1                 this.Accounts = rep.All.OrderBy(a => a.Account).Select(a => a.Account).ToList();
2                 this.Accounts.Insert(0, "");

1 <ComboBox Grid.Column="3" Grid.Row="2" x:Name="Accounts" SelectedItem="{Binding SelectedAccount}" />

 1         public static List<LogisticsTypes?> DeliveryTypes {
 2             get;
 3             set;
 4         }
 5
 6
 7         public LogisticsTypes? SelectedDeliveryType {
 8             get;
 9             set;
10         }

DeliveryTypes = Enum.GetValues(typeof(LogisticsTypes)).Cast<LogisticsTypes?>().ToList();
DeliveryTypes.Insert(0, null);

1 <ComboBox Grid.Row="1" Grid.Column="5" ItemsSource="{Binding Source={x:Static model:OrderQueryViewModel.DeliveryTypes}}" SelectedItem="{Binding SelectedDeliveryType,Mode=TwoWay}">
2                     <ComboBox.ItemTemplate>
3                         <DataTemplate>
4                                 <TextBlock Text="{Binding . , Converter={StaticResource EnumDesc}}" />
5                         </DataTemplate>
6                     </ComboBox.ItemTemplate>
7                 </ComboBox>

 1                 <ComboBox Grid.Row="1" Grid.Column="5" SelectedItem="{Binding SelectedDeliveryType}">
 2                     <ComboBox.ItemsSource>
 3                         <CompositeCollection>
 4                             <ComboBoxItem Content="" />
 5                             <CollectionContainer Collection="{Binding Source={x:Static model:OrderQueryViewModel.DeliveryTypes}}" />
 6                         </CompositeCollection>
 7                     </ComboBox.ItemsSource>
 8                     <ComboBox.ItemTemplate>
 9                         <DataTemplate>
10                             <TextBlock Text="{Binding . , Converter={StaticResource EnumDesc}}" />
11                         </DataTemplate>
12                     </ComboBox.ItemTemplate>
13                 </ComboBox>

 1     public class CanNullConverter : IValueConverter {
 2         public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
 3             return value;
 4         }
 5
 6         public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
 7             NullableConverter nullableConvert;
 8             var toType = targetType;
 9             if (targetType.IsGenericType && targetType.GetGenericTypeDefinition().Equals(typeof(Nullable<>))) {
10                 nullableConvert = new NullableConverter(targetType);
11                 toType = nullableConvert.UnderlyingType;
12             }
13
14             return value.GetType().Equals(toType) ? value : null;
15         }
16     }

<ac:CanNullConverter x:Key="CanNull" />
....
....
....
<ComboBox Grid.Row="1" Grid.Column="5" SelectedItem="{Binding SelectedDeliveryType, Converter={StaticResource CanNull}}">