[Solved] Cannot access class from another class c#

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Obviously your main window is a YourProgress but you could try to get a reference to the ChooseExercises window using the Application.Current.Windows collection:

private void TabThursday_Loaded(object sender, RoutedEventArgs e)
{
    ChooseExercises ce = Application.Current.Windows.OfType<ChooseExercises>().FirstOrDefault();
    ...
}

2

solved Cannot access class from another class c#