Question

I need to retrieve all items from two lists that contains a given value.

Example:

var list1 = {
    new Dummy(){ Name = "Dummy1", Number = 1 },
    new Dummy(){ Name = "Dummy2", Number = 2 },
    new Dummy(){ Name = "Dummy3", Number = 3 }
};

var list2 = {
    new Dummy(){ Name = "Dummy4", Number = 4 },
    new Dummy(){ Name = "Dummy5", Number = 2 },
    new Dummy(){ Name = "Dummy6", Number = 6 }
};

var list3 = GetAllDummiesWithNumbersContainedInBothLists();

I want list3 to contain Dummy2 and Dummy5, since both have the same number.

How do i do this? It should be simple but i cant figure it out…

Answer 1

I’m not entirely sure what your requirements are, but something like this perhaps?

var commonIds = list1.Select(d => d.Number)
                     .Intersect(list2.Select(d => d.Number));

var commonIdsSet = new HashSet<int>(commonIds);

var list3 = list1.Concat(list2)
                 .Where(d => commonIdsSet.Contains(d.Number))
                 .ToList();

if you can clarify the exact requirements (do the results need to be grouped by theNumber, IsNumberunique for an item within a list etc.), we can provide better solutions.