No generic implementation of OrderedDictionary?

Question

There doesn't appear to be a generic implementation of OrderedDictionary (which is in the System.Collections.Specialized namespace) in .NET 3.5. Is there one that I'm missing?

I've found implementations out there to provide the functionality, but wondered if/why there isn't a generic implementation out-of-the-box and if anyone knows whether it's something in .NET 4.0?

Answer 1

I implemented generic OrderedDictionary<TKey, TValue> by wraping around SortedList<TKey, TValue> and adding private Dictionary<TKey, int> _order. Then I created internal implementation of Comparer<TKey> passing reference to the _order dictionary. Then I use this comparer for internal SortedList. This class keeps order of elements passed to constructor and order of additions.

This implementation has almost the same BigO characteristics as SortedList<TKey, TValue> since Adding and Removing to _order is O(1). Each element will take (according to the book 'C# 4 in a Nutshell', p. 292, table 7-1) additional memory space of 22 (overhead) + 4 (int order) + TKey size (let assume 8) = 34. Together with SortedList<TKey, TValue> overhead of 2 bytes the total overhead is 36 bytes, while the same book says that non-generic OrderedDictionary has overhead of 59 bytes.

If I pass sorted=true to constructor, then _order is not used at all, the OrderedDictionary<TKey, TValue> is exactly SortedList<TKey, TValue> with minor overhead for wrapping, if at all meaningful.

I am going to store not-so-many large reference objects in the OrderedDictionary<TKey, TValue>, so for me this c.36 bytes overhead is tolerable.

The main code is below. The complete updated code is on this gist.

public class OrderedList<TKey, TValue> : IDictionary<TKey, TValue>, IDictionary
{
    private readonly Dictionary<TKey, int> _order;
    private readonly SortedList<TKey, TValue> _internalList;

    private readonly bool _sorted;
    private readonly OrderComparer _comparer;

    public OrderedList(IDictionary<TKey, TValue> dictionary, bool sorted = false)
    {
        _sorted = sorted;

        if (dictionary == null) dictionary = new Dictionary<TKey, TValue>();

        if (_sorted)
        {
            _internalList = new SortedList<TKey, TValue>(dictionary);
        }
        else
        {
            _order = new Dictionary<TKey, int>();
            _comparer = new OrderComparer(ref _order);
            _internalList = new SortedList<TKey, TValue>(_comparer);
            // keep prder of the IDictionary
            foreach (var kvp in dictionary)
            {
                Add(kvp);
            }
        }
    }

    public OrderedList(bool sorted = false)
        : this(null, sorted)
    {
    }

    private class OrderComparer : Comparer<TKey>
    {
        public Dictionary<TKey, int> Order { get; set; }

        public OrderComparer(ref Dictionary<TKey, int> order)
        {
            Order = order;
        }

        public override int Compare(TKey x, TKey y)
        {
            var xo = Order[x];
            var yo = Order[y];
            return xo.CompareTo(yo);
        }
    }

    private void ReOrder()
    {
        var i = 0;
        _order = _order.OrderBy(kvp => kvp.Value).ToDictionary(kvp => kvp.Key, kvp => i++);
        _comparer.Order = _order;
        _lastOrder = _order.Values.Max() + 1;
    }

    public void Add(TKey key, TValue value)
    {
        if (!_sorted)
        {
            _order.Add(key, _lastOrder);
            _lastOrder++;
            //very rare event
            if (_lastOrder == int.MaxValue) ReOrder();
        }

        _internalList.Add(key, value);
    }

    public bool Remove(TKey key)
    {
        var result = _internalList.Remove(key);
        if (!_sorted) _order.Remove(key);
        return result;
    }

    // Other IDictionary<> + IDictionary members implementation wrapping around _internalList
    // ...
}

Answer 2

As a follow up to the comment from @V.B. here's an accessible implementation of the System.Runtime.Collections.OrderedDictionary<,>. I was originally going to access it by reflection and provide it via a factory but the dll this is in does not seem to be very accessible at all so I just pulled the source itself.

