How to do something on main thread? - vb.net

Question

I used to be an xcode programmer.
There, when creating application, I tend to do most things on other threads.
Occasionally, such as when I want to access the UI thread, I would then do something at main thread.
Say I did
//Load some heavy resources on the web
doOnMainThread(sub () updateUIandStuff())
//Continue doing other things.
How would I implement doOnMainthread in VB?
There is an easy way to do so in objective-c a long time ago. How to do so in vb.net

Answer 1

There are two common methods to accomplish this; Delegates or Invoke a lambda. The following example will paste straight into a new WinForms project with two labels added in the designer:
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim bw As New System.ComponentModel.BackgroundWorker
AddHandler bw.DoWork, AddressOf BackgroundWorker_DoWork
bw.RunWorkerAsync()
End Sub
Delegate Sub UpdateLabelDelegate(ByVal labelText As String)
Private Sub UpdateLabel(ByVal labelText As String)
Label2.Text = labelText
End Sub
Private Sub BackgroundWorker_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs)
For i As Integer = 1 To 10000
Dim testText As String = "Loop#" & i.ToString
'You can easily check your thread's access to the UI using:
' WinForms "If InvokeRequired Then : End If"
' WPF "If Not Dispatcher.CheckAccess Then : End If"
'Using Invoke Lambda
Invoke(Sub() Label1.Text = testText)
'Using Delegate
Dim updateLbl As UpdateLabelDelegate = AddressOf UpdateLabel
Invoke(updateLbl, testText)
Next
End Sub
End Class
The first example Invoke(Sub() Label1.Text = testText) is my preferred method for when the amount of code that needs to be executed on the main thread is small, like a line or two. If that line is placed in a sub that may be called by the main UI thread or a background thread in different scenarios, then it should be wrapped in a If InvokeRequired Then conditional block. The second example is using a delegate, and this method is better if a larger number of lines of code need to be executed on the main thread. Either method will allow you to call code from a background thread that will be executed on the main UI thread.
As for WPF, the methodology is largely the same, but as Bradley Uffner pointed out in the comments, invoke will be called by Dispatcher