using System;

using System.Linq;

public class Program

{

    public static void Main() {

        var input = new [] {

         new { Key = "A", Value = 1 },

         new{ Key = "A", Value = 2 },

         new{ Key = "B", Value = 1 },

         new{ Key = "C", Value = 2 },

         new{ Key = "C", Value = 3 },

        };

        var output = input.GroupBy(i=> i.Key).ToDictionary(g => g.Key, h => h.Max(p => p.Value));

        foreach (var kvp in output)

{

    Console.WriteLine("Key = {0}, Value = {1}", kvp.Key, kvp.Value);

}

​

    }

}