One thing to note is the indexer here will not throw KeyNotFoundException. I absolutely hate that convention and that was the 1 liberty i took in this implementation. If that's important to you, just replace the line for return default(TValue);. Uses C# 6 (compatible with Visual Studio 2013)

/// <summary>
///     System.Collections.Specialized.OrderedDictionary is NOT generic.
///     This class is essentially a generic wrapper for OrderedDictionary.
/// </summary>
/// <remarks>
///     Indexer here will NOT throw KeyNotFoundException
/// </remarks>
public class OrderedDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IDictionary
{
    private readonly OrderedDictionary _privateDictionary;

    public OrderedDictionary()
    {
        _privateDictionary = new OrderedDictionary();
    }

    public OrderedDictionary(IDictionary<TKey, TValue> dictionary)
    {
        if (dictionary == null) return;

        _privateDictionary = new OrderedDictionary();

        foreach (var pair in dictionary)
        {
            _privateDictionary.Add(pair.Key, pair.Value);
        }
    }

    public bool IsReadOnly => false;
    public int Count => _privateDictionary.Count;
    int ICollection.Count => _privateDictionary.Count;
    object ICollection.SyncRoot => ((ICollection)_privateDictionary).SyncRoot;
    bool ICollection.IsSynchronized => ((ICollection)_privateDictionary).IsSynchronized;

    bool IDictionary.IsFixedSize => ((IDictionary)_privateDictionary).IsFixedSize;
    bool IDictionary.IsReadOnly => _privateDictionary.IsReadOnly;
    ICollection IDictionary.Keys => _privateDictionary.Keys;
    ICollection IDictionary.Values => _privateDictionary.Values;

    void IDictionary.Add(object key, object value)
    {
        _privateDictionary.Add(key, value);
    }

    void IDictionary.Clear()
    {
        _privateDictionary.Clear();
    }

    bool IDictionary.Contains(object key)
    {
        return _privateDictionary.Contains(key);
    }

    IDictionaryEnumerator IDictionary.GetEnumerator()
    {
        return _privateDictionary.GetEnumerator();
    }

    void IDictionary.Remove(object key)
    {
        _privateDictionary.Remove(key);
    }

    object IDictionary.this[object key]
    {
        get { return _privateDictionary[key]; }
        set { _privateDictionary[key] = value; }
    }

    void ICollection.CopyTo(Array array, int index)
    {
        _privateDictionary.CopyTo(array, index);
    }

    public TValue this[TKey key]
    {
        get
        {
            if (key == null) throw new ArgumentNullException(nameof(key));

            if (_privateDictionary.Contains(key))
            {
                return (TValue) _privateDictionary[key];
            }

            return default(TValue);
        }
        set
        {
            if (key == null) throw new ArgumentNullException(nameof(key));

            _privateDictionary[key] = value;
        }
    }

    public ICollection<TKey> Keys
    {
        get
        {
            var keys = new List<TKey>(_privateDictionary.Count);

            keys.AddRange(_privateDictionary.Keys.Cast<TKey>());

            return keys.AsReadOnly();
        }
    }

    public ICollection<TValue> Values
    {
        get
        {
            var values = new List<TValue>(_privateDictionary.Count);

            values.AddRange(_privateDictionary.Values.Cast<TValue>());

            return values.AsReadOnly();
        }
    }

    public void Add(KeyValuePair<TKey, TValue> item)
    {
        Add(item.Key, item.Value);
    }

    public void Add(TKey key, TValue value)
    {
        if (key == null) throw new ArgumentNullException(nameof(key));

        _privateDictionary.Add(key, value);
    }

    public void Clear()
    {
        _privateDictionary.Clear();
    }

    public bool Contains(KeyValuePair<TKey, TValue> item)
    {
        if (item.Key == null || !_privateDictionary.Contains(item.Key))
        {
            return false;
        }

        return _privateDictionary[item.Key].Equals(item.Value);
    }

    public bool ContainsKey(TKey key)
    {
        if (key == null) throw new ArgumentNullException(nameof(key));

        return _privateDictionary.Contains(key);
    }

    public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
    {
        if (array == null) throw new ArgumentNullException(nameof(array));
        if (arrayIndex < 0) throw new ArgumentOutOfRangeException(nameof(arrayIndex));
        if (array.Rank > 1 || arrayIndex >= array.Length
                           || array.Length - arrayIndex < _privateDictionary.Count)
            throw new ArgumentException("Bad Copy ToArray", nameof(array));

        var index = arrayIndex;
        foreach (DictionaryEntry entry in _privateDictionary)
        {
            array[index] = 
                new KeyValuePair<TKey, TValue>((TKey) entry.Key, (TValue) entry.Value);
            index++;
        }
    }

