I used to write this:
Private Sub Example()
Static CachedPeople As List(Of MyApp.Person)
If CachedPeople Is Nothing Then
CachedPeople = New List(Of MyApp.Person)
End If
...rest of code...
End Sub
But then wondered if I could reduce this to:
Private Sub Example()
Static CachedPeople As New List(Of MyApp.Person)
...rest of code...
End Sub
The question is, will the "New" bit only be executed once when the function is first executed but in the next call, it will already exist.
Cheers, Rob.
It'll be executed only once and on next function call, it'll reference the same object, as you mentioned. Your first snippet is not thread-safe, by the way. If two threads call your function at the same time, they might end up running the constructor twice, which is not what you want. Using the second snippet relieves you from manually locking and ensuring thread safety, as the compiler generates the appropriate code for you.
Note that if you had declared it as
Static x As List(Of String)
x = New List(Of String)
It would have been recreated each time.
Related
What I've got is something like this:
Private Sub GiantLegacySub()
... lots of variables and legacy code...
Dim somethingNew = New Func(of String, Boolean)(
Function(stringy as String) As Boolean
... new code that uses the legacy variables ...
End Function)
Dim t = New Thread(AddressOf somethingNew)
End Sub
I am getting an error indicating that somethingNew is being seen as variable name and not a method name and is thus unacceptable by AddressOf. ( I know that somethingNew is a variable, just one that happens to contain a pointer to a method).
Is there a way to do this? I need to leave it inside of GiantLegacySub because of the shear volume of variables in its scope.
Based on Craig's guidance, this was the answer:
Private Sub GiantLegacySub()
... lots of variables and legacy code...
Dim somethingNew = Sub(stringy as String)
... new code that uses the legacy variables ...
End Sub
Dim t = New Thread(somethingNew)
t.Start(someStringForStringy)
End Sub
I have a problem that has been bugging me for a while now. Consider this code:
Public Class Class1
Dim VariableList as New List(of Object) From {MainForm.X, MainForm.Y,
SettingsForm.Z, SettingsForm.Textbox1.Text} '...etc.
Sub SetToZero()
For Each Element in VariableList
Element = 0
Next
End Sub
Sub SetToCustomValue(value As Double)
For Each Element in VariableList
Element = value
Next
End Sub
Sub LoadValuesFromFile()
Dim path As String = MainForm.GetPath()
For Each Element in VariableList
Element = File.Readline()
Next
End Sub
Sub SaveValuesToFile()
Dim path As String = MainForm.GetPath()
For Each Element in VariableList
Element = File.Writeline()
Next
End Sub
'and more similar functions/subs
As you can see, what this class does is that it takes lot of different variables from different places into a collection, and then various functions read or write values to every variable in that collection using loops. In this example, I have just a few variables, but most of the time there are dozens.
Reading the values is not a problem. Writing them, is, because when I declare that VariableList at the top of my class, that List just makes a copy of each variable, rather than maintaining a reference to it. Meaning that if, say, one of the functions modifies the MainForm.X in that List, the actual variable MainForm.X is not modified. To work with references, I would have to forgo loops, and assign every single variable manually, in every function. Which is obviously a lot of bad code. I want to declare that list of variables only once, and then use loops, like in this example code that I wrote above. My question is, how can I make such a container (List, Array, whatever) that would retain the references to the original variables in it, and make the code above possible?
There is no easy way to store pointers to variables in VB.NET. As a workaround, you can use a class to store your variables, as a class is always used as a pointer.
Here's an example of a way to achieve this with a ContainerClass which own a Dictionary of integers. One interest of this method would be that you can declare and name "variables" dynamically. In reality, they will be managed KeyValuePair. Once you have instantiated a copy of this class, you can use it to "manage" your variables by using this class as your pointer.
I included a loop which set all the integers to the same number just for fun, and to demonstrate the kind of manipulation which would end up having an effect similar to one of those described in your question.
Public Class Form2
'This is the container class which will be used to bypass the lack of pointers
'if you wanted to change a property, like the window width, it would be more difficult, but simples variables will be no trouble
Private variableContainer As New VariableContainer
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
variableContainer.AddVar("X", 5)
variableContainer.AddVar("Y", 15)
Debug.Print(variableContainer.GetVar("X"))
Debug.Print(variableContainer.GetVar("Y"))
variableContainer.SetAllVar(42)
Debug.Print("Next line will print 42")
Debug.Print(variableContainer.GetVar("X"))
End Sub
End Class
Public Class VariableContainer
'I know a public variable wouldn't need the fancy functions down there, but it's usually better to encapsulate, especially if you're working with a team
'and "future you" count as a teammate, never forget that...
