WPF(MVVM) Dispatcher question

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MVVM

multi-threading

I am writing a WPF(MVVM) application that would run a process in a background thread.
Here is the particualr method that is running on a different thread than the UI thread.

 private void ExecuteAsync()
{
 IsRunning = true;
 for (int i = 0; i< 1000; i++)
 {
     App.Current.Dispatcher.Invoke(DispatcherPriority.Normal, new Action(
 () =>; TheTextCollection.Add(string.Concat(string.Format("{0} ", i), _theTextFilled))));
     App.Current.Dispatcher.Invoke(DispatcherPriority.Normal,new Action(
                    ()=>;ProgressPercentage=(int)(100*((float)i/(float)1000))));
            }
     App.Current.Dispatcher.Invoke(DispatcherPriority.Normal, new Action(
 () =>; ProgressPercentage = 0));
IsRunning = false;       
}

In the above code:

1) ProgressPercentage is a ViewModel property of type int which is two way data-bound to a progress bar in the UI.

2)TheTextCollection is a property of type ObservableCollection<string> which is one way databound to the items property of a listbox in the UI.

3)There is a button which starts the process and it gets disabled once the process starts (as a typical multithreaded UI would behave.)IsRunning is a variable which is used by the relay command that is bound to the Command property of the button.

The button very finely gets disabled when the process starts but it does not gets enabled on its own after the process ends. However if the some form event is raised it gets enabled (e.g.: scrolling of the listbox or maximizing the form). Any suggestion would be appreciated.