LINQ-如何按字典分组 <string, string[]> ？

Question

I have a dictionary like this:

Dictionary<string, string[]> dic = new Dictionary<string,string[]>(){
            {"A", new string [] {"1", "2", "$", "3", "4"}},
            {"B", new string [] {"5", "6", "$", "7", "8"}},
            {"C", new string [] {"9", "10", "@", "11", "12"}}
        };

and I'd like to turn it in to a new Dictionary like this one:

Dictionary<string, List<string[]>> res = new Dictionary<string,List<string[]>>{
            {"$", new List<string[]> { 
                new string [] {"1", "2", "A", "3", "4"}, 
                new string [] {"5", "6", "B", "7", "8"} 
                }
            },               
            {"@", new List<string[]> { 
                new string [] {"9", "10", "C", "11", "12"}
                }
            }
        };

so the new Key becomes the 3rd element of the old string array, and the old Key is added to the new string array.

note - the old Key does not need to be placed as the new array's 3rd element, but it does need to be in the same index for each new array.

I started trying to use some LINQ for this, but can't wrap my head around the whole thing - this:

Dictionary<string, string[]> test = dic.GroupBy(x => x.Value[2])
.ToDictionary(s => s.Key, s => s.Select(x => x.Key).ToArray());

only works to create another string keyed array valued dictionary where the key's correctly become the 3rd element, but the values are just the old keys.

Answer 1

One possible solution:

Dictionary<string, string[]> dic = new Dictionary<string,string[]>(){
            {"A", new string [] {"1", "2", "$", "3", "4"}},
            {"B", new string [] {"5", "6", "$", "7", "8"}},
            {"C", new string [] {"9", "10", "@", "11", "12"}}
        };

var res = dic.Select(p => new KeyValuePair<string, string[]>(
                              p.Value[2], 
                              p.Value.Select((v,i) => i == 2 ? p.Key : v).ToArray()))
             .GroupBy(p => p.Key)
             .ToDictionary(g => g.Key, g => g.Select(p => p.Value).ToList());

Answer 2

var replacedIndex = 2;

var newDictionary = 
    oldDictionary.GroupBy(x => x.Value.ElementAt(replacedIndex))
                 .ToDictionary(group => group.Key, group => group.Select(x =>
                 {
                     var collection = x.Value;
                     collection[replacedIndex] = x.Key;
                     return collection;
                 }));