Выберите:
Show(Window owner, string messageBoxText);
Show(Window owner, string messageBoxText, string caption);
Show(Window owner, string messageBoxText, string caption, MessageBoxButton button);
Show(Window owner, string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon);
Show(Window owner, string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon, MessageBoxResult defaultResult);
Show(Window owner, string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon, MessageBoxResult defaultResult, MessageBoxOptions options);