C# LINQ where list1 contains list2 without duplicates

Question

I have the following lists

var list1 = new List<Items>()
{
    new Items { Id = 1, Name = "PC" },
    new Items { Id = 2, Name = "Mug" },
    new Items { Id = 3, Name = "Table" },
    new Items { Id = 4, Name = "Table" }
};

var list2 = new List<string> { "Mug", "Table", "Buld" };

I am trying to get a list where list1 contains list2 without duplication

var list1 = new List<Items>()
{
    new Items { Id = 1, Name = "PC" },
    new Items { Id = 2, Name = "Mug" },
    new Items { Id = 3, Name = "Table" }
};

This is what I tried but does not work

var result = list1.Where(t1 => list2.Any(x => x == t1.Name)).ToList();

GroupBy + First, or Distinct with a comparer on Name. More linQ distinct By — Self, Apr 15 '21 at 13:50
Does this answer your question? [How to get a distinct list from a List of objects?](https://stackoverflow.com/questions/4991728/how-to-get-a-distinct-list-from-a-list-of-objects) — Self, Apr 15 '21 at 13:50
And [LINQ's Distinct() on a particular property](https://stackoverflow.com/questions/489258/linqs-distinct-on-a-particular-property) — Self, Apr 15 '21 at 13:51
Why is `{ Id = 1, Name = "PC" }` included in the output, when "PC" isn't in `list2`? — Johnathan Barclay, Apr 15 '21 at 13:52
So I should remove first the duplication and then compare both lists? — Bug, Apr 15 '21 at 13:55
Filter then remove duplicate look the less taxing as you don't have to clear dupe that don't match. — Self, Apr 15 '21 at 13:57
Your `list1.Where(t1 => list2.Any(x => x == t1.Name))` is equivalent to the proposed `list1.Where(item => list2.Contains(item.Name))`. The only missing part is the `.GroupBy(x => x.Property).Select(group => group.First())`. From the dupe target. — Self, Apr 15 '21 at 14:04

score 0 · Accepted Answer · answered Apr 15 '21 at 13:54

Something like this:

   var result = list1
     .Where(item => list2.Contains(item.Name)) // Name must be in list2
     .GroupBy(item => item.Name)
     .Select(group => group.First())  // no duplicates: 1st item from each group
     .ToList();

There's a question: when having duplicates which item should be taken. In the code above I take the item which appears first in the list1. If you want to take, say, the item with the smallest Id you should modify the Select:

      ...
      .Select(group => group.OrderBy(item => item.Id).First())
      ...

C# LINQ where list1 contains list2 without duplicates

1 Answers1