Select Distinct from a list of IEnumerable<T> in .NET 2.0

Question

I am trying to filter an IEnumerable object of the duplicate values, so I would like to get the distinct values from it, for example, lets say that it holds days:

monday tuesday wednesday wednesday

I would like to filter it and return:

monday tuesday wednesday

What is the most efficient way to do this in .net 2.0?

Answer 1

Dictionary<object, object> list = new Dictionary<object, object>();
foreach (object o in enumerable)
    if (!list.ContainsKey(o))
    {
        // Do the actual work.
        list[o] = null;
    }

Dictionary will use a hash table to hold keys therefore lookup is efficient.

Sorting will be O(n log(n)) at best. A hash table with an efficient hash function often outperforms it (O(1) lookups).

Answer 2

Make another IEnumerable. Sort the original. For each item in the original, if the new one doesn't contain the old one, add it.

Answer 3

Another alternative is to use HashSet<T> - a HashSet doesn't allow duplicate items to be used and doesn't require a key/value pair.