Private list As New Dictionary(Of String, Integer)
Public Sub AddVar(ByVal name As String, ByVal value As Integer)
list.Add(name, value)
End Sub
Public Function GetVar(ByVal name As String) As Integer
If list.ContainsKey(name) Then
Return list(name)
Else
'I choose -1 arbitrarily, don't put too much thinking into this detail
Return -1
End If
End Function
Public Sub SetVar(ByVal name As String, ByVal num As Integer)
If list.ContainsKey(name) Then
list(name) = num
End If
End Sub
Public Sub SetAllVar(ByVal num As Integer)
Dim dict As New Dictionary(Of String, Integer)
For Each item As KeyValuePair(Of String, Integer) In list
dict.Add(item.Key, num)
Next
list = dict
End Sub
End Class
Have fun!
I am newbie here. I need your help. I am trying to develop a tool where I want to implement multi-threading in vb.net. I am stuck in a place which I could not resolve. Let me explain you.
I have created a custom class as below:
Public Class TestClass
'Few private Members of my class
Private a As String,b As String
Public Structure abc
Dim xx as object
Dim yy as object
Dim zz as object
End Structure
Public aa(10) As abc
'Now I needed to override the constructor as the initialization of
'instance of this class can be made in two different ways due to
'requirement
Public Sub New(ByVal xy As String,ByRef yz As Object)
'Some internal method to initialize the object
Me.a=xy
Me.b=xy
End Sub
'Another way to create the instance
Public Sub New(ByVal xy As String,ByVal yz As String)
'Some internal method to initialize the object
Me.a=xy
Me.b=yz
End Sub
'Now a public method which I want to call using threading
Public Sub TestSub()
'Do something with Me.a,Me.b and aa(some index)
End Sub
End Class
Now in a different module I am creating the instance of this class as below:
Dim X1 As New TestClass("Some String",<Some Object reference>)
Dim X2 As New TestClass("Some String","Some String")
Now I have declared a sub in the same module like below
Sub DoMyStuff(ByRef A1 As TestClass)
A1.TestSub()
End Sub
After all This I want to create a thread and run the sub "DoMyStuff" by Passing a reference of X1
To Do this I have Imported System Threading:
Imports System.Threading
Inside Module after initialization of X1 and X2 I have written:
Dim T1 as New Threading.Thread(AddressOf DoMyStuff)
T1.Start(X1)
Here I am getting error: Overload Resolution Failed because no 'New' can be called with this arguments: 'Public Sub New(Start As System.Threading.ParameterizedThreadStart, maxStackSize As Integer)': Method 'Public Sub DoMyStuff(ByRef A1 As TestClass)' does not have a signature compatible with delegate 'Delegate Sub ParameterizedThreadStart(obj As Object)'. 'Public Sub New(Start As System.Threading.ThreadStart, maxStackSize As Integer)': Method 'Public Sub DoMyStuff(ByRef A1 As TestClass)' does not have a signature compatible with delegate 'Delegate Sub ThreadStart()'.
If I am writing like this,
Dim T1 As Threading.Thread
T1 = (AddressOf DoMyStuff)
T1.Start(X1)
I am getting the error: 'AddressOf' expression can not be converted to 'System.Threading.Thread' because 'System.Threading.Thread' is not a delegate type.
Might be I am missing some thing, which I am unable to find. I am not so much good in Delegate type concept. Also this is my first project with implementation of Threading. Looked for answers in Google but unable to understand to sort out. Your help highly solicited.
Thanks
Edit:
I have checked your reference Michael Z. and its really helpful. First of all a big Thank you for that :)
However, I am unable to understand a thing in there. If you help me out there, I would be highly grateful. As per my understanding, I have changed my code as below:
Public Delegate Sub Test2(ByRef A1 as TestClass)
Public T1 as Test2 = New Test2(AddressOf DoMyStuff)
Public Sub DoMyStuff(ByRef A1 as TestClass)
A1.TestSub()
End Sub
Now I have Declared in my module as follows:
Dim T as Threading.Thread = New Threading.Thread(AddressOf DoStuff)
Here I am unable to understand how to write the DoStuff sub as in your example you were working with TextBox1 which is an object in UI, so you wrote:
Public Sub DoStuff()
If TextBox1.InvokeRequired Then
TextBox1.Invoke(T1,"Hello")
End If
End Sub
But here I need to work with an object which is custom made i.e. X1 or X2. Also I need to pass the reference of the object in the Thread so that the method TestSub can work with the created object either X1 or X2. Totally lost here :(
Please correct me if anywhere I am wrong to understand your reference, otherwise if correctly understood, can you Please help me out here. Appreciate your help in advance.
