Select distinct by two properties in a list

Question

I have a list<message> that contains properties of type Guid and DateTime (as well as other properties). I would like to get rid of all of the items in that list where the Guid and DateTime are the same (except one). There will be times when those two properties will be the same as other items in the list, but the other properties will be different, so I can't just use .Distinct()

List<Message> messages = GetList();
//The list now contains many objects, it is ordered by the DateTime property

messages = from p in messages.Distinct(  what goes here? ); 

This is what I have right now, but it seems like there ought to be a better way

List<Message> messages = GetList();

for(int i = 0; i < messages.Count() - 1)  //use Messages.Count() -1 because the last one has nothing after it to compare to
{
    if(messages[i].id == messages[i+1}.id && messages[i].date == message[i+1].date)
    {
        messages.RemoveAt(i+1);
    {
    else
    {
         i++
    }
}

Answer 1

LINQ to Objects doesn't provide this functionality easily in a built-in way, but MoreLINQ has a handy DistinctBy method:

messages = messages.DistinctBy(m => new { m.id, m.date }).ToList();

Answer 2

Jon Skeet's DistinctBy is definitely the way to go, however if you are interested in defining your own extension method you might take fancy in this more concise version:

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
    var known = new HashSet<TKey>();
    return source.Where(element => known.Add(keySelector(element)));
}

which has the same signature:

messages = messages.DistinctBy(x => new { x.id, x.date }).ToList();

Answer 3

What about this?

var messages = messages
               .GroupBy(m => m.id)
               .GroupBy(m => m.date)
               .Select(m => m.First());

Answer 4

You can check out my PowerfulExtensions library. Currently it's in a very young stage, but already you can use methods like Distinct, Union, Intersect, Except on any number of properties;

This is how you use it:

using PowerfulExtensions.Linq;
...
var distinct = myArray.Distinct(x => x.A, x => x.B);

Answer 5

Try this,

 var messages = (from g1 in messages.GroupBy(s => s.id) from g2 in g1.GroupBy(s => s.date) select g2.First()).ToList();