Generic group-by in C#
C# interview exercise on Async & Generics, difficulty 3 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write GroupInto(items, key) that returns a Dictionary mapping each key(item) to the list of items that produced it, in input order. A dictionary key can never be null, which the constraint `where TKey : notnull` states.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.