Running threads in vb.net

Question

I have an application in which I am running a separate thread.

Dim thread As New System.Threading.Thread(AddressOf Main)
thread.Start()

However, the thread makes reference to an textbox named Output, and it generates this error upon execution:

System.InvalidOperationException was unhandled
  Message="Cross-thread operation not valid: Control 'Output' accessed from a thread other than the thread it was created on."
  Source="System.Windows.Forms"

(message shortened for space)

How can I make the operations run on another thread, but still use the Output object? I cannot call a subroutine to do that for me, as it generates the exact same error.

The method called is AppendText, by the way.

I am probably missing something important here, so thanks for the help!

Answer 1

You should use Control.Invoke or Control.BeginInvoke to call your subroutine.

Answer 2

Instead of just calling the AppendText method you need to force it to execute on the correct thread. So, if you have a call like this:

myTextBox.AppendText("some text")

...you need to change it to this:

myTextBox.BeginInvoke(New Action(Of String)(AddressOf myTextBox.AppendText), "some text")

You can use either Invoke or BeginInvoke. In this case, since AppendText doesn't have any return value, BeginInvoke is a good choice (the difference is that Invoke will block the current thread while the GUI thread executes the AppendText method, while BeginInvoke will make the call asynchronously instead).

Answer 3

This exception is a very popular exception when using a thread.Some operation are not thread safe (like accessing a control on a thread other than its own thread) so framework is preventing these kind of problems. To solve it you can either use Control.Invoke method to call a delegate that is in the same thread the control or you can use a background worker.

Here you can find a sample of first approach

and

Here you can find a sample of a background worker