The famous, widely needed, DynamicDictionary class.

DynamicDictionary.cs

namespace System.Dynamic
{
	using System;
	using System.Collections.Generic;

	public class DynamicDictionary : DynamicObject
	{
		private Dictionary<string, object> items;		

		public DynamicDictionary()
		{
			this.items = new Dictionary<string, object>();
		}

		public void Add(string key, object value)
		{
			items.Add(key, value);
		}

		public bool ContainsKey(string key)
		{
			return items.ContainsKey(key);
		}

		public bool Remove(string key)
		{
			return items.Remove(key);
		}

		public object this[string key]
		{
			get
			{
				return items[key];
			}
			set
			{
				items[key] = value;
			}
		}

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

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

		public bool IsReadOnly
		{
			get { return ((ICollection<KeyValuePair<string, object>>)items).IsReadOnly; }
		}

		public bool Remove(KeyValuePair<string, object> item)
		{
			return ((ICollection<KeyValuePair<string, object>>)items).Remove(item);
		}

		public override bool TryGetMember(GetMemberBinder binder, out object result)
		{
			return items.TryGetValue(binder.Name, out result);			
		}

		public override bool TrySetMember(SetMemberBinder binder, object value)
		{
			if (items.ContainsKey(binder.Name))
			{
				items[binder.Name] = value;
				return true;
			}
			return false;			
		}

		public override IEnumerable<string> GetDynamicMemberNames()
		{
			var iter= items.GetEnumerator();
			while (iter.MoveNext())
			{
				yield return iter.Current.Key;
			}
		}
	}
}