Question

So far I have seen 1000 examples how to set items in listbox objects from a different thread using the Invoke method.

However, I cannot find any solution on how to simply read the items from a listbox from a background worker tread. How do I invoke this before reading the SelectedItems for example…

foreach (var item in CheckedListBox1.SelectedItems)
{
 //Do something
}

Above code running in the background worker generated the following error:

Cross-thread operation not valid: Control ‘CheckedListBox1’ accessed from a thread other than the thread it was created on.

Answer 1

You may similarly use Invoke to read UI elements from a background thread:

var selectedItems = (IList)this.Invoke(new Func<IList>(() =>
    CheckedListBox1.SelectedItems.Cast<object>().ToList()));

foreach (var item in selectedItems)
{
    //Do something
}

If you know the type of your items, you may specify the type in the Cast call, and return an IList<YourType> rather than the non-generic IList.