    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
    {
        foreach (DictionaryEntry entry in _privateDictionary)
        {
            yield return 
                new KeyValuePair<TKey, TValue>((TKey) entry.Key, (TValue) entry.Value);
        }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public bool Remove(KeyValuePair<TKey, TValue> item)
    {
        if (false == Contains(item)) return false;

        _privateDictionary.Remove(item.Key);

        return true;
    }

    public bool Remove(TKey key)
    {
        if (key == null) throw new ArgumentNullException(nameof(key));

        if (false == _privateDictionary.Contains(key)) return false;

        _privateDictionary.Remove(key);

        return true;
    }

    public bool TryGetValue(TKey key, out TValue value)
    {
        if (key == null) throw new ArgumentNullException(nameof(key));

        var keyExists = _privateDictionary.Contains(key);
        value = keyExists ? (TValue) _privateDictionary[key] : default(TValue);

        return keyExists;
    }
}

Pull requests/discussion accepted on GitHub

Answer 3

You're right. There's no generic equivalent of OrderedDictionary in the framework itself.

(That's still the case for .NET4 too, as far as I'm aware.)

EDIT: But you can vote for it at Visual Studio's UserVoice!

Answer 4

Implementing a generic OrderedDictionary isn't terribly difficult, but it's unnecessarily time consuming and frankly this class is a huge oversight on Microsoft's part. There are multiple ways of implementing this, but I chose to use a KeyedCollection for my internal storage. I also chose to implement various methods for sorting the way that List<T> does since this is essentially a hybrid IList and IDictionary. I've included my implementation here for posterity.

Here's the interface. Notice that it includes System.Collections.Specialized.IOrderedDictionary, which is the non-generic version of this interface that was provided by Microsoft.

// http://unlicense.org
using System;
using System.Collections.Generic;
using System.Collections.Specialized;

namespace mattmc3.Common.Collections.Generic {

    public interface IOrderedDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IOrderedDictionary {
        new TValue this[int index] { get; set; }
        new TValue this[TKey key] { get; set; }
        new int Count { get; }
        new ICollection<TKey> Keys { get; }
        new ICollection<TValue> Values { get; }
        new void Add(TKey key, TValue value);
        new void Clear();
        void Insert(int index, TKey key, TValue value);
        int IndexOf(TKey key);
        bool ContainsValue(TValue value);
        bool ContainsValue(TValue value, IEqualityComparer<TValue> comparer);
        new bool ContainsKey(TKey key);
        new IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator();
        new bool Remove(TKey key);
        new void RemoveAt(int index);
        new bool TryGetValue(TKey key, out TValue value);
        TValue GetValue(TKey key);
        void SetValue(TKey key, TValue value);
        KeyValuePair<TKey, TValue> GetItem(int index);
        void SetItem(int index, TValue value);
    }

}

Here's the implementation along with helper classes:

// http://unlicense.org
using System;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Collections;
using System.Collections.Specialized;
using System.Collections.Generic;
using System.Linq;

namespace mattmc3.Common.Collections.Generic {

    /// <summary>
    /// A dictionary object that allows rapid hash lookups using keys, but also
    /// maintains the key insertion order so that values can be retrieved by
    /// key index.
    /// </summary>
    public class OrderedDictionary<TKey, TValue> : IOrderedDictionary<TKey, TValue> {

        #region Fields/Properties

        private KeyedCollection2<TKey, KeyValuePair<TKey, TValue>> _keyedCollection;

        /// <summary>
        /// Gets or sets the value associated with the specified key.
        /// </summary>
        /// <param name="key">The key associated with the value to get or set.</param>
        public TValue this[TKey key] {
            get {
                return GetValue(key);
            }
            set {
                SetValue(key, value);
            }
        }

        /// <summary>
        /// Gets or sets the value at the specified index.
        /// </summary>
        /// <param name="index">The index of the value to get or set.</param>
        public TValue this[int index] {
            get {
                return GetItem(index).Value;
            }
            set {
                SetItem(index, value);
            }
        }

        public int Count {
            get { return _keyedCollection.Count; }
        }

        public ICollection<TKey> Keys {
            get {
                return _keyedCollection.Select(x => x.Key).ToList();
            }
        }

        public ICollection<TValue> Values {
            get {
                return _keyedCollection.Select(x => x.Value).ToList();
            }
        }

        public IEqualityComparer<TKey> Comparer {
            get;
            private set;
        }

