Dispatcher.BeginInvoke , trying to use lambda to get string set from textblock, but getting conversion error

I am trying to call a selected listbox item from a button, not the listbox.selecteditemchanged method in wpf. So when i try

string yadda = listbox.SelectedItem.ToString();

i get an exception:

The calling thread cannot access this object because a different thread owns it.

So, what i was trying to do is the following:

Dispatcher.BeginInvoke(() =>
                    {
                        lbxSelectedItem =  (lbxFileList.SelectedItem as TextBlock).Text;
                    });

That is not working either because i get another exception:

Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type