Question

I have two collections that I want to intersect, and perform a sum operation on matching elements.

For example the collections are (in pseudo code):

col1 = { {"A", 5}, {"B", 3}, {"C", 2} }
col2 = { {"B", 1}, {"C", 8}, {"D", 6} }

and the desired result is:

intersection = { {"B", 4}, {"C", 10} }

I know how to use an IEqualityComparer to match the elements on their name, but how to sum the values while doing the intersection?

EDIT:

The starting collections haven’t two items with the same name.

Answer 1

Let’s say your input data looks like this:

IEnumerable<Tuple<string, int>> firstSequence = ..., secondSequence = ...;

If the strings are unique in each sequence (i.e there can be no more than a single {“A”, XXX} in either sequence) you can join like this:

var query = from tuple1 in firstSequence
            join tuple2 in secondSequence on tuple1.Item1 equals tuple2.Item1
            select Tuple.Create(tuple1.Item1, tuple1.Item2 + tuple2.Item2);

You might also want to consider using a group by, which would be more appropriate if this uniqueness doesn’t hold:

var query = from tuple in firstSequence.Concat(secondSequence)
            group tuple.Item2 by tuple.Item1 into g
            select Tuple.Create(g.Key, g.Sum());

If neither is what you want, please clarify your requirements more precisely.

EDIT: After your clarification that these are dictionaries – your existing solution is perfectly fine. Here’s another alternative with join:

var joined = from kvp1 in dict1
             join kvp2 in dict2 on kvp1.Key equals kvp2.Key
             select new { kvp1.Key, Value = kvp1.Value + kvp2.Value };

var result = joined.ToDictionary(t => t.Key, t => t.Value);

or in fluent syntax:

var result = dict1.Join(dict2,
                        kvp => kvp.Key,
                        kvp => kvp.Key,
                        (kvp1, kvp2) => new { kvp1.Key, Value = kvp1.Value + kvp2.Value })
                  .ToDictionary(a => a.Key, a => a.Value);