public partial class MainWindow : Window
   {
      private PatientsEntities _context = new PatientsEntities();
      private CollectionViewSource patientViewSource;
 
      public MainWindow()
      {
         InitializeComponent();
      }
      
      private void Window_Loaded(object sender, RoutedEventArgs e)
      {
         var countryList = from c in _context.Countrys orderby c.CountryEn select c;        
 
         CollectionViewSource patientViewSource = ((CollectionViewSource)(this.FindResource("patientViewSource")));
 
         _context.Patients.Load();
 
         patientViewSource.Source = _context.Patients.Local;
 
         CollectionViewSource countryViewSource = ((CollectionViewSource)(this.FindResource("countryViewSource")));
         countryViewSource.Source = countryList.ToList();
      }

 public partial class Patients
    {
        public int PatientID { get; set; }
        public string Name { get; set; }
        public string Vorname { get; set; }
        public Nullable<System.DateTime> Birthdate { get; set; }
        public string CountryID { get; set; }
    
        public virtual Country Country { get; set; }

public partial class Country
    {
        public Country()
        {
            this.Countrys = new ObservableCollection<Patients>();
        }
    
        public string CountryID { get; set; }
        public string CountryEn { get; set; }
    
        public virtual ObservableCollection<Patient> Patients { get; set; }
    }

<Window.Resources>
      <CollectionViewSource x:Key="patientsViewSource" d:DesignSource="{d:DesignInstance {x:Type local:Patients}, CreateList=True}"/>
      <CollectionViewSource x:Key="countrysViewSource" d:DesignSource="{Binding Patients, Source={StaticResource patientsViewSource}}"/>
   </Window.Resources>

<ComboBox x:Name="countryComboBox" DisplayMemberPath="CountryEn" ItemsSource="{Binding Source={StaticResource countryViewSource}}" SelectedValue="{Binding /CountryID}" SelectedValuePath="{Binding /CountryID, Source={StaticResource patientViewSource}}">