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

I'm not sure exactly how this differs from your previous question.

If you're just asking how to return a Dictionary> rather than Dictionary> then all you need is a call to the ToList method.

Stealing and amending Mehrdad's answer to your other question:

var classToStudent = studentToClass
    .SelectMany(
        pair => pair.Value.Select(val => new { Key = val, Value = pair.Key }))
    .GroupBy(item => item.Key)
    .ToDictionary(gr => gr.Key, gr => gr.Select(item => item.Value).ToList());