Thanks
Firstly, ensure you're calling this from a method:
Dim T1 as New Threading.Thread(AddressOf DoMyStuff)
T1.Start(X1)
Next remove the ByRef in the method parameter like so Sub DoMyStuff(A1 As TestClass)
Essentially make your second Module look like:
Module Module2
Dim X1 As New TestClass("Some String", "asd")
Dim X2 As New TestClass("Some String", "Some String")
Sub TestCall
Dim T1 as New Thread(AddressOf DoMyStuff)
T1.Start(X1)
End Sub
Private Sub DoMyStuff(A1 As TestClass)
A1.TestSub()
End Sub
End Module
And you can call it like so:
TestCall()
Try using a simple Call method;
Call New Threading.Thread(AddressOf DoMyStuff).Start(X1)
Looking at what you wanting, there seems no reason to assign this to a variable?
Also - remember, when working with multiple contructors, you need to ensure the [TYPES] you pass in are correct and expected (this could be why your current process isn't working).
See this link for a similar issue.
Can someone help me understand how synclock statements work? I know there are some other posts about synclock here but I was not able to follow the answers very well. I am trying to get a simple multithreading example up and running but am having trouble. At first I tried the following code.
Public Class Class1
Public list As List(Of Integer) = New List(Of Integer)
Public Sub addInt(ByVal i As Integer)
SyncLock list
list.Add(i)
End SyncLock
End Sub
Public Sub MainLoop()
While list.Count < 50
SyncLock list
For Each i As Integer In list
Debug.WriteLine(i.ToString())
Next
End SyncLock
End While
End Sub
End Class
I had a simple winform with two buttons. I used the first button to create a obj of Class1 and start the MainLoop method in a new thread. And I used the seconded button to call the addInt method. However the code might work for a press or two and then lock up. After reading the other questions from this form I realized that the lockobject for the synclock statement did not work as I initially thought, and was just an arbitrary reference object that should not ever be changed. I think the syncLock statement just forces the whole code block to execute before passing processor control to a new thread. So I tried this using another lock object but now it just locks up.
Public Class Class1
Private padLock As String = "PADLOCK"
Public list As List(Of Integer) = New List(Of Integer)
Public Sub addInt(ByVal i As Integer)
SyncLock padLock
list.Add(i)
End SyncLock
End Sub
Public Sub MainLoop()
While list.Count < 50
SyncLock padLock
For Each i As Integer In list
Debug.WriteLine(i.ToString())
Next
End SyncLock
End While
End Sub
End Class
Can someone explain why the above code does not work? In a similar post someone mentioned that the Interlocked class would be useful but I could not figure out what that is or how that is used. A brief "Interlocked Class For Dummies" explanation would also be appreciated.
Ok I think I figured out the problem. I don't think my thread was deadlocking I think it was just starving my Application.Run() Thread of resources. When I added a Thread.Sleep(500) call after the End SyncLock in the MainLoop method the program worked as expected.
I'm looking to pass two or more parameters to a thread in VB 2008.
The following method (modified) works fine without parameters, and my status bar gets updated very cool-y.
But I can't seem to make it work with one, two or more parameters.
This is the pseudo code of what I'm thinking should happen when the button is pressed:
Private Sub Btn_Click()
Dim evaluator As New Thread(AddressOf Me.testthread(goodList, 1))
evaluator.Start()
Exit Sub
This is the testthread method:
Private Sub testthread(ByRef goodList As List(Of OneItem), ByVal coolvalue As Integer)
StatusProgressBar.Maximum = 100000
While (coolvalue < 100000)
coolvalue = coolvalue + 1
StatusProgressBar.Value = coolvalue
lblPercent.Text = coolvalue & "%"
Me.StatusProgressBar.Refresh()
End While
End Sub
First of all: AddressOf just gets the delegate to a function - you cannot specify anything else (i.e. capture any variables).
Now, you can start up a thread in two possible ways.
Pass an Action in the constructor and just Start() the thread.
