如何转换清单 <Tuple<int, int> &gt;到字典 <int, List<int> &gt;？

Question

I have duplicate keys with different values and I want to convert it to a dictionary with 1 key and its values.

The next example will explain best what I mean:

var tup = new List<Tuple<int, int>>();
tup.Add(new Tuple<int, int>(1, 1));
tup.Add(new Tuple<int, int>(1, 2));

var dic = new Dictionary<int, List<int>>();

What is an elegant way to convert the tup to dic?

I managed to do this with foreach but would like to write it in LINQ.

foreach (var item in tup)
{
    if (dic.ContainsKey(item.Item1))
    {
        dic[item.Item1].Add(item.Item2);
    }
    else
    {
        dic.Add(item.Item1, new List<int> { item.Item2 });
    }
}

Answer 1

var list = tup.GroupBy(x => x.Item1)
              .ToDictionary(
                    x => x.Key, 
                    x => x.Select(y => y.Item2).ToList());

First, we group by GroupBy item 1. This should be obvious enough.

Then, we call ToDictionary and pass in a keySelector and an elementSelector . They select the key and value respectively, given an IGrouping<int, Tuple<int, int>> .

For reference, this particular overload of ToDictionary is used .

Alternatively, as Iridium has said in the comments, this works as well:

var list = tup.GroupBy(x => x.Item1, x => x.Item2)
              .ToDictionary(x => x.Key, x => x.ToList());

This overload of GroupBy allows you to select 2 things!

Answer 2

You first need to group by the first tuple element in order to find all elements that have the same key in the dictionary. And then just collect the second tuple elements and make a list out of it:

tup.GroupBy(t => t.Item1)
   .ToDictionary(g => g.Key, g => g.Select(t => t.Item2).ToList());

Answer 3

You can use GroupBy to resolve this problem, like:

var tup = new List<Tuple<int, int>>();
tup.Add(new Tuple<int, int>(1, 1));
tup.Add(new Tuple<int, int>(1, 2));

var dic = tup
         .GroupBy(x => x.Item1)
         .ToDictionary(x => x.Key, tuples => tuples.Select(x => x.Item2).ToList());

BTW, in some cases you can use NameValueCollection , but this is not save your target type, for example

var nvc = tup.Aggregate(new NameValueCollection(),
  (seed, current) =>
  {
    seed.Add(current.Item1.ToString(), current.Item2.ToString());
    return seed;
  });

foreach (var item in nvc)
{
  Console.WriteLine($"Key = {item} Value = {nvc[item.ToString()]}");
}