StackOverflow-Getting Nested Types from Higher in Hierarchy

public class CustomSortDictionary<T> : Dictionary<ID, Comparison<T>>
{
    #region Fields
    private Type sortMethod;
    private static BindingFlags bindingFlags = BindingFlags.Public | BindingFlags.Static | BindingFlags.FlattenHierarchy;
    #endregion

    #region Properties
    public Type SortMethod { get { return sortMethod; } }
    #endregion

    #region Constructors
    /// <summary>
    /// Creates a new CustomSortDictionary.
    /// </summary>
    /// <param name="sortMethod">The SortMethod class to use.</param>
    public CustomSortDictionary(Type sortMethod) : base()
    {
        this.sortMethod = sortMethod;

        MethodInfo m = SortMethod.GetMethod("ConstructCustomSortMethods", bindingFlags);
        m.Invoke(null, null);
    }
    #endregion

    #region Methods
    /// <summary>
    /// Adds a custom sort method to the dictionary and the CitationCollection class.
    /// </summary>
    /// <param name="customSortMethod">The comparison by which to sort in the custom sort.</param>
    /// <param name="name">The name that will be used to refer to the custom sort in the CitationCollection class.</param>
    public void Add(ref string name, Comparison<T> customSortMethod)
    {
        ID key = ((IDCollection)SortMethod.GetNestedType("CustomSort", bindingFlags).GetField("Methods").GetValue(null)).Add(ref name);
        this.Add(key, customSortMethod);
    }
    #endregion
}

/// <summary>
/// Abstract class for SortMethod classes.
/// </summary>
/// <remarks>Added IDs must start at the value 3.</remarks>
public abstract class ASortMethod
{
    public static readonly ID Manual = 1;

    private static readonly ID CustomSortEnum = 2;

    /// <summary>
    /// Used for custom sorting methods.
    /// </summary>
    public static class CustomSort
    {
        public static readonly ID None = CustomSortEnum[0];

        /// <summary>
        /// An IDCollection containing the custom sorting methods that correspond to CitationCollection.CustomSorts.
        /// </summary>
        public static IDCollection Methods;
    }

    /// <summary>
    /// If SortMethod.CustomSort.Methods is not initialized, it is initialized.
    /// </summary>
    public static void ConstructCustomSortMethods()
    {
        if (CustomSort.Methods == null)
        {
            CustomSort.Methods = new IDCollection(CustomSortEnum, typeof(CustomSort));
        }
    }

    /// <summary>
    /// Determines whether the given ID exists in SortMethod.
    /// </summary>
    /// <param name="method">The ID to check for.</param>
    /// <returns>Whether or not it exists in either the class or its CustomSort methods.</returns>
    public static bool Exists(ID method)
    {
        List<FieldInfo> fields = (from FieldInfo f in typeof(ASortMethod).GetFields()
                                  where f.FieldType == typeof(ID) &&
                                        (ID)f.GetValue(null) == method
                                  select f).ToList<FieldInfo>();

        return fields.Count > 0;
    }

    /// <summary>
    /// Determines whether the ID with the given name exists in SortMethod.
    /// </summary>
    /// <param name="methodName">The name of the ID.</param>
    /// <returns>Whether or not it exists in either the class or its CustomSort methods.</returns>
    public static bool Exists(string methodName)
    {
        List<FieldInfo> fields = (from FieldInfo f in typeof(ASortMethod).GetFields()
                                  where f.FieldType == typeof(ID) &&
                                        f.Name == methodName
                                  select f).ToList<FieldInfo>();

        return fields.Count > 0;
    }
}

class CitationCollection : List<Citation>
{
    /// <summary>
    /// The methods by which the CitationCollection may be sorted.
    /// </summary>
    public sealed class SortMethod : ASortMethod
    {
        public static readonly ID Bibliography = 3;

        private static readonly ID ByDateAddedEnum = 4;
        public static class ByDateAdded
        {
            public static readonly ID MostRecentFirst = ByDateAddedEnum[0];
            public static readonly ID MostRecentLast = ByDateAddedEnum[1];
        }
    }
}