Mixing numbers and letters in one dictionary seems odd; what are the TKey and TValue types for your dictionaries? Just string?

Yeah I just realized that as well, my example wasnt well constructed. I am essentially trying to get the fields of a type using reflection and they all have a type that implements the same interface. So the type of the dictionary is Dictionary<string, IInterface>

And I need to get the values of multiple of objects of the same type in a single dictionary based on the field name
So the output type is Dictionary<string, List<IInterface>>

Alright I fiddled around and came up with this:

dictionaries
  .SelectMany(dict => dict)
  .ToLookup(pair => pair.Key, pair => pair.Value)
  .ToDictionary(grouping => grouping.Key, grouping => grouping.ToList());

This actually does what I want.

The benchmark results do not favor LINQ here FYI 🙂

(Of course, when do the benchmark results ever favor LINQ?)

its manually triggered by UI interaction, so performance is not a concern