Reverse many-to-many Dictionary>

Question

Actually my previous question got me thinking and I realized that reversing a Dictionary is not trivial. What is the most elegant and readable way to do it?

Answer 1

To reverse a dictionary is very easy:

var newDic = oldDic.ToDictionary(x => x.Value, x => x.Key);

That's all.

Now, your question is different. It is about reversing a many-to-many relationship, established on a dictionary.

So, let's say you have Dictionary>. The idea is to extract from this the "middle table" of the many-to-many relationship. Then you can regroup it by the other side, and retransform into a dictionary.

For the first part, we will use the overload of SelectMany that

"Projects each element of a sequence to an IEnumerable, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein."

var table =
    dict.SelectMany(
        x => x.Value,
        (dictEntry, entryElement) => new
               {
                      Entity1 = dictEntry.Key,
                      Entity2 = entryElement
               }
    );

So, now you just have to regroup this table the way you want, and then convert it to a dictionary.

 var newDict =
     table
         .GroupBy(x => x.Entity2,
                  x => x.Entity1,
                  (entity2, entity1) => new {entity1, entity2})
         .ToDictionary(x => x.entity2, x => x.entity1);