如何在列表中查找属性具有特定值或具有与此值的相关项的所有项？

Question

How to get(or filter) list where there are record with the same userID and artistID

Here is my rating object

public class Rating
{
    public int userID { get; set; }
    public int artistID { get; set; }
    public int rating { get; set; }
}

and here is my data

Rating rate1 = new Rating { artistID = 1, userID = 101, rating = 2 };
Rating rate2 = new Rating { artistID = 1, userID = 102, rating = 4 };
Rating rate3 = new Rating { artistID = 2, userID = 101, rating = 3 };
Rating rate4 = new Rating { artistID = 2, userID = 102, rating = 5 };
Rating rate5 = new Rating { artistID = 3, userID = 102, rating = 1 };

List<Rating> ratings = new List<Rating>(2);
ratings.Add(rate1);
ratings.Add(rate2);
ratings.Add(rate3);
ratings.Add(rate4);
ratings.Add(rate5);

Output if there ara record : where userID = 101, and where (artistID where userID is 101)

Output example that i want to have:

artistID    userID      rating
1           101         2
1           102         4
2           101         3
2           102         5

I want also 1, 102, 4 because there is another rating with this artistID which userID is 101. The same applies to 2, 102, 5 .

Update If You want to include all records where there is another record with a different user but the same artist (quote from @Tim Schmelter). You can found answer from @Tim Schmelter in Update version.

For example, if u change rate5 to Rating rate5 = new Rating { artistID = 3, userID = 101, rating = 1 }; and also add new object rate6 Rating rate6 = new Rating { artistID = 3, userID = 102, rating = 1 };

It will make result:

artistID    userID      rating
1           101         2
1           102         4
2           101         3
2           102         5
3           101         1
3           102         1

Because artistID that rated by userID-101 also rated by userid-102 you can found answer in @Tim Schmelter answer.

Answer 1

So you want all ratings where either the userID is 101 or there is one with the same artistID that has userID = 101? You can use Any in Where :

var query = ratings.Where(r =>  r.userID  == 101 || 
              ratings.Any(rr => rr.userID == 101 && r.artistID == rr.artistID));

This is similar to a correlated subquery in sql.

Test:

foreach (Rating r in query)
    Console.WriteLine("artistID = {0}, userID = {1}, rating = {2}"
                   , r.artistID, r.userID, r.rating);

Result:

artistID = 1, userID = 101, rating = 2
artistID = 1, userID = 102, rating = 4
artistID = 2, userID = 101, rating = 3
artistID = 2, userID = 102, rating = 5

---

Update "i want all records where there is another record with a different user but the same artist"

Now it's clear, you want this:

var query = ratings
    .Where(r => ratings
        .Any(rr => r.artistID == rr.artistID && rr.userID != r.userID));