        #endregion

        #region Constructors

        public OrderedDictionary() {
            Initialize();
        }

        public OrderedDictionary(IEqualityComparer<TKey> comparer) {
            Initialize(comparer);
        }

        public OrderedDictionary(IOrderedDictionary<TKey, TValue> dictionary) {
            Initialize();
            foreach (KeyValuePair<TKey, TValue> pair in dictionary) {
                _keyedCollection.Add(pair);
            }
        }

        public OrderedDictionary(IOrderedDictionary<TKey, TValue> dictionary, IEqualityComparer<TKey> comparer) {
            Initialize(comparer);
            foreach (KeyValuePair<TKey, TValue> pair in dictionary) {
                _keyedCollection.Add(pair);
            }
        }

        #endregion

        #region Methods

        private void Initialize(IEqualityComparer<TKey> comparer = null) {
            this.Comparer = comparer;
            if (comparer != null) {
                _keyedCollection = new KeyedCollection2<TKey, KeyValuePair<TKey, TValue>>(x => x.Key, comparer);
            }
            else {
                _keyedCollection = new KeyedCollection2<TKey, KeyValuePair<TKey, TValue>>(x => x.Key);
            }
        }

        public void Add(TKey key, TValue value) {
            _keyedCollection.Add(new KeyValuePair<TKey, TValue>(key, value));
        }

        public void Clear() {
            _keyedCollection.Clear();
        }

        public void Insert(int index, TKey key, TValue value) {
            _keyedCollection.Insert(index, new KeyValuePair<TKey, TValue>(key, value));
        }

        public int IndexOf(TKey key) {
            if (_keyedCollection.Contains(key)) {
                return _keyedCollection.IndexOf(_keyedCollection[key]);
            }
            else {
                return -1;
            }
        }

        public bool ContainsValue(TValue value) {
            return this.Values.Contains(value);
        }

        public bool ContainsValue(TValue value, IEqualityComparer<TValue> comparer) {
            return this.Values.Contains(value, comparer);
        }

        public bool ContainsKey(TKey key) {
            return _keyedCollection.Contains(key);
        }

        public KeyValuePair<TKey, TValue> GetItem(int index) {
            if (index < 0 || index >= _keyedCollection.Count) {
                throw new ArgumentException(String.Format("The index was outside the bounds of the dictionary: {0}", index));
            }
            return _keyedCollection[index];
        }

        /// <summary>
        /// Sets the value at the index specified.
        /// </summary>
        /// <param name="index">The index of the value desired</param>
        /// <param name="value">The value to set</param>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown when the index specified does not refer to a KeyValuePair in this object
        /// </exception>
        public void SetItem(int index, TValue value) {
            if (index < 0 || index >= _keyedCollection.Count) {
                throw new ArgumentException("The index is outside the bounds of the dictionary: {0}".FormatWith(index));
            }
            var kvp = new KeyValuePair<TKey, TValue>(_keyedCollection[index].Key, value);
            _keyedCollection[index] = kvp;
        }

        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() {
            return _keyedCollection.GetEnumerator();
        }

        public bool Remove(TKey key) {
            return _keyedCollection.Remove(key);
        }

        public void RemoveAt(int index) {
            if (index < 0 || index >= _keyedCollection.Count) {
                throw new ArgumentException(String.Format("The index was outside the bounds of the dictionary: {0}", index));
            }
            _keyedCollection.RemoveAt(index);
        }

        /// <summary>
        /// Gets the value associated with the specified key.
        /// </summary>
        /// <param name="key">The key associated with the value to get.</param>
        public TValue GetValue(TKey key) {
            if (_keyedCollection.Contains(key) == false) {
                throw new ArgumentException("The given key is not present in the dictionary: {0}".FormatWith(key));
            }
            var kvp = _keyedCollection[key];
            return kvp.Value;
        }

        /// <summary>
        /// Sets the value associated with the specified key.
        /// </summary>
        /// <param name="key">The key associated with the value to set.</param>
        /// <param name="value">The the value to set.</param>
        public void SetValue(TKey key, TValue value) {
            var kvp = new KeyValuePair<TKey, TValue>(key, value);
            var idx = IndexOf(key);
            if (idx > -1) {
                _keyedCollection[idx] = kvp;
            }
            else {
                _keyedCollection.Add(kvp);
            }
        }

        public bool TryGetValue(TKey key, out TValue value) {
            if (_keyedCollection.Contains(key)) {
                value = _keyedCollection[key].Value;
                return true;
            }
            else {
                value = default(TValue);
                return false;
            }
        }