Pass a ParameterizedThreadStart and forward one extra object argument to the method pointed to when calling .Start(parameter)
I consider the latter option an anachronism from pre-generic, pre-lambda times - which have ended at the latest with VB10.
You could use that crude method and create a list or structure which you pass to your threading code as this single object parameter, but since we now have closures, you can just create the thread on an anonymous Sub that knows all necessary variables by itself (which is work performed for you by the compiler).
Soo ...
Dim Evaluator = New Thread(Sub() Me.TestThread(goodList, 1))
It's really just that ;)
Something like this (I'm not a VB programmer)
Public Class MyParameters
public Name As String
public Number As Integer
End Class
newThread as thread = new Thread( AddressOf DoWork)
Dim parameters As New MyParameters
parameters.Name = "Arne"
newThread.Start(parameters);
public shared sub DoWork(byval data as object)
{
dim parameters = CType(data, Parameters)
}
Dim evaluator As New Thread(Sub() Me.testthread(goodList, 1))
With evaluator
.IsBackground = True ' not necessary...
.Start()
End With
Well, the straightforward method is to create an appropriate class/structure which holds all your parameter values and pass that to the thread.
Another solution in VB10 is to use the fact that lambdas create a closure, which basically means the compiler doing the above automatically for you:
Dim evaluator As New Thread(Sub()
testthread(goodList, 1)
End Sub)
In addition to what Dario stated about the Delegates you could execute a delegate with several parameters:
Predefine your delegate:
Private Delegate Sub TestThreadDelegate(ByRef goodList As List(Of String), ByVal coolvalue As Integer)
Get a handle to the delegate, create parameters in an array, DynamicInvoke on the Delegate:
Dim tester As TestThreadDelegate = AddressOf Me.testthread
Dim params(1) As Object
params(0) = New List(Of String)
params(1) = 0
tester.DynamicInvoke(params)
Just create a class or structure that has two members, one List(Of OneItem) and the other Integer and send in an instance of that class.
Edit: Sorry, missed that you had problems with one parameter as well. Just look at Thread Constructor (ParameterizedThreadStart) and that page includes a simple sample.
Pass multiple parameter for VB.NET 3.5
Public Class MyWork
Public Structure thread_Data
Dim TCPIPAddr As String
Dim TCPIPPort As Integer
End Structure
Dim STthread_Data As thread_Data
STthread_Data.TCPIPAddr = "192.168.2.2"
STthread_Data.TCPIPPort = 80
Dim multiThread As Thread = New Thread(AddressOf testthread)
multiThread.SetApartmentState(ApartmentState.MTA)
multiThread.Start(STthread_Data)
Private Function testthread(ByVal STthread_Data As thread_Data)
Dim IPaddr as string = STthread_Data.TCPIPAddr
Dim IPport as integer = STthread_Data.TCPIPPort
'Your work'
End Function
End Class
I think this will help you...
Creating Threads and Passing Data at Start Time!
Imports System.Threading
' The ThreadWithState class contains the information needed for
' a task, and the method that executes the task.
Public Class ThreadWithState
' State information used in the task.
Private boilerplate As String
Private value As Integer
' The constructor obtains the state information.
Public Sub New(text As String, number As Integer)
boilerplate = text
value = number
End Sub
' The thread procedure performs the task, such as formatting
' and printing a document.
Public Sub ThreadProc()
Console.WriteLine(boilerplate, value)
End Sub
End Class
' Entry point for the example.
'
Public Class Example
Public Shared Sub Main()
' Supply the state information required by the task.
Dim tws As New ThreadWithState( _
"This report displays the number {0}.", 42)
' Create a thread to execute the task, and then
' start the thread.
Dim t As New Thread(New ThreadStart(AddressOf tws.ThreadProc))
t.Start()
Console.WriteLine("Main thread does some work, then waits.")
t.Join()
Console.WriteLine( _
"Independent task has completed main thread ends.")
End Sub
End Class
' The example displays the following output:
' Main thread does some work, then waits.
' This report displays the number 42.
' Independent task has completed; main thread ends.
With VB 14, you can do the following with Tuples:
Shared Sub _runner(data as (goodList As List(Of OneItem), coolvalue As Integer))
Console.WriteLine($"goodList: {data.goodList}")
Console.WriteLine($"coolvalue: {data.coolvalue}")
' do stuff...
End Sub
Dim thr As New Thread(AddressOf _runner)
thr.Start((myGoodList, cval))