using System;

using System.Collections.Generic;

using System.Linq;

​

public class UniqueKeyKvpList : List<KeyValuePair<char, int>>

{

    private readonly HashSet<char> _keys = new HashSet<char>();

​

    // Hiding base method List<KeyValuePair<char, int>>.Add()

    public new void Add(KeyValuePair<char, int> kvp) => Add(kvp.Key, kvp.Value);

    // Simpler .Add() method; imitates Dictionary.Add() in usage

    public void Add(char key, int value) => AddIfUniqueKey(key, value);

​

    private void AddIfUniqueKey(char key, int value)

    {

        if (!_keys.Contains(key))

        {

            _keys.Add(key);

            base.Add(new KeyValuePair<char, int>(key, value));

        }

    }

}

​

public class Program

{

    public static void Main()

    {

        var myKvpList = new UniqueKeyKvpList();

​

        myKvpList.Add('a', 4);

        myKvpList.Add('b', 7);

        myKvpList.Add('c', 1);

        myKvpList.Add('d', 7);

        myKvpList.Add('e', 2);

        myKvpList.Add('c', 7); // will not be added

        myKvpList.Add('f', 7);

        myKvpList.Add('g', 3);

​

        Console.WriteLine($"{nameof(myKvpList)} contains:");

        Console.WriteLine(string.Join(Environment.NewLine, myKvpList));

        Console.WriteLine();

        // .Net 6 -- due to use of .MaxBy()

        char firstCharWithMaxValue = myKvpList

            .MaxBy(kvp => kvp.Value)

            .Key;

​

        char[] charsWithMaxValue = myKvpList

            .GroupBy(kvp => kvp.Value)

            .MaxBy(gr => gr.Key) // Group key is the kvp value

            .Select(kvp => kvp.Key)

            .ToArray();

​

        Console.WriteLine($"Char of first max value: {firstCharWithMaxValue}");

        Console.WriteLine($"Char of entries with max value: {string.Join(", ", charsWithMaxValue)}");

        // < .Net 6 (alternatives)

        char firstCharWithMaxValue_alt = myKvpList

            .OrderByDescending(kvp => kvp.Value)

            .First()

            .Key;

        char[] charsWithMaxValue_alt = myKvpList

            .GroupBy(kvp => kvp.Value)

            .OrderByDescending(gr => gr.Key) // Group key is the kvp value

            .First()

            .Select(kvp => kvp.Key)

            .ToArray();

        Console.WriteLine($"(alt) Char of first max value: {firstCharWithMaxValue_alt}");

        Console.WriteLine($"(alt) Char of entries with max value: {string.Join(", ", charsWithMaxValue_alt)}");

    }

}

​