Use LINQ to get items in one List<>, that are not in another List<>

user_331463 0
Viewed 569342

I would assume there's a simple LINQ query to do this, I'm just not exactly sure how.

Given this piece of code:

class Program
{
    static void Main(string[] args)
    {
        List<Person> peopleList1 = new List<Person>();
        peopleList1.Add(new Person() { ID = 1 });
        peopleList1.Add(new Person() { ID = 2 });
        peopleList1.Add(new Person() { ID = 3 });

        List<Person> peopleList2 = new List<Person>();
        peopleList2.Add(new Person() { ID = 1 });
        peopleList2.Add(new Person() { ID = 2 });
        peopleList2.Add(new Person() { ID = 3 });
        peopleList2.Add(new Person() { ID = 4 });
        peopleList2.Add(new Person() { ID = 5 });
    }
}

class Person
{
    public int ID { get; set; }
}

I would like to perform a LINQ query to give me all of the people in peopleList2 that are not in peopleList1.

This example should give me two people (ID = 4 & ID = 5)

.net-3.5 linq c#
11 Answers

Once you write a generic FuncEqualityComparer you can use it everywhere.

peopleList2.Except(peopleList1, new FuncEqualityComparer<Person>((p, q) => p.ID == q.ID));

public class FuncEqualityComparer<T> : IEqualityComparer<T>
{
    private readonly Func<T, T, bool> comparer;
    private readonly Func<T, int> hash;

    public FuncEqualityComparer(Func<T, T, bool> comparer)
    {
        this.comparer = comparer;
        if (typeof(T).GetMethod(nameof(object.GetHashCode)).DeclaringType == typeof(object))
            hash = (_) => 0;
        else
            hash = t => t.GetHashCode(); 
    }

    public bool Equals(T x, T y) => comparer(x, y);
    public int GetHashCode(T obj) => hash(obj);
}
user_44917680

first, extract ids from the collection where condition

List<int> indexes_Yes = this.Contenido.Where(x => x.key == 'TEST').Select(x => x.Id).ToList();

second, use "compare" estament to select ids diffent to the selection

List<int> indexes_No = this.Contenido.Where(x => !indexes_Yes.Contains(x.Id)).Select(x => x.Id).ToList();

Obviously you can use x.key != "TEST", but is only a example

user_82200820 
{
    static void Main(string[] args)
    {
        List<Person> peopleList1 = new List<Person>();
        peopleList1.Add(new Person() { ID = 1 });
        peopleList1.Add(new Person() { ID = 2 });
        peopleList1.Add(new Person() { ID = 3 });

        List<Person> peopleList2 = new List<Person>();
        peopleList2.Add(new Person() { ID = 1 });
        peopleList2.Add(new Person() { ID = 2 });
        peopleList2.Add(new Person() { ID = 3 });
        peopleList2.Add(new Person() { ID = 4 });
        peopleList2.Add(new Person() { ID = 5 });
    }

    var leftPeeps = peopleList2.Where(x => !peopleList1.Select(y => y.ID).Contains(x.ID))?.ToList() ?? new List<Person>();
}

class Person
{
    public int ID { get; set; }
}

Notice the !peopleList1.Select(y => y.ID).Contains(x.ID) Select statement. This allows us to grab the indexer we want (ID) and see if it contains the ID of the previous list. ! means we don't want those. This may leave us with no entries. so, we can ensure we have something by checking for null and using a null coalesce.

user_51207190
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