I need to do this all the time and don’t have the best memory in the world. Today, I decided that I looked this up one too many times so here’s my solution to this multithreading problem:
The Problem:
You try to modify UI components created in one thread in another thread. In VB, you can only make UI changes on the same thread that created it so you get a nice “Cross thread operation not valid” exception.
Here’s the wrong code:
thread = New System.Threading.Thread(AddressOf DoStuff)
thread.Start()
Private Sub DoStuff()
'error occurs here'
Me.Text = "Stuff"
End Sub
The Solution:
If you’re super lazy or you’re very sure of what your code is going to do, you can simply disable the exception with:
Private Sub DoStuff()
Me.CheckForIllegalCrossThreadCalls = False
Me.Text = "Stuff"
End Sub
This, of course, doesn’t prevent your code from making a mess. To do it properly, you need a delegate:
thread = New System.Threading.Thread(AddressOf DoStuff)
thread.Start()
Private Delegate Sub DoStuffDelegate()
Private Sub DoStuff()
If Me.InvokeRequired Then
Me.Invoke(New DoStuffDelegate(AddressOf DoStuff))
Else
Me.Text = "Stuff"
End If
End Sub
Wildly simple right?
To understand what the code does, you need to understand what Invoke() does. It can be confusing to google Invoke() because Delegates and Controls have different implementations of Invoke(). In this case, we’re calling Invoke on a subclass of Control. That causes the method on the second thread which is going through DoStuff() to run a method on the first thread (which created the UI element).
Now if you need parameters, here’s another example where “ReallyLongProcess” is a subroutine that raises an event “Done” with a parameter “success”
AddHandler Me.Done, AddressOf WorkFinished
thread = new System.Threading.Thread(AddressOf ReallyLongProcess)
thread.Start()
Private Delegate Sub DoStuffDelegate(ByRef success as Boolean)
Private Sub DoStuff(ByRef success as Boolean)
If Me.InvokeRequired Then
Me.Invoke(New DoStuffDelegate(AddressOf DoStuff), success)
Else
If success Then
'do stuff
Else
'do stuff
End If
End If
End Sub
Vb.net 2015 working sample:
Add button,add progressbar
Imports System.Threading
Public Class Form1
Private trd As Thread
Private Sub ThreadTask()
Dim stp As Integer
Dim newval As Integer
Dim rnd As New Random()
Me.CheckForIllegalCrossThreadCalls = False
Do
stp = ProgressBar1.Step * rnd.Next(-1, 2)
newval = ProgressBar1.Value + stp
If newval > ProgressBar1.Maximum Then
newval = ProgressBar1.Maximum
ElseIf newval < ProgressBar1.Minimum Then
newval = ProgressBar1.Minimum
End If
ProgressBar1.Value = newval
Thread.Sleep(100)
Loop
End Sub
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
trd = New Thread(AddressOf ThreadTask)
trd.IsBackground = True
trd.Start()
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
MessageBox.Show("This is the main thread")
End Sub
End Class