        #endregion

        #region sorting
        public void SortKeys() {
            _keyedCollection.SortByKeys();
        }

        public void SortKeys(IComparer<TKey> comparer) {
            _keyedCollection.SortByKeys(comparer);
        }

        public void SortKeys(Comparison<TKey> comparison) {
            _keyedCollection.SortByKeys(comparison);
        }

        public void SortValues() {
            var comparer = Comparer<TValue>.Default;
            SortValues(comparer);
        }

        public void SortValues(IComparer<TValue> comparer) {
            _keyedCollection.Sort((x, y) => comparer.Compare(x.Value, y.Value));
        }

        public void SortValues(Comparison<TValue> comparison) {
            _keyedCollection.Sort((x, y) => comparison(x.Value, y.Value));
        }
        #endregion

        #region IDictionary<TKey, TValue>

        void IDictionary<TKey, TValue>.Add(TKey key, TValue value) {
            Add(key, value);
        }

        bool IDictionary<TKey, TValue>.ContainsKey(TKey key) {
            return ContainsKey(key);
        }

        ICollection<TKey> IDictionary<TKey, TValue>.Keys {
            get { return Keys; }
        }

        bool IDictionary<TKey, TValue>.Remove(TKey key) {
            return Remove(key);
        }

        bool IDictionary<TKey, TValue>.TryGetValue(TKey key, out TValue value) {
            return TryGetValue(key, out value);
        }

        ICollection<TValue> IDictionary<TKey, TValue>.Values {
            get { return Values; }
        }

        TValue IDictionary<TKey, TValue>.this[TKey key] {
            get {
                return this[key];
            }
            set {
                this[key] = value;
            }
        }

        #endregion

        #region ICollection<KeyValuePair<TKey, TValue>>

        void ICollection<KeyValuePair<TKey, TValue>>.Add(KeyValuePair<TKey, TValue> item) {
            _keyedCollection.Add(item);
        }

        void ICollection<KeyValuePair<TKey, TValue>>.Clear() {
            _keyedCollection.Clear();
        }

        bool ICollection<KeyValuePair<TKey, TValue>>.Contains(KeyValuePair<TKey, TValue> item) {
            return _keyedCollection.Contains(item);
        }

        void ICollection<KeyValuePair<TKey, TValue>>.CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex) {
            _keyedCollection.CopyTo(array, arrayIndex);
        }

        int ICollection<KeyValuePair<TKey, TValue>>.Count {
            get { return _keyedCollection.Count; }
        }

        bool ICollection<KeyValuePair<TKey, TValue>>.IsReadOnly {
            get { return false; }
        }

        bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> item) {
            return _keyedCollection.Remove(item);
        }

        #endregion

        #region IEnumerable<KeyValuePair<TKey, TValue>>

        IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator() {
            return GetEnumerator();
        }

        #endregion

        #region IEnumerable

        IEnumerator IEnumerable.GetEnumerator() {
            return GetEnumerator();
        }

        #endregion

        #region IOrderedDictionary

        IDictionaryEnumerator IOrderedDictionary.GetEnumerator() {
            return new DictionaryEnumerator<TKey, TValue>(this);
        }

        void IOrderedDictionary.Insert(int index, object key, object value) {
            Insert(index, (TKey)key, (TValue)value);
        }

        void IOrderedDictionary.RemoveAt(int index) {
            RemoveAt(index);
        }

        object IOrderedDictionary.this[int index] {
            get {
                return this[index];
            }
            set {
                this[index] = (TValue)value;
            }
        }

        #endregion

        #region IDictionary

        void IDictionary.Add(object key, object value) {
            Add((TKey)key, (TValue)value);
        }

        void IDictionary.Clear() {
            Clear();
        }

        bool IDictionary.Contains(object key) {
            return _keyedCollection.Contains((TKey)key);
        }

        IDictionaryEnumerator IDictionary.GetEnumerator() {
            return new DictionaryEnumerator<TKey, TValue>(this);
        }

        bool IDictionary.IsFixedSize {
            get { return false; }
        }

        bool IDictionary.IsReadOnly {
            get { return false; }
        }

        ICollection IDictionary.Keys {
            get { return (ICollection)this.Keys; }
        }

        void IDictionary.Remove(object key) {
            Remove((TKey)key);
        }

        ICollection IDictionary.Values {
            get { return (ICollection)this.Values; }
        }

        object IDictionary.this[object key] {
            get {
                return this[(TKey)key];
            }
            set {
                this[(TKey)key] = (TValue)value;
            }
        }

