The objective in this question is to create a code block that executes when a datatable has changed content (just the fact that it has changed, not the specific row/column etc). This is VB Winforms NET.
This works:
Public Class Form1
Sub testhandler()
Dim locallyDeclaredDT as new datatable
AddHandler locallyDeclaredDT.ColumnChanged, New DataColumnChangeEventHandler(AddressOf Column_Changed)
End Sub
Private Shared Sub Column_Changed(sender As Object, e As DataColumnChangeEventArgs)
MessageBox.Show("The table has changed")
End Sub
End Class
But I need a datatable that is accessible outside of the sub's scope so I try this, which doesn't work.
Public Class Form1
Dim globallyDeclaredDT as new datatable
Sub testhandler()
AddHandler globallyDeclaredDT.ColumnChanged, New DataColumnChangeEventHandler(AddressOf Column_Changed)
End Sub
Private Shared Sub Column_Changed(sender As Object, e As DataColumnChangeEventArgs)
MessageBox.Show("The table has changed")
End Sub
End Class
The problem is caused by the AddHandler line: the ColumnChanged property is underlined and the error is
'ColumnChanged' is not an event of 'Object'.
How can I fix this?
Related
I'm currently trying to implement the second response from this thread How can I handle ComboBox selected index changing? in vb (the response that suggests subclassing ComboBox to introduce new SelectedIndexChangingEvent). The event handler
Private Sub MyComboBox1_SelectedIndexChanging(sender As Object, e As System.ComponentModel.CancelEventArgs) Handles MyComboBox1.SelectedIndexChanging
MsgBox("Changing")
End Sub
never gets hit. I'm thinking it has something to do with the way I'm initializing the selectedIndexChanging (lowercase first letter) variable. Any thoughts?
Imports System.ComponentModel
Public Class MyComboBox
Inherits ComboBox
Public Event SelectedIndexChanging as CancelEventHandler
Public LastAcceptedSelectedIndex As Integer
Public Sub New()
LastAcceptedSelectedIndex = -1
End Sub
Protected Sub OnSelectedIndexChanging(e As CancelEventArgs)
Dim selectedIndexChanging as CancelEventHandler = SelectedIndexChanging
If Not SelectedIndexChanging Is Nothing Then
selectedIndexChanging(Me, e)
End If
End Sub
Protected Overrides Sub OnSelectedIndexChanged(e As EventArgs)
If LastAcceptedSelectedIndex <> SelectedIndex Then
Dim cancelEventArgs = New CancelEventArgs
OnSelectedIndexChanging(cancelEventArgs)
If Not cancelEventArgs.Cancel Then
LastAcceptedSelectedIndex = SelectedIndex
MyBase.OnSelectedIndexChanged(e)
Else
SelectedIndex = LastAcceptedSelectedIndex
End If
End If
End Sub
End Class
VB handles event declaration a bit different than C#. The VB RaiseEvent keyword effectively generates code you attempted to translate for the `OnSelectedIndexChanging' method.
The correct VB implementation would be:
Protected Sub OnSelectedIndexChanging(e As CancelEventArgs)
RaiseEvent SelectedIndexChanging(Me, e)
End Sub
You could follow the original pattern, by using the hidden variable VB creates that is the real CancelEventHandler variable. These hidden variables follow the naming pattern of eventNameEvent. So the real CancelEventHandler variable is named: SelectedIndexChangingEvent.
Protected Sub OnSelectedIndexChanging(e As CancelEventArgs)
Dim selectedIndexChanging As CancelEventHandler = Me.SelectedIndexChangingEvent
If Not selectedIndexChanging Is Nothing Then
selectedIndexChanging(Me, e)
End If
End Sub
I want some values in a class to decrease whenever the timer in the main form ticks. I am creating multiple instances of the same class as my program is a simulation application and I am not storing these instances in an array or any list in that matter. I simply declare them and add their picture box to the controls on the main form. However I am hoping to have a sub routine inside the class that triggers whenever the timer in the main form ticks. I thought of something like this:
Public Class Jimmy
Dim _a As Integer = 10
Sub decreseNum(sender As Object, e As EventArgs) Handles mainapp.tmrLog.Tick
_a -= 1
End Sub
End Class
with mainapp being the name of the main form and tmrLog being the timer I want to associate my sub routine with. However the above code doesn't work
You could try defining a local reference to the timer in the Jimmy class:
Public Class Jimmy
Dim _a As Integer = 10
Private WithEvents tmr As Timer
Public Sub New(ByRef MainTmr As Timer)
tmr = MainTmr
End Sub
Sub decreseNum(sender As Object, e As EventArgs) Handles tmr.Tick
_a -= 1
End Sub
End Class
If you want all your classes react to timer.elapsed event, just sign up for it. The program below is fully operational. It is example what you can do to have your children to react to timer events of single parent/timer
Imports System
imports system.timers
Public Module Module1
Public Sub Main()
dim mc as new MainClass()
mc.CreateChildren(5)
System.Threading.Thread.Sleep(60000) ' wait and monitor output of childern
mc.Stop()
Console.WriteLine("All should stop now...")
