Mapping hack Dictionary - Death to switch statements!

Transpose.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Utils
{
    public class Transpose<TKey, TValue> : Dictionary<TKey, Func<TValue>>
    {
        public IEnumerable<TValue> Intersect(IEnumerable<TKey> keys)
        {
            return Map(keys).Where(IsNotDefault);
        }

        public IEnumerable<TValue> Union(IEnumerable<TKey> keys)
        {
            return Map(keys).Union(Values.Select(func => func()));
        }

        public IEnumerable<TKey> SymmetricExceptWith(IEnumerable<TKey> keys)
        {
            var enumerable = keys as TKey[] ?? keys.ToArray();
            return Keys.Except(enumerable).Union(enumerable.Except(Keys));
        }

        public IEnumerable<TValue> Map(IEnumerable<TKey> keys)
        {
            return keys.Select(Map);
        }

        public TValue Map(TKey key)
        {
            return Map(key, () => default(TValue));
        }

        public TValue Map(TKey key, Func<TValue> defaultValue)
        {
            return HasKey(key)
                ? this[key]()
                : defaultValue();
        }

        public readonly Func<TKey, bool> HasKey;
        public readonly Func<TValue, bool> IsDefault;
        public readonly Func<TValue, bool> IsNotDefault;

        public Transpose(Func<TKey, bool> hasKey = null, Func<TValue, bool> isDefault = null, Func<TValue, bool> isNotDefault = null)
        {
            HasKey = hasKey ?? ContainsKey;
            IsDefault = isDefault ?? (value => EqualityComparer<TValue>.Default.Equals(value, default(TValue)));
            IsNotDefault = isNotDefault ?? (value => !IsDefault(value));
        }
    }
}