Generic Key/Value pair collection in that preserves insertion order?

Here is a wrapper for the non-generic Systems.Collections.Specialized.OrderedDictionary type.

This type will return keys/value/pairs sequences in insertion order, much like Ruby 2.0 hashes.

It does not require C#6 magic, conforms to IDictionary<TKey,TValue> (which also means that accessing a non-assigned key throws an exception), and ought to be serializable.

It is given the name 'IndexedDictionary' per note on Adrian's answer.

YMMV.

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

/// <summary>
/// A dictionary that maintains insertion ordering of keys.
/// 
/// This is useful for emitting JSON where it is preferable to keep the key ordering
/// for various human-friendlier reasons.
/// 
/// There is no support to manually re-order keys or to access keys
/// by index without using Keys/Values or the Enumerator (eg).
/// </summary>
[Serializable]
public sealed class IndexedDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
    // Non-generic version only in .NET 4.5
    private readonly OrderedDictionary _backing = new OrderedDictionary();

    private IEnumerable<KeyValuePair<TKey, TValue>> KeyValuePairs
    {
        get
        {
            return _backing.OfType<DictionaryEntry>()
                .Select(e => new KeyValuePair<TKey, TValue>((TKey)e.Key, (TValue)e.Value));
        }
    }

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

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

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

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

    public bool Contains(KeyValuePair<TKey, TValue> item)
    {
        return _backing.Contains(item.Key);
    }

    public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
    {
        KeyValuePairs.ToList().CopyTo(array, arrayIndex);
    }

    public bool Remove(KeyValuePair<TKey, TValue> item)
    {
        TValue value;
        if (TryGetValue(item.Key, out value)
            && Equals(value, item.Value))
        {
            Remove(item.Key);
            return true;
        }
        return false;
    }

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

    public bool IsReadOnly
    {
        get { return _backing.IsReadOnly; }
    }

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

    public void Add(TKey key, TValue value)
    {
        _backing.Add(key, value);
    }

    public bool Remove(TKey key)
    {
        var result = _backing.Contains(key);
        if (result) {
            _backing.Remove(key);
        }
        return result;
    }

    public bool TryGetValue(TKey key, out TValue value)
    {
        object foundValue;
        if ((foundValue = _backing[key]) != null
            || _backing.Contains(key))
        {
            // Either found with a non-null value, or contained value is null.
            value = (TValue)foundValue;
            return true;
        }
        value = default(TValue);
        return false;
    }

    public TValue this[TKey key]
    {
        get
        {
            TValue value;
            if (TryGetValue(key, out value))
                return value;
            throw new KeyNotFoundException();
        }
        set { _backing[key] = value; }
    }

    public ICollection<TKey> Keys
    {
        get { return _backing.Keys.OfType<TKey>().ToList(); }
    }

    public ICollection<TValue> Values
    {
        get { return _backing.Values.OfType<TValue>().ToList(); }
    }
}

If you need constant complexity of Add, Remove, ContainsKey and order preservation, then there's no such generic in .NET Framework 4.5.

If you're okay with 3rd party code, take a look at my repository (permissive MIT license): https://github.com/OndrejPetrzilka/Rock.Collections

There's OrderedDictionary<K,V> collection:

• source code based on classic Dictionary<K,V> (from .NET Core)
• preserves order of insertions and allows manual reordering
• features reversed enumeration
• has same operation complexities as Dictionary<K,V>
• Add and Remove operations are ~20% slower compared to Dictionary<K,V>
• consumes 8 more bytes of memory per item