using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<Thing> listThings = new List<Thing>() {

            new Thing(1,"Did action A",1),

            new Thing(1,"Did action B",2),

            new Thing(1,"Did action C",3),

            new Thing(2,"Did action A",1),

            new Thing(2,"Did action B",2)

        };

        List<int> userNums=listThings.Select(x => x.UserNumber).Distinct().ToList();

        List<Thing> listFilteredThings = new List<Thing>();

        foreach(int i in userNums){

            List<Thing> tmp = listThings.Where(x => x.UserNumber==i).ToList();

            listFilteredThings.AddRange(tmp.Where(x => x.ActionNum==tmp.Max(y => y.ActionNum)).ToList());

        }

        foreach(Thing thing in listFilteredThings){

            Console.WriteLine(thing.UserNumber +", " + thing.Name);

        }

    }

}

​

public class Thing {

    public int UserNumber;

    public string Name;

    public int ActionNum;

    public Thing(int userNum, string name, int actionNum){

        this.UserNumber=userNum;

        this.Name=name;

        this.ActionNum=actionNum;

    }

}