Hierarchise a collection

hierarchiseMethod.cs

/// <summary>
/// Takes an enumerable of items and builds a hierarchy from two properties present on the item
/// </summary>
/// <typeparam name="TOuter">The type of the outer key</typeparam>
/// <typeparam name="TInner">The type of the inner key</typeparam>
/// <typeparam name="TValue">The type of the item being processed</typeparam>
/// <param name="items">The enumerable to process</param>
/// <param name="outerSelector">Function for accessing the outer key</param>
/// <param name="innerSelector">Function for accessing the inner key</param>
public static Dictionary<TOuter, Dictionary<TInner, TValue>> Hierarchise<TOuter, TInner, TValue>(this IEnumerable<TValue> items,
                                                                                                 Func<TValue, TOuter> outerSelector,
                                                                                                 Func<TValue, TInner> innerSelector)
{
    return items.GroupBy(outerSelector)
                .Select(og => new KeyValuePair<TOuter, IEnumerable<KeyValuePair<TInner, TValue>>>(
                            og.Key, og.Select(gi => new KeyValuePair<TInner, TValue>(innerSelector(gi), gi))))
                .ToDictionary(kvp => kvp.Key, kvp => kvp.Value.ToDictionary(k => k.Key, k => k.Value));
}

Using it is as simple as: Customers.Hierarchise(c => c.LastName, c => c.FirstName)

Just watch out for colliding keys as they're not dealt with.