[OperationContract]
        List<WeatherDataDTO> GetDataByDate(DateTime start, DateTime end);

 public class WeatherDataDTO
    {
        public int Id { get; set; }
        public DateTime Datum { get; set; }
        public TimeSpan Zeit { get; set; }
        public double Temperatur { get; set; }
        public int Luftdruck { get; set; }
        public int Regen { get; set; }
        public double Wind { get; set; }
        public int Richtung { get; set; }
        public int Feuchtigkeit { get; set; }
    }

public List<WeatherDataDTO> GetDataByDate(DateTime start, DateTime end)
        {
            using (var context = new WeatherdataEntities1())
            {
                var resList = (from b in context.WeatherDatas
                               where b.Datum >= start && b.Datum <= end
                               select b).ToList();

                var dtoList = new List<WeatherDataDTO>();

                foreach (var b in resList.GetRange(0, 50))
                {
                    dtoList.Add(new WeatherDataDTO
                    {
                        Id = b.Id,
                        Datum = b.Datum.Value,
                        Zeit = b.Zeit.Value,
                        Temperatur = b.Temperatur.Value,
                        Luftdruck = b.Luftdruck.Value,
                        Regen = b.Regen.Value,
                        Wind = b.Wind.Value,
                        Richtung = b.Richtung.Value,
                        Feuchtigkeit = b.Feuchtigkeit.Value
                    });
                }
                return dtoList;
            }
        }

<DatePicker Name="start" HorizontalAlignment="Left" Margin="100,10,10,0" VerticalAlignment="Top" SelectedDate="1/1/2013"/>
        <DatePicker Name="end" HorizontalAlignment="Left" Margin="100,40,0,0" VerticalAlignment="Top" SelectedDate="1/2/2013"/>
        <Label Content="Startdate:" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" FontSize="14"/>
        <Label Content="Enddate:" HorizontalAlignment="Left" Margin="10,40,0,0" VerticalAlignment="Top" FontSize="14"/>
        <Button Content="Send" HorizontalAlignment="Left" Margin="10,70,0,0" VerticalAlignment="Top" RenderTransformOrigin="-0.573,-0.562" Width="50" Click="Button_Click"/>
        <DataGrid Name="Grid"  Margin="10,100,10,10"/>

private void Button_Click(object sender, RoutedEventArgs e)
        {
            ServiceReference1.Service1Client service1Client = new ServiceReference1.Service1Client();
            List<ServiceReference1.WeatherDataDTO> list = service1Client.GetDataByDate((DateTime)start.SelectedDate, (DateTime)end.SelectedDate).ToList();
            Grid.ItemsSource = list;

        }