Why am I getting this error:"Cross-thread operation not valid: Control lbFolders accessed from a thread other than the thread it was created on."?

Question

This is baffling me, maybe somebody can shine the light of education on my ignorance. This is in a C# windows app. I am accessing the contents of a listbox from a thread. When I try to access it like this

prgAll.Maximum = lbFolders.SelectedItems.Count;

I get the error. However, here is the part I don't get. If I comment out that line, the very next line

foreach (string dir in lbFolders.SelectedItems)

executes just fine.

Edit: As usual, my communication skills are lacking. Let me clarify.

I know that accessing GUI items from threads other than the ones they were created on causes problems. I know the right way to access them is via delegate.

My question was mainly this: Why can I access and iterate through the SelectedItems object just fine, but when I try to get (not set) the Count property of it, it blows up.

Answer 1

prgAll.Maximum = lbFolders.SelectedItems.Count;

On that line you perform an assignment (set/add), which by default is not thread-safe.

On the second line it's just a get operation, where thread-safety merely doesn't matter.

EDIT: I don't mean access to the prgAll element.

Accessing the Count property changes the internal state of the ListBox inner collection, that is why it throws the exception.

Answer 2

The Count property of SelectedItems is not thread-safe, so you can't use it cross-thread.

Answer 3

You can't touch a GUI object from a thread that isn't the main GUI thread. See here for more details and the solution.

Answer 4

You're trying to write to a control from a thread other than the main thread. Use Invoke or BeginInvoke.

void SetMax()
{
    if (prgAll.InvokeRequired)
    {
        prgAll.BeginInvoke(new MethodInvoker(SetMax));
        return;
    }

    prgAll.Maximum = lbFolders.SelectedItems.Count;
}

Answer 5

You can't access GUI elements from a separate thread. Use a delegate to make the change.

eg.

lblStatus.Invoke((Action)(() => lblStatus.Text = counter.ToString()));

or older skool:

lblTest.Invoke((MethodInvoker)(delegate() 
{ 
  lblTest.Text = i.ToString(); 
}));

I've got a blog post on how to do this in all the .Net releases here.

Answer 6

Because you created a control in a thread and you're trying to reach it from another one. Call the InvokeRequired property as shown here:

private void RunMe()
{
    if (!InvokeRequired)
    {
        myLabel.Text = "You pushed the button!";
    }
    else
    {
        Invoke(new ThreadStart(RunMe));
    }
}