How to use LINQ to select some fields.

Question

Hello:
I have this requirements, the following is the data:
var A1 = ("A", "EUR/USD", "EURO");
var A2 = ("A", "CHF/USD", "CHF");
var A3 = ("A", "EUR/GBP", "EURO");
var A4 = ("B", "EUR/USD", "EURO");
var A5 = ("B", "CHF/USD", "CHF");
var A6 = ("B", "EUR/GBP", "EURO");
var A7 = ("C", "EUR/USD", "EURO");
var A8 = ("C", "CHF/USD", "CHF");
List<(string, string, string)> forex_pairs = new()
{
A1, A2, A3, A4, A5, A6, A7, A8
};
I want to select the first item from the list of tuples, where the last 2 items are in the same group.
In the above example, I want to get the following results:
Group#1: ("EUR/USD", "EURO") have the following HashSet<string> members: ("A", "B", "C");
Group#2: ("CHF/USD", "CHF") have the following HashSet<string> members: ("A", "B", "C");
Group#3: ("EUR/GBP", "EURO") have the following HashSet<string> members: ("A", "B");
I know how to group by the last 2 items, but I don’t know how to find the first item and put them into a HashSet. Please advise,
Thanks,

Answer 1

Once you've grouped by the second & third items you'll have an IEnumerable<> of your items per group, so you can just .ToHashSet() them:

   var A1 = ("A", "EUR/USD", "EURO");  
   var A2 = ("A", "CHF/USD", "CHF");  
   var A3 = ("A", "EUR/GBP", "EURO");  
   var A4 = ("B", "EUR/USD", "EURO");  
   var A5 = ("B", "CHF/USD", "CHF");  
   var A6 = ("B", "EUR/GBP", "EURO");  
   var A7 = ("C", "EUR/USD", "EURO");  
   var A8 = ("C", "CHF/USD", "CHF");  
     
   List<(string, string, string)> forex_pairs = new()  
   {  
   	A1, A2, A3, A4, A5, A6, A7, A8  
   };  
     
   var groups = new List<Group>();  
     
   foreach (var group in forex_pairs.GroupBy(p => new { Code = p.Item2, Name = p.Item3 })) {  
   	groups.Add(new Group(  
   		group.Key.Code,  
   		group.Key.Name,  
   		group.Select(g => g.Item1).ToHashSet()  
   	));  
   }  
     
   foreach (var group in groups) {  
   	Console.WriteLine($"{group.Code}+{group.Name}: {string.Join(", ", group.GroupIds)}");  
   }  
     
   record Group(string Code, string Name, HashSet<string> GroupIds);  

Answer 2

just select first of group:

var list = forex_pairs  
    .GroupBy(r => new { r.Item2, r.Item3 }, r => r)  
    .Select(r => r.First());