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//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            List<MyClass> mytable = new List<MyClass>();
            mytable.Add(new MyClass{client_id = 1014, _month = "2009-11"});
            mytable.Add(new MyClass{client_id = 1015, _month = "2009-11"});
            mytable.Add(new MyClass{client_id = 2560, _month = "2009-11"});
            mytable.Add(new MyClass{client_id = 2562, _month = "2009-11"});
            mytable.Add(new MyClass{client_id = 2570, _month = "2009-11"});
            mytable.Add(new MyClass{client_id = 4123, _month = "2010-11"});
            mytable.Add(new MyClass{client_id = 4122, _month = "2010-11"});
            mytable.Add(new MyClass{client_id = 2560, _month = "2010-11"});
            mytable.Add(new MyClass{client_id = 1015, _month = "2010-11"});
            mytable.Add(new MyClass{client_id = 2560, _month = "2007-11"});            
            
            var v = mytable
                .GroupBy(x => x.client_id)
                .Where(x => x.Min(y => DateTime.ParseExact(y._month,"yyyy-MM",null)) == new DateTime(2009,11,1))
                .Select(x=>x.Key);
            foreach(var k in v)
            {
                Console.WriteLine(k);
            }
            
            Console.WriteLine("Hello, world!");
        }
    }
    
    public class MyClass
    {
        public int client_id {get;set;}
        public string _month {get;set;}
    }
}

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Compilation time: 0.3 sec, absolute running time: 0.14 sec, cpu time: 0.11 sec, average memory usage: 18 Mb, average nr of threads: 5