        #endregion

        #region ICollection

        void ICollection.CopyTo(Array array, int index) {
            ((ICollection)_keyedCollection).CopyTo(array, index);
        }

        int ICollection.Count {
            get { return ((ICollection)_keyedCollection).Count; }
        }

        bool ICollection.IsSynchronized {
            get { return ((ICollection)_keyedCollection).IsSynchronized; }
        }

        object ICollection.SyncRoot {
            get { return ((ICollection)_keyedCollection).SyncRoot; }
        }

        #endregion
    }

    public class KeyedCollection2<TKey, TItem> : KeyedCollection<TKey, TItem> {
        private const string DelegateNullExceptionMessage = "Delegate passed cannot be null";
        private Func<TItem, TKey> _getKeyForItemDelegate;

        public KeyedCollection2(Func<TItem, TKey> getKeyForItemDelegate)
            : base() {
            if (getKeyForItemDelegate == null) throw new ArgumentNullException(DelegateNullExceptionMessage);
            _getKeyForItemDelegate = getKeyForItemDelegate;
        }

        public KeyedCollection2(Func<TItem, TKey> getKeyForItemDelegate, IEqualityComparer<TKey> comparer)
            : base(comparer) {
            if (getKeyForItemDelegate == null) throw new ArgumentNullException(DelegateNullExceptionMessage);
            _getKeyForItemDelegate = getKeyForItemDelegate;
        }

        protected override TKey GetKeyForItem(TItem item) {
            return _getKeyForItemDelegate(item);
        }

        public void SortByKeys() {
            var comparer = Comparer<TKey>.Default;
            SortByKeys(comparer);
        }

        public void SortByKeys(IComparer<TKey> keyComparer) {
            var comparer = new Comparer2<TItem>((x, y) => keyComparer.Compare(GetKeyForItem(x), GetKeyForItem(y)));
            Sort(comparer);
        }

        public void SortByKeys(Comparison<TKey> keyComparison) {
            var comparer = new Comparer2<TItem>((x, y) => keyComparison(GetKeyForItem(x), GetKeyForItem(y)));
            Sort(comparer);
        }

        public void Sort() {
            var comparer = Comparer<TItem>.Default;
            Sort(comparer);
        }

        public void Sort(Comparison<TItem> comparison) {
            var newComparer = new Comparer2<TItem>((x, y) => comparison(x, y));
            Sort(newComparer);
        }

        public void Sort(IComparer<TItem> comparer) {
            List<TItem> list = base.Items as List<TItem>;
            if (list != null) {
                list.Sort(comparer);
            }
        }
    }

    public class Comparer2<T> : Comparer<T> {
        //private readonly Func<T, T, int> _compareFunction;
        private readonly Comparison<T> _compareFunction;

        #region Constructors

        public Comparer2(Comparison<T> comparison) {
            if (comparison == null) throw new ArgumentNullException("comparison");
            _compareFunction = comparison;
        }

        #endregion

        public override int Compare(T arg1, T arg2) {
            return _compareFunction(arg1, arg2);
        }
    }

    public class DictionaryEnumerator<TKey, TValue> : IDictionaryEnumerator, IDisposable {
        readonly IEnumerator<KeyValuePair<TKey, TValue>> impl;
        public void Dispose() { impl.Dispose(); }
        public DictionaryEnumerator(IDictionary<TKey, TValue> value) {
            this.impl = value.GetEnumerator();
        }
        public void Reset() { impl.Reset(); }
        public bool MoveNext() { return impl.MoveNext(); }
        public DictionaryEntry Entry {
            get {
                var pair = impl.Current;
                return new DictionaryEntry(pair.Key, pair.Value);
            }
        }
        public object Key { get { return impl.Current.Key; } }
        public object Value { get { return impl.Current.Value; } }
        public object Current { get { return Entry; } }
    }
}

And no implementation would be complete without a few tests (but tragically, SO won't let me post that much code in one post), so I'll have to leave you to write your tests. But, I left a few of them in so that you could get an idea of how it works:

