm trying to get a worker thread to raise an event to the main thread to display data on the screen (the word "HIT" everytime the worker thread completes a cycle). My code is below.
I'm not sure why but the main thread's Private Sub CounterClass_GivingUpdate() Handles _counter.AboutToDistributeNewupdate isn't executing.
Imports System.Threading
Module Module1
Private WithEvents _counter As CounterClass
Private trd As Thread
Sub Main()
While True
Dim s As String = Console.ReadLine()
Dim started As Boolean
Select Case s
Case "status"
WriteStatusToConsole("You typed status")
Case "startcounter"
If started = False Then
starttheThread()
started = True
WriteStatusToConsole("You Have Started The Timer")
Else
WriteStatusToConsole("YOU HAVE ALREADY STARTED THE TIMER!!!")
End If
End Select
End While
End Sub
Private Sub CounterClass_GivingUpdate() Handles _counter.AboutToDistributeNewupdate
WriteStatusToConsole("Hit")
End Sub
Private Sub starttheThread()
Dim c As New CounterClass
trd = New Thread(AddressOf c.startProcess)
trd.Start()
End Sub
Sub WriteStatusToConsole(ByVal stringToDisplay As String)
Console.WriteLine(stringToDisplay)
End Sub
End Module
Public Class CounterClass
Public Event AboutToDistributeNewupdate()
Public Sub sendStatusUpdateEvent(ByVal updatestatus As String)
RaiseEvent AboutToDistributeNewupdate()
End Sub
Public Sub startProcess()
Dim i As Int64
Do
Thread.Sleep(1000)
i = i + 1
sendStatusUpdateEvent(i.ToString)
Loop
End Sub
End Class
Your CounterClass_GivingUpdate() only handles the _counter variable's event (the variable that you do not use!). Every time you declare a new CounterClass it has its own instance of the event that it raises.
You know have two options:
Option 1
Subscribe to the event for each new CounterClass instance you create. Meaning you must use the AddHandler statement every time you create a new instance of your class:
Private Sub starttheThread()
Dim c As New CounterClass
AddHandler c.AboutToDistributeNewupdate, AddressOf CounterClass_GivingUpdate
trd = New Thread(AddressOf c.startProcess)
trd.Start()
End Sub
Option 2
Mark the event as Shared to make it available without needing to create an instance of the class. For this you must also change how you subscribe to the event, by subscribing to it in your method Main():
Sub Main()
AddHandler CounterClass.AboutToDistributeNewupdate, AddressOf CounterClass_GivingUpdate
...the rest of your code...
End Sub
Private Sub CounterClass_GivingUpdate() 'No "Handles"-statement here.
WriteStatusToConsole("Hit")
End Sub
Public Class CounterClass
Public Shared Event AboutToDistributeNewupdate() 'Added the "Shared" keyword.
...the rest of your code...
End Class
I am trying to create a new thread using an anonymous function but I keep getting errors. Here is my code:
New Thread(Function()
'Do something here
End Function).Start
Here are the errors I get:
New:
Syntax Error
End Function:
'End Function' must be preceded by a matching 'Function'.
There's two ways to do this;
With the AddressOf operator to an existing method
Sub MyBackgroundThread()
Console.WriteLine("Hullo")
End Sub
And then create and start the thread with;
Dim thread As New Thread(AddressOf MyBackgroundThread)
thread.Start()
Or as a lambda function.
Dim thread as New Thread(
Sub()
Console.WriteLine("Hullo")
End Sub
)
thread.Start()
It is called a lambda expression in VB. The syntax is all wrong, you need to actually declare a variable of type Thread to use the New operator. And the lambda you create must be a valid substitute for the argument you pass to the Thread class constructor. None of which take a delegate that return a value so you must use Sub, not Function. A random example:
Imports System.Threading
Module Module1
Sub Main()
Dim t As New Thread(Sub()
Console.WriteLine("hello thread")
End Sub)
t.Start()
t.Join()
Console.ReadLine()
End Sub
End Module
What is called has to be a functinon not a sub.
Single line(has to return value):
Dim worker As New Thread(New ThreadStart(Function() 42))
Multiline:
Dim worker As New Thread(New ThreadStart(Function()
' Do something here
End Function))
Source: Threading, Closures, and Lambda Expressions in VB.Net
I need to raise an event from a form on a new thread.
(I don't believe the reason for this is relevant, but just in case: I'll be raising events from code within a form's WndProc sub. If the code handling the event blocks with something on a form [such as a msgbox], then all sorts of trouble occurs with disconnected contexts and what not. I've confirmed that raising events on new threads fixing the problem.)
This is what I am currently doing:
Public Event MyEvent()
Public Sub RaiseMyEvent()
RaiseEvent MyEvent
End Sub
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
Dim t As New Threading.Thread(AddressOf RaiseMyEvent)
t.Start()
End Sub
Is there a better way?
It is my understanding that events in VB are actually made up of delegates in the background. Is there any way to raise events in new threads without creating subs for each? Or, is there a more appropriate method I should be using?
You can eliminate the RaiseMyEvent sub like this:
Public Class Class1
Public Event MyEvent()
Sub Demo()
Dim t As New Threading.Thread(Sub() RaiseEvent MyEvent())
t.Start()
End Sub
End Class
Don't know if this will help, but i'll always do threading and events like this:
Event MyEvent(ByVal Var1 As String, ByVal Var2 As String)
Private Delegate Sub del_MyEvent(ByVal Var1 As String, ByVal Var2 As String)
Private Sub StartNewThread()
'MAIN UI THREAD
Dim sVar1 As String = "Test"
Dim sVar2 As String = "Second Var"
Dim oThread As New Threading.Thread(New Threading.ParameterizedThreadStart(AddressOf StartNewThread_Threaded))
With oThread
.IsBackground = True
.Priority = Threading.ThreadPriority.BelowNormal
.Name = "StartNewThread_Threaded"
.Start(New Object() {sVar1, sVar2})
End With
End Sub
Private Sub StartNewThread_Threaded(ByVal o As Object)
'CHILD THREAD
Dim sVar1 As String = o(0)
Dim sVar2 As String = o(1)
'Do threaded operation
Threading.Thread.Sleep(1000)
'Raise event
RaiseEvent_MyEvent(sVar1, sVar2)
End Sub
Public Sub RaiseEvent_MyEvent(ByVal Var1 As String, ByVal Var2 As String)
If Me.InvokeRequired Then
'Makes the sub threadsafe (I.e. the event will only be raised in the UI Thread)
Dim oDel As New del_MyEvent(AddressOf RaiseEvent_MyEvent)
Me.Invoke(oDel, Var1, Var2)
Exit Sub
End If
'MAIN UI THREAD
RaiseEvent MyEvent(Var1, Var2)
End Sub