Question

I’m looking for a generic method to implement a wait screen during long operations. I have used threading a few times before, but I have the feeling that I implemented it either very poorly, or with way too much hassle (and copy/pasting – the horror!).

I want to keep this as generic and simple as possible, so I won’t have to implement loads of BackgroundWorkers handling all kinds of crap, making things hard to maintain.

Here’s what I would like to do — please note this might differ from what’s actually possible/best practise/whatever — using VB.NET, Framework 2.0 (so no anonymous methods):

  Private Sub HandleBtnClick(sender as Object, e as EventArgs) Handles Button.Click       LoadingScreen.Show()        'Do stuff here, this takes a while!'       Dim Result as Object = DoSomethingTakingALongTime(SomeControl.SelectedObject)        LoadingScreen.Hide()        ProcessResults(Result)   End Sub 

The application is now completely single-threaded, so everything runs on the GUI thread. I need to be able to access objects in DoSomethingTakingALongTime() without getting cross-thread exceptions. The GUI thread waits for some method (which takes a long time) to complete, while the LoadingScreen Form should stay responsive (it’s animated/has a progressbar/etc.).

Is this a doable/good approach or am I seeing this way too simplistic? What is the best practise concerning this matter? And most importantly: how could I implement such a system? As I already mentioned, I have very little experience with threading, so be gentle please 🙂

Answer 1

Your problem is that your getting a cross thread exception when your trying to pass your Worker thread data to your ui thread. what you need to do is check InvokeRequired and begininvoke before setting the controls on your ui so you don’t get the error like so:

Private Sub work_CrossThreadEvent(ByVal sender As Object, ByVal e As System.EventArgs) Handles work.CrossThreadEvent         If Me.InvokeRequired Then            Me.BeginInvoke(New EventHandler(AddressOf work_CrossThreadEvent), New Object() {sender, e})            Return        End If        Me.Text = 'Cross Thread'  End Sub 

just change the New EventHandler part to the event handler your using.

Also i think using a background worker isn’t a bad method for your worker classes, just create a class for your work and use the background worker to do the threading stuff a bit like this:

Public MustInherit Class Worker      Protected WithEvents worker As BackgroundWorker      Public Sub New()          worker = New BackgroundWorker()         worker.WorkerReportsProgress = True         worker.WorkerSupportsCancellation = True      End Sub      Public Sub Start()          If (Not worker.IsBusy AndAlso Not worker.CancellationPending) Then             worker.RunWorkerAsync()         End If      End Sub      Public Sub Cancel()         If (worker.IsBusy AndAlso Not worker.CancellationPending) Then             worker.CancelAsync()         End If     End Sub      Protected MustOverride Sub Work()      Private Sub OnDoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles worker.DoWork         Work()     End Sub      Public Event WorkCompelted As RunWorkerCompletedEventHandler     Private Sub OnRunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs) Handles worker.RunWorkerCompleted         OnRunWorkerCompleted(e)     End Sub     Protected Overridable Sub OnRunWorkerCompleted(ByVal e As RunWorkerCompletedEventArgs)         RaiseEvent WorkCompelted(Me, e)     End Sub      Public Event ProgressChanged As ProgressChangedEventHandler     Private Sub OnProgressChanged(ByVal sender As Object, ByVal e As ProgressChangedEventArgs) Handles worker.ProgressChanged         OnProgressChanged(e)     End Sub     Protected Overridable Sub OnProgressChanged(ByVal e As ProgressChangedEventArgs)         RaiseEvent ProgressChanged(Me, e)     End Sub  End Class  Public Class ActualWork     Inherits Worker      Public Event CrossThreadEvent As EventHandler      Protected Overrides Sub Work()          'do work here'         WorkABit()         worker.ReportProgress(25)          WorkABit()         worker.ReportProgress(50)          WorkABit()         worker.ReportProgress(75)          WorkABit()         worker.ReportProgress(100)      End Sub      Private Sub WorkABit()          If worker.CancellationPending Then Return         Thread.Sleep(1000)         RaiseEvent CrossThreadEvent(Me, EventArgs.Empty)      End Sub  End Class 

disclaimer.. bit rusty with vb but you should get the idea.