// http://unlicense.org
using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using mattmc3.Common.Collections.Generic;

namespace mattmc3.Tests.Common.Collections.Generic {
    [TestClass]
    public class OrderedDictionaryTests {

        private OrderedDictionary<string, string> GetAlphabetDictionary(IEqualityComparer<string> comparer = null) {
            OrderedDictionary<string, string> alphabet = (comparer == null ? new OrderedDictionary<string, string>() : new OrderedDictionary<string, string>(comparer));
            for (var a = Convert.ToInt32('a'); a <= Convert.ToInt32('z'); a++) {
                var c = Convert.ToChar(a);
                alphabet.Add(c.ToString(), c.ToString().ToUpper());
            }
            Assert.AreEqual(26, alphabet.Count);
            return alphabet;
        }

        private List<KeyValuePair<string, string>> GetAlphabetList() {
            var alphabet = new List<KeyValuePair<string, string>>();
            for (var a = Convert.ToInt32('a'); a <= Convert.ToInt32('z'); a++) {
                var c = Convert.ToChar(a);
                alphabet.Add(new KeyValuePair<string, string>(c.ToString(), c.ToString().ToUpper()));
            }
            Assert.AreEqual(26, alphabet.Count);
            return alphabet;
        }

        [TestMethod]
        public void TestAdd() {
            var od = new OrderedDictionary<string, string>();
            Assert.AreEqual(0, od.Count);
            Assert.AreEqual(-1, od.IndexOf("foo"));

            od.Add("foo", "bar");
            Assert.AreEqual(1, od.Count);
            Assert.AreEqual(0, od.IndexOf("foo"));
            Assert.AreEqual(od[0], "bar");
            Assert.AreEqual(od["foo"], "bar");
            Assert.AreEqual(od.GetItem(0).Key, "foo");
            Assert.AreEqual(od.GetItem(0).Value, "bar");
        }

        [TestMethod]
        public void TestRemove() {
            var od = new OrderedDictionary<string, string>();

            od.Add("foo", "bar");
            Assert.AreEqual(1, od.Count);

            od.Remove("foo");
            Assert.AreEqual(0, od.Count);
        }

        [TestMethod]
        public void TestRemoveAt() {
            var od = new OrderedDictionary<string, string>();

            od.Add("foo", "bar");
            Assert.AreEqual(1, od.Count);

            od.RemoveAt(0);
            Assert.AreEqual(0, od.Count);
        }

        [TestMethod]
        public void TestClear() {
            var od = GetAlphabetDictionary();
            Assert.AreEqual(26, od.Count);
            od.Clear();
            Assert.AreEqual(0, od.Count);
        }

        [TestMethod]
        public void TestOrderIsPreserved() {
            var alphabetDict = GetAlphabetDictionary();
            var alphabetList = GetAlphabetList();
            Assert.AreEqual(26, alphabetDict.Count);
            Assert.AreEqual(26, alphabetList.Count);

            var keys = alphabetDict.Keys.ToList();
            var values = alphabetDict.Values.ToList();

            for (var i = 0; i < 26; i++) {
                var dictItem = alphabetDict.GetItem(i);
                var listItem = alphabetList[i];
                var key = keys[i];
                var value = values[i];

                Assert.AreEqual(dictItem, listItem);
                Assert.AreEqual(key, listItem.Key);
                Assert.AreEqual(value, listItem.Value);
            }
        }

        [TestMethod]
        public void TestTryGetValue() {
            var alphabetDict = GetAlphabetDictionary();
            string result = null;
            Assert.IsFalse(alphabetDict.TryGetValue("abc", out result));
            Assert.IsNull(result);
            Assert.IsTrue(alphabetDict.TryGetValue("z", out result));
            Assert.AreEqual("Z", result);
        }

        [TestMethod]
        public void TestEnumerator() {
            var alphabetDict = GetAlphabetDictionary();

            var keys = alphabetDict.Keys.ToList();
            Assert.AreEqual(26, keys.Count);

            var i = 0;
            foreach (var kvp in alphabetDict) {
                var value = alphabetDict[kvp.Key];
                Assert.AreEqual(kvp.Value, value);
                i++;
            }
        }

        [TestMethod]
        public void TestInvalidIndex() {
            var alphabetDict = GetAlphabetDictionary();
            try {
                var notGonnaWork = alphabetDict[100];
                Assert.IsTrue(false, "Exception should have thrown");
            }
            catch (Exception ex) {
                Assert.IsTrue(ex.Message.Contains("index is outside the bounds"));
            }
        }

