using System;

using System.Collections.Generic;

using System.Linq;

public class CustomerComparer : IComparer<KeyValuePair<int, Data>>

{

    private List<string> orderedLetters = new List<string>() { "C", "A", "S" };

​

    public int Compare(KeyValuePair<int, Data> str1, KeyValuePair<int, Data> str2)

    {

        return orderedLetters.IndexOf(str1.Value.Identity) - orderedLetters.IndexOf(str2.Value.Identity);

    }

}

​

public class Program

{

    public static void Main()

    {

        Data value1 = new Data { Name = "Name1", Identity = "S", LastUsed = new DateTime().AddMinutes(1)};

        Data value2 = new Data { Name = "Name2", Identity = "A", LastUsed = new DateTime().AddMinutes(2)};

        Data value3 = new Data { Name = "Name3", Identity = "C", LastUsed = new DateTime().AddMinutes(3)};

        Data value4 = new Data { Name = "Name4", Identity = "C", LastUsed = new DateTime().AddMinutes(4)};

​

        Dictionary<int, Data> unsortedDictionary = new Dictionary<int, Data>();

        unsortedDictionary.Add(1, value1);

        unsortedDictionary.Add(2, value2);

        unsortedDictionary.Add(3, value3);

        unsortedDictionary.Add(4, value4);

​

        //var customSortedValues = unsortedDictionary.Values.OrderBy(item => item, new CustomerComparer()).ToArray();

        var customSortedValues = unsortedDictionary.Values.OrderByDescending(item => item.LastUsed).ToArray();

​

        Console.WriteLine(customSortedValues[2].ToString()); //Random Access, lookup via key - O(1)

        Console.WriteLine(customSortedValues.First().ToString()); //Get least recently used key

    }

}

​

​

​

public class Data

{

    public string Name { get; set; }

    public string Identity { get; set; }

    public DateTime LastUsed { get; set; }

    public virtual string ToString(){

        return $"{this.Name} : {this.Identity} : {this.LastUsed}";

    }

}