Crossthread operation not valid... - VB.NET

Question

I am using vb.net, and in my program I get this 'crossthread operation not valid' error when I run my backgroundworker that will make this textbox enabled true. My main sub will first turn the enabled to false, and when the backgroundworker runs it will turn it back true then exit. Why does it give me an error? FYI: There is more code to this but I don't want to make it any more confusing...

Here is the stack trace:

at System.Windows.Forms.Control.get_Handle()
   at System.Windows.Forms.Control.OnEnabledChanged(EventArgs e)
   at System.Windows.Forms.Control.set_Enabled(Boolean value)
   at Helium.Form1.BackgroundWorker1_DoWork(Object sender, DoWorkEventArgs e) in C:\Users\Kevin\documents\visual studio 2010\Projects\Helium\Helium\Form1.vb:line 167
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

and here is the exact error message:

{"Cross-thread operation not valid: Control 'mainText' accessed from a thread other than the thread it was created on."}

Can someone please help me out!

Thanks,

KEvin

Answer 1

The purpose of the BackgroundWorker class is to perform work on a non-GUI thread while the GUI remains responsive. Unless you set Control.CheckForIllegalCrossThreadCalls to false (which you shouldn't do), or use Invoke as suggested in the other answers (which I also wouldn't recommend), you're going to get an illegal cross-thread operation exception.

If you want GUI-related "stuff" to happen while your BackgroundWorker is running, I'd generally recommend using the BackgroundWorker.ReportProgress method and attaching an appropriate handler to the BackgroundWorker.ProgressChanged event. If you want something on the GUI to happen once the BackgroundWorker is finished, then simply attach your handler to update the GUI to the BackgroundWorker.RunWorkerCompleted event.

Answer 2

Type the following code in the Form1_Load (or whatever your form is) sub:

System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = False

It fixes all problems with blocked cross-thread operations.