using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Dictionary<DateTime, NotificationType> NotificationHistory = new Dictionary<DateTime, NotificationType>

        {

            {DateTime.Parse("10/17/2021"), NotificationType.TeamsEnvironmentInactivityFirstDisableWarning}, 

            {DateTime.Parse("10/30/2021"), NotificationType.TeamsEnvironmentInactivitySecondDisableWarning}, 

            {DateTime.Parse("11/20/2021"), NotificationType.TeamsEnvironmentInactivitySecondDisableWarning}

        };

        var res = NotificationHistory.GroupBy(x => x.Value).Select(g => g.OrderByDescending(x => x.Key).First()).ToDictionary(x => x.Key, x => x.Value);

        Console.WriteLine(res.Keys.Count);

        Console.WriteLine(res.Keys.First());

        Console.WriteLine(res.Values.First());

    }

​

    public enum NotificationType

    {

        /// <summary>

        /// The teams environment inactivity first disable warning

        /// </summary>

        TeamsEnvironmentInactivityFirstDisableWarning,

        /// <summary>

        /// The teams environment inactivity second disable warning

        /// </summary>

        TeamsEnvironmentInactivitySecondDisableWarning,

        /// <summary>

        /// The teams environment inactivity disabled

        /// </summary>

        TeamsEnvironmentInactivityDisabled,

        /// <summary>

        /// The teams environment inactivity first delete warning

        /// </summary>

        TeamsEnvironmentInactivityFirstDeleteWarning,

        /// <summary>

        /// The teams environment inactivity second delete warning

        /// </summary>

        TeamsEnvironmentInactivitySecondDeleteWarning,

        /// <summary>

        /// The teams environment inactivity deleted

        /// </summary>

        TeamsEnvironmentInactivityDeleted,

    }

}