Binding ComboBoxes to enums… in Silverlight!

Agh, I spoke too soon! There is a perfectly good solution, at least in Silverlight 3. (It might only be in 3, since this thread indicates that a bug related to this stuff was fixed in Silverlight 3.)

Basically, you need a single converter for the ItemsSource property, but it can be entirely generic without using any of the prohibited methods, as long as you pass it the name of a property whose type is MyEnum. And databinding to SelectedItem is entirely painless; no converter needed! Well, at least it is as long as you don’t want custom strings for each enum value via e.g. the DescriptionAttribute, hmm… will probably need another converter for that one; hope I can make it generic.

Update: I made a converter and it works! I have to bind to SelectedIndex now, sadly, but it’s OK. Use these guys:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Windows.Data;

namespace DomenicDenicola.Wpf
{
    public class EnumToIntConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            // Note: as pointed out by Martin in the comments on this answer, this line
            // depends on the enum values being sequentially ordered from 0 onward,
            // since combobox indices are done that way. A more general solution would
            // probably look up where in the GetValues array our value variable
            // appears, then return that index.
            return (int)value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return Enum.Parse(targetType, value.ToString(), true);
        }
    }
    public class EnumToIEnumerableConverter : IValueConverter
    {
        private Dictionary<Type, List<object>> cache = new Dictionary<Type, List<object>>();

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            var type = value.GetType();
            if (!this.cache.ContainsKey(type))
            {
                var fields = type.GetFields().Where(field => field.IsLiteral);
                var values = new List<object>();
                foreach (var field in fields)
                {
                    DescriptionAttribute[] a = (DescriptionAttribute[])field.GetCustomAttributes(typeof(DescriptionAttribute), false);
                    if (a != null && a.Length > 0)
                    {
                        values.Add(a[0].Description);
                    }
                    else
                    {
                        values.Add(field.GetValue(value));
                    }
                }
                this.cache[type] = values;
            }

            return this.cache[type];
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

With this sort of binding XAML:

<ComboBox x:Name="MonsterGroupRole"
          ItemsSource="{Binding MonsterGroupRole,
                                Mode=OneTime,
                                Converter={StaticResource EnumToIEnumerableConverter}}"
          SelectedIndex="{Binding MonsterGroupRole,
                                  Mode=TwoWay,
                                  Converter={StaticResource EnumToIntConverter}}" />

And this sort of resource-declaration XAML:

<Application xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:ddwpf="clr-namespace:DomenicDenicola.Wpf">
    <Application.Resources>
        <ddwpf:EnumToIEnumerableConverter x:Key="EnumToIEnumerableConverter" />
        <ddwpf:EnumToIntConverter x:Key="EnumToIntConverter" />
    </Application.Resources>
</Application>

Any comments would be appreciated, as I’m somewhat of a XAML/Silverlight/WPF/etc. newbie. For example, will the EnumToIntConverter.ConvertBack be slow, so that I should consider using a cache?