using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        TestLinq();

    }

​

    static void TestLinq()

    {

        var validKeys = new List<string>()

        {

            "a-1",

            "b-1",

            "c-1",

            "d-1",

            "e-1",

            "f-1",

            "g-1",

            "h-1",

            "i-1",

            "j-1",

            "a-2",

            "b-2",

            "c-2",

            "d-2",

            "e-2",

            "f-2",

            "g-2",

            "h-2",

            "i-2",

            "j-2"

        };

        foreach (var keyGroup in validKeys.GroupBy(k => GetGroup(k)))

        {

            Console.WriteLine($"Group: {keyGroup.Key}");

            foreach (var key in keyGroup)

            {

                Console.WriteLine($"Hashslot: {keyGroup.Key}, Key: {key}");

            }

        }

    }

​

    static string GetGroup(string key)

    {

        Console.WriteLine($"Check Key for slot: {key}");

        return key[..1];

    }

}