Intersect between 2 collections in C#

Question

I have:

            List<INFRAESTRUCTURA> l1 = listQ1.ToList();
            List<INFRAESTRUCTURA> l2 = listQ2.ToList();

And I need to intersect it comparing ids. Something like that:

 l1.Intersect(l2, l1[].id_infraestructura == l2[].id_infraestructura)

But I don't know which method I must use and it sintax.

I found this:

var ids = list1.Select(a => a.id).Intersect(list2.Select(b => b.id));

But this return a list of ids and i need a list of elements contained in both lists.

Thank you!

So the result is `List` with only those where the ID appears in both collections? — Tim Schmelter, Jan 27 '16 at 09:20

Tim Schmelter · Answer 1 · 2016-01-27T09:28:10.533

I would use Enumerable.Join:

var intersecting = from i1 in l1
                   join i2 in l2
                   on i1.id_infraestructura equals i2.id_infraestructura
                   select i1;
List<INFRAESTRUCTURA> result = intersecting.ToList();

If you would override Equals + GetHashCode in INFRAESTRUCTURA or provide a custom IEqualityComparer<INFRAESTRUCTURA> you could use Enumerable.Intersect directly:

List<INFRAESTRUCTURA> result = l1.Intersect(l2).ToList();

Here's a possible implementation:

public class InfrastructureComparer : IEqualityComparer<INFRAESTRUCTURA>
{
    public bool Equals(INFRAESTRUCTURA x, INFRAESTRUCTURA y)
    {
        if (x == null && y == null) return true;
        if (x == null || y == null) return false;
        return x.id_infraestructura == y.id_infraestructura;
    }

    public int GetHashCode(INFRAESTRUCTURA obj)
    {
        if (obj == null) return 0;
        return obj.id_infraestructura;
    }
}

you can use the overloads which take an IEqualityComparer<T> like here:

List<INFRAESTRUCTURA> result = l1.Intersect(l2, new InfrastructureComparer()).ToList();

If you want both objects in the result you could use an anonymous type:

var intersecting = from i1 in l1
                   join i2 in l2
                   on i1.id_infraestructura equals i2.id_infraestructura
                   select new { i1, i2 };

score 1 · Answer 2 · answered Jan 27 '16 at 09:20

1

You can use Linq Join

l1.Join(l2, l => l.id_infraestructura, r => r.id_infraestructura, (l,r) => l.id_infraestructura);

answered Jan 27 '16 at 09:20

wertzui

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score 1 · Accepted Answer · edited May 23 '17 at 10:27

The other answers are correct, but you can use Intersect with your custom comparer. You can create custom comparer by implementing IEqualityComparer<> interface. And for implementing this interface we must implmenet two methods, Equals and GetHashCode.

public class InfraestructuraComparer: IEqualityComparer<INFRAESTRUCTURA>
{
    /// <summary>
    /// Whether the two INFRAESTRUCTURA are equal.
    /// </summary>
    public bool Equals(INFRAESTRUCTURA firstObj, INFRAESTRUCTURA secondObj)
    {
        if (firstObj == null && secondObj == null)
            return true;

        if (firstObj == null || secondObj == null)
            return false;

        // Your equality logic goes to here
        return firstObj.ID == secondObj.ID;
    }

    /// <summary>
    /// Return the hash code for this instance.
    /// </summary>
    public int GetHashCode(INFRAESTRUCTURA obj)
    {
        // Don't compute hash code on null object.
        if (obj == null) return 0;

        unchecked
        {
            var hash = 17;

            hash = hash * 23 + obj.Id.GetHashCode();

            return hash;    
        }
    }
}

And then:

var result = list1.Intersect(list2, new InfraestructuraComparer());

You can also use this comparer in Except method, for finding the difference of two sequences.

var result = list1.Except(list2, new InfraestructuraComparer());

Additionally:

From the first point of view you may misunderstood GetHashCode(). You can read about this method in many question of StackOverflow. You can read the answer to this question.

Intersect between 2 collections in C#

3 Answers3