Console.Read()
End Sub
End Module
public class MainClass 'This class could be your form
private _timer as new Timer(5000)
public sub CreateChildren(count as integer)
For i as integer = 1 to count
dim c as new Child(i)
Addhandler _timer.Elapsed, addressof c.DoWhentimerTicks
next
Console.WriteLine("timer should run now...")
_timer.Start()
end sub
public sub [Stop]()
_timer.Stop()
End Sub
End class
public class Child
private _myNO as integer
public sub new (no as integer)
_myNo = no
end sub
public sub DoWhentimerTicks(sender as object , e as ElapsedEventArgs)
Console.WriteLine(string.format("Child #{0} just ticked. Time = {1}", _myNo, e.signaltime))
end sub
End class
I found my solution, posting here for further reference.
My situation was trying to have my timer in the mainform triggering a sub in a class, and I used the following solution.
Class:
Sub addHandlesToSub
AddHandler Form1.Timer1.Tick, AddressOf subToBeTriggered
End Sub
Sub subToBeTriggered(sender As Object, e As EventArgs)
'My code
End Sub
The parameters in subToBeTriggered are useful when you want to remove the handler with
RemoveHandler Form1.Timer1.Tick, AddressOf subToBeTriggered
Otherwise, there will be an error without the parameters.
Thanks for all the answers though.
I have a listbox on my main vb.net form which I am using to display status messages from the server program I am running. My actual program consists of many different classes (in separate files) and what I would like to be able to do is to call the Sub frm.UpdateList("With Info in Here") from each of the classes to write to the listbox.
If I call the frm.UpdateList or UpdateList from the frm class, it writes to the listbox fine, but if I call it from any other class nothing happens (I don't get an error either).
I have tried with and without making it shared (and changing frm to me) but neither works as I would hope.
Would anyone be able to help me understand why this is not working, I have invoked the item, and it does get added to but just not from a separate class (which is what I need it to do).
Many Thanks!
Private Delegate Sub UpdateListDelegate(ByVal itemName As String)
Public Shared Sub UpdateList(ByVal itemName As String)
If frm.InvokeRequired Then
frm.Invoke(New UpdateListDelegate(AddressOf UpdateList), itemName)
Else
frm.ListBox1.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)
End If
End Sub
Edit: Try 2, with the following thanks to Idle_Mind works on the frm class (frm is the main form and only form) but it still does not write to the listbox when called from other classes (and no errors occur):
Public Shared Sub UpdateList(ByVal itemName As String)
Dim frm As Form = My.Application.ApplicationContext.MainForm
If Not IsNothing(frm) Then
Dim matches() As Control = frm.Controls.Find("ListBox1", True)
If matches.Length > 0 AndAlso TypeOf matches(0) Is ListBox Then
Dim LB As ListBox = DirectCast(matches(0), ListBox)
LB.Invoke(New MethodInvoker(Sub() LB.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)))
End If
End If
End Sub
I have a listbox on my main vb.net form
This will only work on the startup form, and is not really a good design. Consider other approaches as well:
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim soc As New SomeOtherClass
soc.Foo()
End Sub
End Class
Public Class SomeOtherClass
Public Sub Foo()
Dim msg As String = "Hello?!"
Helper.UpdateList(msg) ' <-- You can do this from any class...
End Sub
End Class
Public Class Helper
Public Shared Sub UpdateList(ByVal itemName As String)
Dim frm As Form = My.Application.ApplicationContext.MainForm
If Not IsNothing(frm) Then
Dim matches() As Control = frm.Controls.Find("ListBox1", True)
If matches.Length > 0 AndAlso TypeOf matches(0) Is ListBox Then
Dim LB As ListBox = DirectCast(matches(0), ListBox)
LB.Invoke(New MethodInvoker(Sub() LB.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)))
End If
End If
End Sub
End Class
Other correct approaches, which would require more work on your part, might include:
(1) Pass a reference to your main form into the other classes as you create them. Then those classes can either up the ListBox directly, or possibly call a method in it as suggested by Plutonix. Here's an example of this in action:
Public Class Form1
Public Sub UpdateList(ByVal itemName As String)
ListBox1.Invoke(New MethodInvoker(Sub() ListBox1.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)))
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim soc As New SomeOtherClass(Me)
soc.Foo()
End Sub
End Class
Public Class SomeOtherClass
Private _Main As Form1
Private Sub New()
End Sub
Public Sub New(ByVal MainForm As Form1)
_Main = MainForm
End Sub
Public Sub Foo()
If Not IsNothing(_Main) Then
_Main.UpdateList("Hello?!")
End If
End Sub
End Class
You'd have to modify all of your other classes in a similar fashion so that they can receive an instance of your form.
(2) Make the other classes raise a custom event that the main form subscribes to when those classes are created.
I am having an issue when trying to delete ListView Items from a second form.
For example, if I use the following command on Form1 it works:
Listview1.SelectedItems(0).Remove
However, if I attempt to remove from Form2 like so:
Form1.Listview1.SelectedItems(0).Remove
I get the following error:
"Invalid argument=value of '0' is not valid for 'index'. Parameter name: index"
I then tried to get a count of items from the listview on Form2 and it gives me a return of 0
Form1.Listview1.Items.Count
I'm not sure what my problem is.
