Intersect not giving expected results

Question

In the below code if I iterated over L3 I would expect to have 2 results available. However it only has one result and that result is the objTest with a Id = 9. I thought it should be a result set with two objtest s' Id = 9 and Id = 10.

class Program
{
public class objTest 
{
    public int Value { get; set; }
    public bool On  { get; set; }
    public int Id { get; set; }
}
class PramComp : EqualityComparer<objTest>
{
    public override bool Equals(objTest x, objTest y)
    {
        return x.Value == y.Value;
    }
    public override int GetHashCode(objTest obj)
    {
        return obj.Value.GetHashCode();
    }
}
 static void Main(string[] args)
{
    List<objTest> L1 = new List<objTest>();
    L1.Add(new objTest { Value = 1, On = true ,Id =1});
    L1.Add(new objTest { Value = 2, On = false ,Id =2});
    L1.Add(new objTest { Value = 3, On = false, Id = 3 });
    L1.Add(new objTest { Value = 4, On = false ,Id =4});
    L1.Add(new objTest { Value = 5, On = false ,Id =5});

    List<objTest> L2 = new List<objTest>();
    L2.Add(new objTest { Value = 6, On = false ,Id =6});
    L2.Add(new objTest { Value = 7, On = false ,Id=7});
    L2.Add(new objTest { Value = 8, On = false,Id =8 });
    L2.Add(new objTest { Value = 1, On = true,Id =9 });
    L2.Add(new objTest { Value = 1, On = true, Id =10 });

    var L3 = L2.Intersect(L1, new PramComp());

}
}

So I have made a mistake with my code if I want to return the two results Id=9 and Id=10. Could someone tell me where my mistake is?

Answer 1

Enumerable.Intersect is designed to produce:

the set intersection of two sequences.

A "set" does not allow duplicate values, which means you'll only get one of the two values.

If you want to return both of the ones that match, you could use:

var L3 = L2.Where(item => L1.Any(one => one.Value == item.Value));

This will go through the L2 collection and find all items which have any matching value in L1, but preserve the duplicates.