LINQ's Distinct() on a particular property

Question

I am playing with LINQ to learn about it, but I can't figure out how to use Distinct when I do not have a simple list (a simple list of integers is pretty easy to do, this is not the question). What I if want to use Distinct on a List<TElement> on one or more properties of the TElement?

Example: If an object is Person, with property Id. How can I get all Person and use Distinct on them with the property Id of the object?

Person1: Id=1, Name="Test1"
Person2: Id=1, Name="Test1"
Person3: Id=2, Name="Test2"

How can I get just Person1 and Person3? Is that possible?

If it's not possible with LINQ, what would be the best way to have a list of Person depending on some of its properties?

Answer 1

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.