gistfile1.cs

public class ControllerDictionary<TKey, TValue, TCast> : Dictionary<TKey, TValue> where TValue : UIViewController where TCast : TValue
{
	public new TValue this[TKey key]
	{
		get
		{
			if (ContainsKey(key))
			{
				return base[key] as TCast;
			}
			else
			{
				return default(TValue) as TCast;
			}
		}

		set
		{ 
			if (ContainsKey)
			{
				base[key] = value;
			}
			else
			{
				Add(key, value);
			}
		}
	}
}