/// <summary>
/// implementiert die Anforderung LIST(Suppliers) findAllPreferredSuppliers()
/// </summary>
/// <returns> eine Liste von Supplier, die mindestens einen Eintrag in preferredSupplier haben </returns>
public List <Supplier> findAllPreferredSuppliers()
{
List <Supplier> result = new List <Supplier>();
var allProducts = _supplierContext.Product;
foreach (Product p in allProducts)
{
Supplier s = _supplierContext.Find <Supplier>(p.preferredSupplier);
if (!result.Contains(s))
{
result.Add(s);
}
}
return(result);
}