        [TestMethod]
        public void TestMissingKey() {
            var alphabetDict = GetAlphabetDictionary();
            try {
                var notGonnaWork = alphabetDict["abc"];
                Assert.IsTrue(false, "Exception should have thrown");
            }
            catch (Exception ex) {
                Assert.IsTrue(ex.Message.Contains("key is not present"));
            }
        }

        [TestMethod]
        public void TestUpdateExistingValue() {
            var alphabetDict = GetAlphabetDictionary();
            Assert.IsTrue(alphabetDict.ContainsKey("c"));
            Assert.AreEqual(2, alphabetDict.IndexOf("c"));
            Assert.AreEqual(alphabetDict[2], "C");
            alphabetDict[2] = "CCC";
            Assert.IsTrue(alphabetDict.ContainsKey("c"));
            Assert.AreEqual(2, alphabetDict.IndexOf("c"));
            Assert.AreEqual(alphabetDict[2], "CCC");
        }

        [TestMethod]
        public void TestInsertValue() {
            var alphabetDict = GetAlphabetDictionary();
            Assert.IsTrue(alphabetDict.ContainsKey("c"));
            Assert.AreEqual(2, alphabetDict.IndexOf("c"));
            Assert.AreEqual(alphabetDict[2], "C");
            Assert.AreEqual(26, alphabetDict.Count);
            Assert.IsFalse(alphabetDict.ContainsValue("ABC"));

            alphabetDict.Insert(2, "abc", "ABC");
            Assert.IsTrue(alphabetDict.ContainsKey("c"));
            Assert.AreEqual(2, alphabetDict.IndexOf("abc"));
            Assert.AreEqual(alphabetDict[2], "ABC");
            Assert.AreEqual(27, alphabetDict.Count);
            Assert.IsTrue(alphabetDict.ContainsValue("ABC"));
        }

        [TestMethod]
        public void TestValueComparer() {
            var alphabetDict = GetAlphabetDictionary();
            Assert.IsFalse(alphabetDict.ContainsValue("a"));
            Assert.IsTrue(alphabetDict.ContainsValue("a", StringComparer.OrdinalIgnoreCase));
        }

        [TestMethod]
        public void TestSortByKeys() {
            var alphabetDict = GetAlphabetDictionary();
            var reverseAlphabetDict = GetAlphabetDictionary();
            Comparison<string> stringReverse = ((x, y) => (String.Equals(x, y) ? 0 : String.Compare(x, y) >= 1 ? -1 : 1));
            reverseAlphabetDict.SortKeys(stringReverse);
            for (int j = 0, k = 25; j < alphabetDict.Count; j++, k--) {
                var ascValue = alphabetDict.GetItem(j);
                var dscValue = reverseAlphabetDict.GetItem(k);
                Assert.AreEqual(ascValue.Key, dscValue.Key);
                Assert.AreEqual(ascValue.Value, dscValue.Value);
            }
        }

-- UPDATE --

Source for this and other really useful missing core .NET libraries here: https://github.com/mattmc3/dotmore/blob/master/dotmore/Collections/Generic/OrderedDictionary.cs

Answer 5

Right, it's an unfortunate omission. I miss Python's OrderedDict

A dictionary that remembers the order that keys were first inserted. If a new entry overwrites an existing entry, the original insertion position is left unchanged. Deleting an entry and reinserting it will move it to the end.

So I wrote my own OrderedDictionary<K,V> class in C#. How does it work? It maintains two collections - a vanilla unordered dictionary and an ordered list of keys. With this solution, the standard dictionary operations keep their fast complexities, and look up by index is fast too.

https://gist.github.com/hickford/5137384

Here's the interface

/// <summary>
/// A dictionary that remembers the order that keys were first inserted. If a new entry overwrites an existing entry, the original insertion position is left unchanged. Deleting an entry and reinserting it will move it to the end.
/// </summary>
/// <typeparam name="TKey">The type of keys</typeparam>
/// <typeparam name="TValue">The type of values</typeparam>
public interface IOrderedDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
    /// <summary>
    /// The value of the element at the given index.
    /// </summary>
    TValue this[int index] { get; set; }

    /// <summary>
    /// Find the position of an element by key. Returns -1 if the dictionary does not contain an element with the given key.
    /// </summary>
    int IndexOf(TKey key);

    /// <summary>
    /// Insert an element at the given index.
    /// </summary>
    void Insert(int index, TKey key, TValue value);

    /// <summary>
    /// Remove the element at the given index.
    /// </summary>
    void RemoveAt(int index);
}