Getting TKey from a dictionary with common values where TValue is List<string>

Question

I have a dictionary which looks like this:

Dictionary<string, List<string>> dict = new Dictionary<string, List<string>>()
{
    {"a" , new List<string> { "Red","Yellow"} },
    {"b" , new List<string> { "Blue","Red"} },
    {"c" , new List<string> { "Green","Orange"} },
    {"d" , new List<string> { "Black","Green"} },
};

I need output as a dictionary from the dict where common value in the List<string> should be the key and the value should be the list of keys.

E.g.:

Red: [a,b]
Green: [c,d]

I don't know how to solve this problem with list in dictionary as TValue.

Please explain me how to handle lists with in dictionary.

Answer 1

You can flattern your dictionary with SelectMany and get plain list that looks like

"a" - "Red"
"a" - "Yellow"
"b" - "Blue"
"b" = "Red"
// and so on

then group by value and build a new dictionary from those groups. Try this code:

var commonValues = dict.SelectMany(kv => kv.Value.Select(v => new {key = kv.Key, value = v}))
    .GroupBy(x => x.value)
    .Where(g => g.Count() > 1)
    .ToDictionary(g => g.Key, g => g.Select(x => x.key).ToList());