LINQ's Distinct() on a particular property

What if I want to obtain a distinct list based on one or more properties?

Simple! You want to group them and pick a winner out of the group.

List<Person> distinctPeople = allPeople
  .GroupBy(p => p.PersonId)
  .Select(g => g.First())
  .ToList();

If you want to define groups on multiple properties, here's how:

List<Person> distinctPeople = allPeople
  .GroupBy(p => new {p.PersonId, p.FavoriteColor} )
  .Select(g => g.First())
  .ToList();

EDIT: This is now part of MoreLINQ.

What you need is a "distinct-by" effectively. I don't believe it's part of LINQ as it stands, although it's fairly easy to write:

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
    (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
    HashSet<TKey> seenKeys = new HashSet<TKey>();
    foreach (TSource element in source)
    {
        if (seenKeys.Add(keySelector(element)))
        {
            yield return element;
        }
    }
}

So to find the distinct values using just the Id property, you could use:

var query = people.DistinctBy(p => p.Id);

And to use multiple properties, you can use anonymous types, which implement equality appropriately:

var query = people.DistinctBy(p => new { p.Id, p.Name });

Untested, but it should work (and it now at least compiles).

It assumes the default comparer for the keys though - if you want to pass in an equality comparer, just pass it on to the HashSet constructor.

Use:

List<Person> pList = new List<Person>();
/* Fill list */

var result = pList.Where(p => p.Name != null).GroupBy(p => p.Id).Select(grp => grp.FirstOrDefault());

The where helps you filter the entries (could be more complex) and the groupby and select perform the distinct function.