View

    public partial class CreateMeterWindow : Window
    {
        private CreateMeterViewModel _CreateMeterViewModel;

        public CreateMeterViewModel CreateMeterViewModel
        {
            get
            {
                return _CreateMeterViewModel;
            }
        }
        
        public CreateMeterWindow()
        {
            _CreateMeterViewModel = new CreateMeterViewModel(this);
            InitializeComponent();
        }

        private void ComboBox_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
        {
            _CreateMeterViewModel.ComboBox_SelectionChanged();
        }
    }

View (xaml)

<Window x:Class="Views.CreateMeterWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"
        Title="Voeg een nieuwe meter toe" Height="300" Width="600"
        DataContext="{Binding CreateMeterViewModel, RelativeSource={RelativeSource Self}}">
        
        
        <xctk:DecimalUpDown Visibility="{Binding IsDoubleMeter}" HorizontalAlignment="Left" Watermark="1234567,89" Height="23" Margin="150,105,0,0" Value="{Binding NightReading.Reading}" VerticalAlignment="Top" Width="200" />
        <DatePicker Visibility="{Binding IsDoubleMeter}" Margin="355,105,10,0" VerticalAlignment="Top" SelectedDate="{Binding NightReading.Date}"/>
        <Label Visibility="{Binding IsDoubleMeter}" Content="Huidige stand (nacht)" HorizontalAlignment="Left" Margin="10,101,0,0" VerticalAlignment="Top"/>
        
</Window>

Viewmodel

    public class CreateMeterViewModel : INotifyPropertyChanged
    {
        private Window _Window;
        private Meter _Meter;
        private CounterReading _NightReading;
        private MeterType _MeterType;
        private bool _IsDoubleMeter;

        public Meter Meter
        {
            get
            {
                if (_Meter == null)
                {
                    if(_MeterType != MeterType.Unknown)
                    {
                        _Meter = MeterFactory.Create(_MeterType);
                    }
                }
                return _Meter;
            }
        }

        public CounterReading NightReading
        {
            get
            {
                if (_NightReading == null)
                {
                    _NightReading = new CounterReading();
                }
                return _NightReading;
            }
        }

        public MeterType MeterType
        {
            get
            {
                return _MeterType;
            }
            set
            {
                _MeterType = value;
            }
        }

        public bool IsDoubleMeter
        {
            get
            {
                return _IsDoubleMeter;
            }
            set
            {
                _IsDoubleMeter = value;
                NotifyPropertyChanged("IsDoubleMeter");

            }
        }

        public CreateMeterViewModel(Window window)
        {
            _Window = window;
        }

        internal void ComboBox_SelectionChanged()
        {
            if (_MeterType != MeterType.Unknown)
            {
                _Meter = MeterFactory.Create(_MeterType);
                IsDoubleMeter = _Meter is DoubleElectricityMeter;
            } else
            {
                _Meter = null;
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public void NotifyPropertyChanged(String info)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(info));
            }
        }
    }