public override void Search()
 {
     Proxy proxy = new Proxy();
     Result = proxy.SearchInvoice(DateFrom, DateTo, AmountFrom,AmountTo,SearchContact);
     Items.Clear();
     if (Result.Invoice != null)
     {
         foreach (var obj in Result.Invoice)
         {
             Items.Add(new InvoiceViewModel(obj));
         }
     }
     else
     {
         Invoice obj = new Invoice();
         obj.Nummer = "Keinen Eintrag gefunden!";
         obj.ID = "x";
         Items.Add(new InvoiceViewModel(obj));
     }
 }