>>>>Using a Dictionary < Tkey, TValue > would be better
>>>
>>>Thanks, I remember having read some of that in previous threads. I might have to consider that.
>>
>>
>>You can update the value of a dictionary
>>
>>Here, I use a Tuple of three integers - for simplicity the first one is the key
>>
>>
>>

>>		public static void Main()
>>		{
>>
>>
>>			Dictionary<int, Tuple<int, int, int>> dict = new Dictionary<int, Tuple<int, int, int>>();
>>
>>			Tuple<int, int, int> t;  // key is tuple.Item1
>>
>>			t = new Tuple<int, int, int>(1, 1, 1);
>>			UpdateDictionary(dict, t);
>>
>>			t = new Tuple<int, int, int>(2, 1, 1);
>>			UpdateDictionary(dict, t);
>>
>>			PrintDictionary("Before any update", dict);
>>
>>			t = new Tuple<int, int, int>(2, 2, 2);
>>			UpdateDictionary(dict, t);
>>
>>			PrintDictionary("After update", dict);
>>			Console.ReadLine();
>>		}
>>		static void UpdateDictionary(Dictionary<int, Tuple<int, int, int>> dict, Tuple<int, int, int> t)
>>		{
>>			if (dict.ContainsKey(t.Item1) )
>>			{
>>				dict[t.Item1] = t;
>>			}
>>			else
>>				dict.Add(t.Item1, t);
>>		}
>>		static void PrintDictionary(string msg, Dictionary<int, Tuple<int, int, int>> dict)
>>		{
>>			Console.WriteLine(msg);
>>			foreach (KeyValuePair<int, Tuple<int, int, int>> x in dict)
>>			{
>>				Console.WriteLine("{0} {1} {2}", x.Value.Item1, x.Value.Item2, x.Value.Item3);
>>			}
>>
>>		}
>>

Since the key is in the data I'd consider using KeyedCollection instead of Dictionary ?
>UPDATE : Assuming the key is an int.....

Well - I made the key part of the data for simplicity - it doesn't need to be

If you go for a KeyedCollection, wouldn't you need to subclass KeyedCollection first (and implement GetKeyForItem() ? http://msdn.microsoft.com/en-us/library/ms132454.aspx)