Update
I have posted a brief example of my code (using your suggestion as I can understand it):
frmShowMessages
Private Sub ViewMessage()
Dim frm As New frmViewMailMessage
frm.Show()
End Sub
Public Sub DeleteItem(ByVal index As Integer)
lsvReceivedMessages.Items(index).Remove()
End Sub
frmViewMessage
Private instanceForm as frmShowMessages
Private Sub frmViewMailMessage_Load(sender As Object, e As EventArgs) Handles MyBase.Load
instanceForm = New frmShowMessages()
End Sub
Private Sub cmdDelete_Click(sender As Object, e As EventArgs) Handles cmdDelete.Click
instanceForm.DeleteItem(_index)
End Sub
Hopefully my code can help identify where my issue is.
In VB.net usually you get a default Form instance for each of your Form. Probably you are creating an instance of Form1 and then you are trying to access ListView1 of default instance.
E.g.
Sub ButtonClick()
Dim f As New Form1()
f.Show()
' at this point if you access f's ListView you will get correct count
f.ListView1.Items.Count
' however if you try to access default instance it will NOT have any item
Form1.ListView.Items.Count
End Sub
It means your instance f is NOT equal to default Form1 instance.
Solution can be, make the f variable as class level variable and use it everywhere. Or if Form1 will have only 1 instance, then you can use the default instance everywhere.
Personally I would NOT go with direct control accessing over forms. I would create a Public method which should return the data as list to the caller, in this case your Form2.
UPDATED-2:
As per your given scenario, I am simplifying things for you, and doing implementation using Event.
Public Class frmShowMessages
Private Sub btnOpenMessage_Click(sender As System.Object, e As System.EventArgs) Handles btnOpenMessage.Click
Dim frmView As New frmViewMessage(Me.ListView1.SelectedItems(0).Index)
AddHandler frmView.MessageDeleted, AddressOf DeleteMessageHandler
frmView.Show()
End Sub
Private Sub DeleteMessageHandler(sender As Object, e As frmViewMessage.MessageDeletedEventArgs)
Me.ListView1.Items.RemoveAt(e.MessageIndex)
End Sub
End Class
Public Class frmViewMessage
' a class which will be used for Event communication
Public Class MessageDeletedEventArgs
Inherits EventArgs
Public Property MessageIndex As Integer
Public Sub New(ByVal iIndex As Integer)
MyBase.New()
Me.MessageIndex = iIndex
End Sub
End Class
' main event which will alert the parent that a message deletion should be done
Public Event MessageDeleted As EventHandler(Of MessageDeletedEventArgs)
' private variable that will hold the MessageIndex
Private Property MessageIndex As Integer
' method that is responsible to raise event
Protected Overridable Sub OnMessageDeleted()
RaiseEvent MessageDeleted(Me, New MessageDeletedEventArgs(Me.MessageIndex))
End Sub
' we want to create this Form using the MessageIndex of ListView
Public Sub New(ByVal iMessageIndex As Integer)
Me.InitializeComponent()
Me.MessageIndex = iMessageIndex
End Sub
' the delete button will raise the event to indicate parent that
' a deletion of message should be done
Private Sub btnDelete_Click(sender As System.Object, e As System.EventArgs) Handles btnDelete.Click
Me.OnMessageDeleted()
End Sub
End Class
Please have a look at the code below:
Public Delegate Sub TestButtonClick(ByVal test As Integer)
Public Class Person
Private Name As String
Private ID As Integer
Public Event ButtonClick As TestButtonClick
Public Sub DelegateTest1(ByVal Test As Integer)
MsgBox(Test)
End Sub
Public Sub ChangeName()
RaiseEvent ButtonClick(1)
End Sub
Public Sub DelegateTest2()
MsgBox("Delegate Test 2")
End Sub
Public Sub DelegateTest3()
MsgBox("Delegate Test 3")
End Sub
End Class
Public Class Form1
Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Dim p1 As Person = New Person
AddHandler p1.ButtonClick, AddressOf p1.DelegateTest1
AddHandler p1.ButtonClick, AddressOf p1.DelegateTest2
AddHandler p1.ButtonClick, AddressOf p1.DelegateTest3
p1.ChangeName()
End Sub
End Class
The output is:
1
DelegateTest2
DelegateTest3
I do not understand why this application compiles i.e. the delegate accepts an integer in its signature but Person.DelegateTest2 and Person.DelegateTest3 do not.
If I change Person.DelegateTest2() to the following then I do get an error as I would expect:
Public Sub DelegateTest2(ByVal Test As Integer, ByVal Test2 As Integer)
MsgBox("Delegate Test 2")
End Sub
Why does the Delegate allow you to pass zero arguments when it has arguments i.e. an integer in my case?
Don't forget that VB.NET inherits all the legacy baggage from the beloved VB. You could make it strict by putting the following to the top of your file so that it behaves as a real .NET programming language and not some hybrid crap:
Option Strict On
Also I would recommend you setting this to be the default option so that you don't find yourself in the wilderness.