How to keep ProgressBar Indeterminate animating

How to keep ProgressBar Indeterminate animating

I've been learning about how this issue is confronted via Threads
(BackgroundWorker and others) and how to keep the UI responsive. I've
managed to do so but then I realized that is not what I want. What I need
actually is just to display an animating ProgressBar while a long
operation is performed. (I don't need/want to update the UI because in
that operation I'm exporting all graphical representation and the plotter
is updated constantly).
I have thought about a Dialog popping-up with the progress bar while the
operation is being performed. The problem is that I get this exception:
The calling thread cannot access this object because a different thread
owns it
There are plenty of questions about this and the answer is to use Dispatcher:
Dispatcher.CurrentDispatcher.Invoke(() => myOperation(),
DispatcherPriority.Normal);
Here is what've done:
I'm using Modern UI, there is a Dialog called ModernDialog, it's just a
fancy dialog:
class DialogProgress
{
public ModernDialog progressDlg = new ModernDialog();
public DialogProgress()
{
ProgressBar bar = new ProgressBar();
bar.IsIndeterminate = true;
bar.Width = 150;
StackPanel content = new StackPanel();
content.Children.Add(bar);
progresoDlg.Content = content ;
//Thread paralel = new Thread(() => myOperation());
//paralel.SetApartmentState(ApartmentState.STA);
//paralel.Start();
Dispatcher.CurrentDispatcher.Invoke(() => myOperation(),
DispatcherPriority.Normal);
}
void myOperation()
{
progresoDlg.ShowDialog();
}
}
I know I'm mixing stuff there, there are Threads and Dispatcher, but I
can't figure out how to use them.
Here is how I call this dialog:
public void MyLongMethod()
{
DialogProgress progress = new DialogProgress();
}
If I use only Dispatcher, the dialog shows up and the bar is being
animated but MyLongMethod is not working (it starts after closing the
dialog).
If I use Threads I get the mentioned exception.
How can I solve this problem?
(P.D. using a dialog Is only a suggestion, I would be happy too if the
progress bar is in the UI and I switch the visibility when the long method
starts/ends)