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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 of an Object on one or more properties of the object?

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 in .NET 3.5?

2020/05/06
1
1143
5/6/2020 8:49:42 AM

Accepted Answer

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.

2015/11/04
1291
11/4/2015 12:25:47 PM


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.

2018/12/31

You could also use query syntax if you want it to look all LINQ-like:

var uniquePeople = from p in people
                   group p by new {p.ID} //or group by new {p.ID, p.Name, p.Whatever}
                   into mygroup
                   select mygroup.FirstOrDefault();
2017/01/20

I think it is enough:

list.Select(s => s.MyField).Distinct();
2015/01/23

Solution first group by your fields then select firstordefault item.

    List<Person> distinctPeople = allPeople
   .GroupBy(p => p.PersonId)
   .Select(g => g.FirstOrDefault())
   .ToList();
2017/07/13

You can do this with the standard Linq.ToLookup(). This will create a collection of values for each unique key. Just select the first item in the collection

Persons.ToLookup(p => p.Id).Select(coll => coll.First());
2016/01/16